Hi-Fi amplifier based on complementary field-effect transistors. Hi-fi amplifier using complementary field-effect transistors Hi-fi amplifier using complementary field-effect transistors

Hi-Fi AMPLIFIER WITH COMPLEMENTARY FIELD TRANSISTORS

E.PIRET.

A 100 Ohm linearizing resistor R11 is included in the emitter circuit of the input stage, and a general negative feedback of about 30 dB is connected to the emitters. “Inside” the cascade, between the collector of the “lower” transistor (T2) and the emitter of the “upper” transistor (T3), there is a second (“internal”) feedback loop of about 18 dB. This means that with the exception of transistors T1, T2, both loops have the same effect on all other stages.

Fig.1

Let's stop for a moment! The temperature coefficient Kt of field-effect transistors (i.e., the ratio of gate voltage/drain current) is close to zero. For small currents it is small and negative, for large currents it is small and positive. A sign change occurs for high-power transistors at a current of about 100 mA. The final amplifier operates at a quiescent current of 100 mA. Field-effect transistors are “swung” through transistor emitter followers, which, as is known, have a positive CT. Therefore, it is necessary to use a pre-biased circuit that compensates for the temperature dependence.

The temperature dependence of the emitter followers is compensated by diodes D3 and D4.

The quiescent current of the field-effect transistors of the final amplifier is set by potentiometer P at a level of about 100 mA.

Resistors (R29, R30) are installed in the gate circuits of field-effect transistors to prevent self-excitation. A circuit consisting of diodes and zener diodes (D5...D8) prevents the appearance of a gate-source voltage that is dangerous for field-effect transistors.

In the source circuit of the field-effect transistors there are resistors (R31 and R32) with a nominal value of 0.47 Ohms. Of these, R32 is marked with an asterisk - in the prototype its value was zero. This resistor smooths out possible differences in the transconductance of the field-effect transistors. As a rule, turning on R32 does not have a catastrophic effect on the gain; you can expect an increase in distortion by an amount of about 20...30%.

As usual, the RCL link at the amplifier output protects it from self-excitation under extremely high reactive load impedance.

The Rx resistance in the T1 emitter circuit at the amplifier input is used to accurately balance the amplifier. If we take R13 and R14 of the same size (6.8 kOhm), and short-circuit Rx, then the output bias will be quite satisfactory. But if it is necessary to improve it, then R13 is reduced to 6.2 kOhm, and instead of Rx, a 1 kOhm potentiometer is temporarily connected. After approximately 30 minutes of “warming up” the amplifier, this potentiometer sets the output voltage level to zero. The resistance of the potentiometer is measured, and a resistor with a value that is closest to the measured one is soldered as Rx. Typically, when replacing D1 or D2, it becomes necessary to replace Rx.

Capacitor C9 performs frequency correction of the amplifier. It causes a double effect: on the one hand, it carries out a “lag” correction with a capacitive load of the collectors T7 and T8 and, on the other hand, a “advanced” correction, being connected not to ground, but to R21.

Resistor R34 prevents the occurrence of two different ground loops in the case when two or more UMZCHs are powered from the same power supply. The input ground is connected to the metal case or chassis and to the preamplifier, and the other grounds, which are essentially return wires for zero currents, are connected separately to the neutral point of the power supply.

Installation. The amplifier is assembled on a double-sided printed circuit board, the drawing of which is shown in Fig. 2-3.

On the parts side there is a continuous grounding foil. Countersinking at the places where the leads of the parts “enter” the board prevents short circuits. The leads of the parts connecting to the ground are soldered directly (without holes) to the grounding foil. On the assembly drawing these points are marked in black.

Two final field-effect transistors are installed on aluminum corners, which are connected to the heatsink, creating a thermal bridge, and both are attached to the board. They must be isolated from the corners and the board. The resistor in the emitter circuit “hangs in the air” because it is mounted mounted. To shorten the leads, resistors R29 and R30 are soldered on the side of the board tracks. The heat sinks should not form a “false ground” with the “zero” foil, so the “zero” foil is interrupted by a deep scratch running parallel to the heat sinks. For normal cooling of field-effect transistors, a cooling surface of about 400 cm 2 is sufficient. Transistors T9 and T10 are attached to the “zero” foil through a thin mica plate. A short circuit can very easily occur here, so the installation must be carefully checked with an ohmmeter.

The 10 mm diameter L1 coil consists of approximately 15 tightly wound turns of 0.5 mm diameter wire (without core). Resistor R33 is located along the L1 axis, and its leads are soldered together with the coil leads and then attached to the board.

The three wires going to the power supply are twisted together. The two wires leading to the speaker are also twisted into a separate bundle (regardless of the previous ones). Since large currents flow here, their magnetic fields can significantly increase distortion - mainly at high frequencies.

Twisting wires together causes the magnetic fields of currents flowing in opposite directions to cancel each other out.

The neutral point of the power supply and the speaker terminal are not connected to the case, and the wires going to them are not laid together with other wires.

Power unit. The power supply circuit is the simplest (Fig. 4). A transformer tapped from the middle of the secondary winding feeds a full-wave rectifier consisting of two groups of 2 diodes. Ripple smoothing is carried out by capacitors with a capacity of at least 4700 µF (40 V). Such a unit can provide power to two final amplifiers.

The field effect transistors used in the final amplifier are very expensive. For the price of one such transistor you can purchase the rest of the set of parts. The question inevitably arises whether the excess costs are compensated by the expected improvement in quality. The answer to this question depends on many circumstances, because:

We are talking about subjectively perceived distortion, so the sound experience will vary from person to person;

The perception of distortion depends on the music being played. When playing purely “authored” electronic music, it makes no sense to talk about distortions, because it is impossible to know whether or not these distortions were in the source material;

Playing music coming from a CD is problematic. According to “critical ears” and the author, this music has a specific coloring. Playing back from a good analog record or directly from a concert gives excellent quality.

Translation by A. Belsky.

The amplifier circuit is shown in Fig. 1. Through the RC low-pass filter chain, the signal enters the complementary input stage (T1, T2, T3, T4). If desired, you can increase the capacitance of the coupling capacitor C1, but this only makes sense if the cutoff frequency of the sound-emitting system is very low. A 100 Ohm linearizing resistor R11 is included in the emitter circuit of the input stage, and a general negative feedback of about 30 dB is connected to the emitters. “Inside” the cascade, between the collector of the “lower” transistor (T2) and the emitter of the “upper” transistor (T3), there is a second (“internal”) feedback loop of about 18 dB. This means that with the exception of transistors T1, T2, both loops have the same effect on all other stages.

Through an emitter follower (the main role of which is to shift the DC voltage level), the signal from the input stage is fed to a voltage amplifier (T7, T8). Linearizing resistors are again installed in the emitters of the transistors. The collector current of these transistors flows through circuits that regulate the quiescent current of the field-effect transistors of the final amplifier. Let's stop for a moment! The temperature coefficient Kt of field-effect transistors (i.e., the ratio of gate voltage/drain current) is close to zero. For small currents it is small and negative, for large currents it is small and positive. A sign change occurs for high-power transistors at a current of about 100 mA. The final amplifier operates at a quiescent current of 100 mA. Field-effect transistors are “swung” through transistor emitter followers, which, as is known, have a positive CT. Therefore, it is necessary to use a pre-biased circuit that compensates for the temperature dependence. The temperature dependence of the emitter followers is compensated by diodes D3 and D4. The quiescent current of the field-effect transistors of the final amplifier is set by potentiometer P at a level of about 100 mA. Resistors (R29, R30) are installed in the gate circuits of field-effect transistors to prevent self-excitation. A circuit consisting of diodes and zener diodes (D5...D8) prevents the appearance of a gate-source voltage that is dangerous for field-effect transistors. In the source circuit of the field-effect transistors there are resistors (R31 and R32) with a nominal value of 0.47 Ohms. Of these, R32 is marked with an asterisk - in the prototype its value was zero. This resistor smooths out possible differences in the transconductance of the field-effect transistors. As a rule, turning on R32 does not have a catastrophic effect on the gain; you can expect an increase in distortion by an amount of about 20...30%. As usual, the RCL link at the amplifier output protects it from self-excitation under extremely high reactive load impedance. The Rx resistance in the T1 emitter circuit at the amplifier input is used to accurately balance the amplifier. If we take R13 and R14 of the same size (6.8 kOhm), and short-circuit Rx, then the output bias will be quite satisfactory. But if it is necessary to improve it, then R13 is reduced to 6.2 kOhm, and instead of Rx, a 1 kOhm potentiometer is temporarily connected. After approximately 30 minutes of “warming up” the amplifier, this potentiometer sets the output voltage level to zero. The resistance of the potentiometer is measured, and a resistor with a value that is closest to the measured one is soldered as Rx. Typically, when replacing D1 or D2, it becomes necessary to replace Rx. Capacitor C9 performs frequency correction of the amplifier. It causes a double effect: on the one hand, it carries out a “lag” correction with a capacitive load of the collectors T7 and T8 and, on the other hand, a “advanced” correction, being connected not to ground, but to R21. Resistor R34 prevents the occurrence of two different ground loops in the case when two or more UMZCHs are powered from the same power supply. The input ground is connected to the metal case or chassis and to the preamplifier, and the other grounds, which are essentially return wires for zero currents, are connected separately to the neutral point of the power supply.

Installation. The amplifier is assembled on a double-sided printed circuit board, the drawing of which is shown in Fig. 2-3. On the parts side there is a continuous grounding foil. Countersinking at the places where the leads of the parts “enter” the board prevents short circuits. The leads of the parts connecting to the ground are soldered directly (without holes) to the grounding foil. On the assembly drawing these points are marked in black. Two final field-effect transistors are installed on aluminum corners, which are connected to the heatsink, creating a thermal bridge, and both are attached to the board. They must be isolated from the corners and the board. The resistor in the emitter circuit “hangs in the air” because it is mounted mounted. To shorten the leads, resistors R29 and R30 are soldered on the side of the board tracks. The heat sinks should not form a “false ground” with the “zero” foil, so the “zero” foil is interrupted by a deep scratch running parallel to the heat sinks. For normal cooling of field-effect transistors, a cooling surface of about 400 cm2 is sufficient. Transistors T9 and T10 are attached to the “zero” foil through a thin mica plate. A short circuit can very easily occur here, so the installation must be carefully checked with an ohmmeter. The 10 mm diameter L1 coil consists of approximately 15 tightly wound turns of 0.5 mm diameter wire (without core). Resistor R33 is located along the L1 axis, and its leads are soldered together with the coil leads and then attached to the board. The three wires going to the power supply are twisted together. The two wires leading to the speaker are also twisted into a separate bundle (regardless of the previous ones). Since large currents flow here, their magnetic fields can significantly increase distortion - mainly at high frequencies. Twisting wires together causes the magnetic fields of currents flowing in opposite directions to cancel each other out. The neutral point of the power supply and the speaker terminal are not connected to the case, and the wires going to them are not laid together with other wires.

Power unit. The power supply circuit is the simplest (Fig. 4). A transformer tapped from the middle of the secondary winding feeds a full-wave rectifier consisting of two groups of 2 diodes. Ripple smoothing is carried out by capacitors with a capacity of at least 4700 µF (40 V). Such a unit can provide power to two final amplifiers.

The upper limit of the voltage of the secondary winding of the transformer is determined by the type of transistors T7, T8 used. In the case of using the BC 546/556 pair, the supply voltage (in the absence of a signal) should not exceed 30...32 V. These transistors “do not tolerate higher voltages”. With a supply voltage of ±30 V, you can use a transformer of 220/2x22.5 V or 230/2x24 V. An amplifier with a supply voltage of ±30 V can deliver about 24 W (at 8 Ohms) to the load. The field effect transistors used in the final amplifier are very expensive. For the price of one such transistor you can purchase the rest of the set of parts. The question inevitably arises whether the excess costs are compensated by the expected improvement in quality. The answer to this question depends on many circumstances, because:

We are talking about subjectively perceived distortions, so the sound sensations will be different for different people;

the perception of distortion depends on the music being played. When playing purely “authored” electronic music, it makes no sense to talk about distortions, because it is impossible to know whether or not these distortions were in the source material;

It is problematic to play music coming from a CD. According to “critical ears” and the author, this music has a specific coloring. Playing back from a good analog record or directly from a concert gives excellent quality.

larger

Installation. The amplifier is assembled on a double-sided printed circuit board, the drawing of which is shown in Fig. 2-3. On the parts side there is a continuous grounding foil. Countersinking at the places where the leads of the parts “enter” the board prevents short circuits. The leads of the parts connecting to the ground are soldered directly (without holes) to the grounding foil. On the assembly drawing these points are marked in black. Two final field-effect transistors are installed on aluminum corners, which are connected to the heatsink, creating a thermal bridge, and both are attached to the board. They must be isolated from the corners and the board. The resistor in the emitter circuit “hangs in the air” because it is mounted mounted. To shorten the leads, resistors R29 and R30 are soldered on the side of the board tracks. The heat sinks should not form a “false ground” with the “zero” foil, so the “zero” foil is interrupted by a deep scratch running parallel to the heat sinks. For normal cooling of field-effect transistors, a cooling surface of about 400 cm 2 is sufficient. Transistors T9 and T10 are attached to the “zero” foil through a thin mica plate. A short circuit can very easily occur here, so the installation must be carefully checked with an ohmmeter. The 10 mm diameter L1 coil consists of approximately 15 tightly wound turns of 0.5 mm diameter wire (without core). Resistor R33 is located along the L1 axis, and its leads are soldered together with the coil leads and then attached to the board. The three wires going to the power supply are twisted together. The two wires leading to the speaker are also twisted into a separate bundle (regardless of the previous ones). Since large currents flow here, their magnetic fields can significantly increase distortion - mainly at high frequencies. Twisting wires together causes the magnetic fields of currents flowing in opposite directions to cancel each other out. The neutral point of the power supply and the speaker terminal are not connected to the case, and the wires going to them are not laid together with other wires.

The upper limit of the voltage of the secondary winding of the transformer is determined by the type of transistors T7, T8 used. In the case of using the BC 546/556 pair, the supply voltage (in the absence of a signal) should not exceed 30...32 V. These transistors “do not tolerate higher voltages”. With a supply voltage of ±30 V, you can use a transformer of 220/2x22.5 V or 230/2x24 V. An amplifier with a supply voltage of ±30 V can deliver about 24 W (at 8 Ohms) to the load. The field effect transistors used in the final amplifier are very expensive. For the price of one such transistor you can purchase the rest of the set of parts. The question inevitably arises whether the excess costs are compensated by the expected improvement in quality. The answer to this question depends on many circumstances, because:

We are talking about subjectively perceived distortions, so the sound sensations will be different for different people;

the perception of distortion depends on the music being played. When playing purely “authored” electronic music, it makes no sense to talk about distortions, because it is impossible to know whether or not these distortions were in the source material;

It is problematic to play music coming from a CD. According to “critical ears” and the author, this music has a specific coloring. Playing back from a good analog record or directly from a concert gives excellent quality.

Translation by A. Belsky. Radiotechnika, No. 7, 96

Fig.1.
Through an emitter follower (the main role of which is to shift the DC voltage level), the signal from the input stage is fed to a voltage amplifier (T7, T8). Linearizing resistors are again installed in the emitters of the transistors. The collector current of these transistors flows through circuits that regulate the quiescent current of the field-effect transistors of the final amplifier. Let's stop for a moment! The temperature coefficient Kt of field-effect transistors (i.e., the ratio of gate voltage/drain current) is close to zero. For small currents it is small and negative, for large currents it is small and positive. A sign change occurs for high-power transistors at a current of about 100 mA. The final amplifier operates at a quiescent current of 100 mA. Field-effect transistors are “swung” through transistor emitter followers, which, as is known, have a positive CT. Therefore, it is necessary to use a pre-biased circuit that compensates for the temperature dependence. The temperature dependence of the emitter followers is compensated by diodes D3 and D4. The quiescent current of the field-effect transistors of the final amplifier is set by potentiometer P at a level of about 100 mA. Resistors (R29, R30) are installed in the gate circuits of field-effect transistors to prevent self-excitation. A circuit consisting of diodes and zener diodes (D5...D8) prevents the appearance of a gate-source voltage that is dangerous for field-effect transistors. In the source circuit of the field-effect transistors there are resistors (R31 and R32) with a nominal value of 0.47 Ohms. Of these, R32 is marked with an asterisk - in the prototype its value was zero. This resistor smooths out possible differences in the transconductance of the field-effect transistors. As a rule, turning on R32 does not have a catastrophic effect on the gain; you can expect an increase in distortion by an amount of about 20...30%. As usual, the RCL link at the amplifier output protects it from self-excitation under extremely high reactive load impedance. The Rx resistance in the T1 emitter circuit at the amplifier input is used to accurately balance the amplifier. If we take R13 and R14 of the same size (6.8 kOhm), and short-circuit Rx, then the output bias will be quite satisfactory. But if it is necessary to improve it, then R13 is reduced to 6.2 kOhm, and instead of Rx, a 1 kOhm potentiometer is temporarily connected. After approximately 30 minutes of “warming up” the amplifier, this potentiometer sets the output voltage level to zero. The resistance of the potentiometer is measured, and a resistor with a value that is closest to the measured one is soldered as Rx. Typically, when replacing D1 or D2, it becomes necessary to replace Rx. Capacitor C9 performs frequency correction of the amplifier. It causes a double effect: on the one hand, it carries out a “lag” correction for the capacitive load of the collectors T7 and T8 and, on the other hand, a “advanced” correction, being connected not to ground, but to R21. Resistor R34 prevents the occurrence of two different ground loops in the case when two or more UMZCHs are powered from the same power supply. The input ground is connected to the metal case or chassis and to the preamplifier, and the other grounds, which are essentially return wires for zero currents, are connected separately to the neutral point of the power supply.

Installation. The amplifier is assembled on a double-sided printed circuit board, the drawing of which is shown in Fig. 2-3. On the parts side there is a continuous grounding foil. Countersinking at the “entry” points of the parts’ leads into the board prevents short circuits. The leads of the parts connecting to the ground are soldered directly (without holes) to the grounding foil. On the assembly drawing these points are marked in black. Two final field-effect transistors are installed on aluminum corners, which are connected to the heatsink, creating a thermal bridge, and both are attached to the board. They must be isolated from the corners and the board. The resistor in the emitter circuit “hangs in the air” because it is mounted mounted. To shorten the leads, resistors R29 and R30 are soldered on the side of the board tracks. The heat sinks should not form a “false ground” with the “zero” foil, so the “zero” foil is interrupted by a deep scratch running parallel to the heat sinks. For normal cooling of field-effect transistors, a cooling surface of about 400 cm2 is sufficient. Transistors T9 and T10 are attached to the “zero” foil through a thin mica plate. A short circuit can very easily occur here, so the installation must be carefully checked with an ohmmeter. The 10 mm diameter L1 coil consists of approximately 15 tightly wound turns of 0.5 mm diameter wire (without core). Resistor R33 is located along the L1 axis, and its leads are soldered together with the coil leads and then attached to the board. The three wires going to the power supply are twisted together. The two wires leading to the speaker are also twisted into a separate bundle (regardless of the previous ones). Since large currents flow here, their magnetic fields can significantly increase distortion - mainly at high frequencies. Twisting wires together causes the magnetic fields of currents flowing in opposite directions to cancel each other out. The neutral point of the power supply and the speaker terminal are not connected to the case, and the wires going to them are not laid together with other wires.
Power unit. The power supply circuit is the simplest (Fig. 4). A transformer tapped from the middle of the secondary winding feeds a full-wave rectifier consisting of two groups of 2 diodes. Ripple smoothing is carried out by capacitors with a capacity of at least 4700 µF (40 V). Such a unit can provide power to two final amplifiers.

Fig.4.
The upper limit of the voltage of the secondary winding of the transformer is determined by the type of transistors T7, T8 used. In the case of using the BC 546/556 pair, the supply voltage (in the absence of a signal) should not exceed 30...32 V. These transistors “do not tolerate higher voltages”. With a supply voltage of ±30 V, you can use a transformer of 220/2x22.5 V or 230/2x24 V. An amplifier with a supply voltage of ±30 V can deliver about 24 W (at 8 Ohms) to the load. The field effect transistors used in the final amplifier are very expensive. For the price of one such transistor you can purchase the rest of the set of parts. The question inevitably arises whether the excess costs are compensated by the expected improvement in quality. The answer to this question depends on many circumstances, because:
— we are talking about subjectively perceived distortions, so the sound sensations will be different for different people;
— the perception of distortion depends on the music being played. When playing purely “authored” electronic music, it makes no sense to talk about distortions, because it is impossible to know whether or not these distortions were in the source material;
— it is problematic to play music coming from a CD. According to “critical ears” and the author, this music has a specific coloring. Playing back from a good analog record or directly from a concert gives excellent quality.

The stereo power amplifier brought to the attention of readers is designed for a car cassette player, but, naturally, can also be used in portable equipment with a supply voltage of 9...13 V. The amplifier contains a minimum of parts, simple......

Many radio amateurs have heard about the beautiful sound achieved with tube amplifiers, but hearing from someone is one thing, but making and listening to it yourself is a completely different matter. Besides…….

The proposed UMZCH (Fig. 1) is built on the basis of the KR544UD2 operational amplifier. UMZCH parameters Operating frequency range, Hz, no less than 15...30000 Nonlinearity of amplitude-frequency characteristic, dB, no more than 2 Rated power at the load: -......

I once had a fascination with totally directly heated circuits. Strictly speaking, it has not gone away so far, but it has taken a soft, compromise form, when I easily get along, for example, with…….

We continue the topic of low-power amplifiers on integrated circuits. This time, let's look at the amplifier based on the MAX9751 chip. What is remarkable about this microcircuit? Well, first of all – low supply voltage – 5 volts, unipolar,…….

When creating high-power amplifiers in the output stage, it is necessary to use parallel connection of specially selected and matched groups of transistors, which significantly complicates and increases the cost of manufacturing the amplifier. It is much simpler and cheaper to use gain leaders in this cascade and powerfully sti - bipolar transistors with insulated gate(IGBT), since there are no questions about selecting and installing groups of transistors. But it is believed that such transistors can only operate in switching modes. In addition, there are practically no complementary pairs among them.

Currently, there is a strong opinion that only cascades with symmetrical output on complementary transistors are capable of providing high parameters of UMZCH. This is due to the fact that almost all of them repeat the topology developed by Lin at the company RCA back in 1956, - a differential input stage, a second voltage amplification stage and a symmetrical push-pull output stage - a current amplifier.But this structure is far from optimal if one of the arms of the output stage is built according to the Shikpai circuit, as happens when designing an UMZCH with powerful transistors of the same conductivity.

The main problem of an amplifier with an output stage based on transistors of the same conductivity is the potential instability generated by the fact that one of the arms of the output stage is covered by local negative feedback. As a result, the phase-frequency characteristics of the arms differ significantly. And this gives rise to ringing and parasitic generation in the output stage and requires additional correction that balances such an output stage, which reduces the overall cutoff frequency of the UMZCH and ultimately leads to increased distortion. Although such circuits do not arouse enthusiasm among designers, nevertheless, transistors of the same conductivity are widely used in the output stages of powerful UMZCH microcircuits due to their low cost of production. Of course, among bipolar transistors there are quite a lot of complementary pairs, and difficulties arise only with the selection of pairs of complementary transistors of the group IGBT, the attractiveness of using them is obvious. This hinders the use of such transistors, despite their undeniable advantages over bipolar and field-effect transistors.There are bridge circuits of powerful cascades that do not require complementary pairs of transistors. But they are quite complex, and it is difficult to use effective feedback in them.,as a result, bridge circuits are not widely used except in car radios, where they are used due to limited supply voltage.

Let us consider separately the asymmetrical push-pull output stage on IGBT (Fig. 1), when the upper transistor is connected according to a circuit with a common collector and the lower transistor is connected according to a circuit with a common emitter.

The dependence of the output voltage on the control current for the upper transistor will be: U H = l e (1+R 3 * S) *R n, and for the lower transistor - U H = l e * R 3 * S * R H . It can be noted that these dependences of the output voltage are very close, and with equal slope values and high resistance of resistors in the gate circuit(R 1, R 2) The output stage is almost symmetrical. But symmetry and linearity are different properties. And the remarkable property of this circuit is that the difference in the transconductance of the transistors can be compensated by selecting resistors. Such symmetry is unattainable for complementary field-effect transistors. The difference in transconductance between complementary pairs of field-effect transistors reaches 300%, and the difference in their input capacitance is approximately the same.

Of course, symmetry is high only at low frequencies, which are also audio frequencies. The challenge is to build a circuit that maintains symmetry over the widest frequency range. And here Lin’s topology is no longer optimal.

But let's return to the diagram in Fig. 1. The disadvantage of the cascade is that each arm requires its own signal generator, and as a result, difficulties arise in ensuring thermal stability of the quiescent current of the cascade. Much more convenient is the cascade excitation circuit in Fig. 2. Its attractiveness is that now two signal sources are not required, and control of such a cascade is much simpler. Moreover, here the change in the resistance of the signal source R changes the current from the current source to the resistors in the gate circuit of the transistors, and the change in resistance R r leads to an out-of-phase change in voltage at the gates of transistors. When increasing Rr the upper transistor is unlocked and the lower one is locked, when decreasing Rr the upper transistor is locked and the lower one is unlocked. The total value of the currents on the gate resistors, at any value Rr, remains unchanged and is determined by the current source. ,,,That is, here the input signal is converted into a control symmetrical antiphase current, but a control voltage that differs tens of times, for the upper and lower arms of the asymmetrical output stage, which is necessary to control the asymmetrical output stage. This is how the push-pull mode of operation of a powerful asymmetrical output stage is implemented.The initial current of the output transistors and thermal stabilization of the quiescent current is achieved by changing the current of one current source, since when the current of the current source decreases, both transistors are turned off.

The construction of an output stage using transistors of the same conductivity structure according to the proposed circuit is quite attractive due to its simplicity, especially with a high output power of the amplifier (more than 100 W), when IGBT- transistors have a number of advantages over bipolar and field-effect transistors. In addition, according to the company's developers PLINIUS The sound with amplifiers based on pnp transistors is better than those on pnp transistors, and in expensive models they prefer an asymmetrical output stage. This is explained by the fact that transistors of the preferred structure are more linear and have better frequency properties, as well as a higher gain.

For effective use IGBT, as well as field-effect transistors of the same conductivity, I propose a new structure of the UMZCH - an input cascode amplifier, then a composite stage on transistors of different conductivity with a current source and a zener diode, and, finally, a push-pull asymmetrical output stage with transistors of the same structure. This structure with voltage boost th and auxiliary circuits is shown in Fig. 3. The new structure creates the shortest signal path to the bottom transistor, which has T the worst frequency properties and, despite its simplicity, has a large overall gain.

Let's look at the diagram in Fig. 3 more details. Input signal, via resistor R 1 , which determines the input resistance of the amplifier, goes to the base of the transistor VT 1. Including this transistor in the cascode allows you to use a low-voltage, high-frequency, low-noise transistor at the input and neutralize the Miller effect, as well as reduce the influence of common-mode voltage. Transistor VT 2 should withstand the required voltage, i.e..be relatively high voltage. Using a "broken cascode" instead of a regular one protects the transistors VT 1 and VT 2 from breakdown, since when the input signal is overloaded, the current increases VT 1 and VT 2 limited by resistor R 3.

Using a differential input amplifier instead of a cascode amplifier will reduce the transconductance of the input stage by half and increase the noise of the input stage by 2 dB, and this will ultimately lead to increased distortion. There will also be a need to select a pair of input transistors.

From the output of the cascode amplifier, the signal goes to a composite cascade using transistors VT 3 VT 4, which perform the function Rr. These transistors are connected according to the OB-OE structure with emitter integration, which is optimal for the selection and use of transistors. Voltage and power gains of transistors VT 3 and VT 4 vary greatly, this requires application as VT 3 high-voltage, medium-power transistors, the frequency properties of which are, as a rule, much worse than low-power, low-voltage transistors. Therefore, turning it on in OB mode is more effective than in OE mode. Voltage gain for VT 4 not as big as for VT 3. Therefore, turning it on in the OE mode will not degrade the overall frequency response too much.

Selecting a suitable low-cost high-voltage p-p-p transistor structure for VT 3 does not cause problems, but the transistor VT 4 - low-voltage, low-power p-p-p structure of high-frequency transistors for wide application.

Field effect transistors as VT 1...VT 4 it is not advisable to use, since they have a lower slope than bipolar T transistors, which will be equivalent to reducing the gain of the stages and the linearity of the amplifier as a whole.

In order to increase the maximum voltage amplitude for half-cycles of positive polarity, a voltage additive was introduced in the form of a circuit R 6, C1. Although instead of a voltage boost, additional power can be used, which will expand the range of the amplifier to the low frequency region. Zener diode VD 1 compensates for residual voltage drop across transistors VT 3, VT 4 in half cycles minus polar With ty and thereby reduces the saturation voltage at the minus supply.

The use of parallel OOS, instead of the more common serial OOS, makes the amplifier less sensitive (in terms of linearity) to changes in the resistance of the signal source. Thus, as it increases, the nonlinear distortions of the amplifier do not increase as happens when using serial OOS.

A remarkable property of the proposed structure is the “natural” limitation of the maximum output current.The fact is that the voltage across the resistors R 5, R 7 can maximally take only twice the value of the original, and by choosing the resistance of the emitter resistors R 8, R 9 you can limit the maximum current of transistors by calculating it using the formula: Imax = (2 U initial – Umax)/R e,

where U start - gate-emitter voltage of transistors VT 5, VT 6, in which a given initial current flows through the transistors; Umax - gate-emitter voltage of transistors VT 5, VT 6 when maximum current flows through them; R e - resistor resistance R 8, R 9.

Due to the fact that the maximum voltage across the resistors R 5, R 7 does not exceed twice the initial value (for example: if U ze start 5.7 V, then U ze max = 11.4 V), there is no point in installing gate overvoltage protection.And since the currents of all amplifier devices are limited, there is no need for additional cascade protection circuits, which significantly simplifies the amplifier.

In practice, the gate-emitter voltage of transistors when the maximum current flows through them is not known in advance, therefore experimental selection R e selection is made I max.

As is easy to notice, R 8 and R 9 perform not only a restrictive, but also a linearizing function for VT 5 and VT 6, creating a local environmental protection system in itself And x nonlinear elements.

A variant of a practical scheme for implementing a powerful UMZCH is shown in Fig. 4.

As can be seen from the given parameters of the technical characteristics, the described amplifier is not inferior in quality to the best amplifiers with a symmetrical structure, and such a high output power is realized with only eight transistors! Not a bad result at a component cost of about 10 USD, taking into account the fact that selection and selection of groups of transistors is not necessary. In general, the scheme is one of the best in terms of cost/quality ratio.

The most detailed features of the operation of the UMZCH can be described using the full diagram (Fig. 4) as follows.Input signal, through circuit C1, R 1, setting the lower limit frequency and input resistance, goes to the base of the transistor VT 1. As an input to s microwave transistor KT368A is used (for quick recovery from saturation after an overload when the output signal is limited). The base of the same transistor receives a feedback signal through circuit C2, R 3.

Chain SZ, R 2, R 4, R 7 designed to set zero offset voltage at the amplifier output.Since the tuning resistor R 7 Over time, the resistance may change; instead, it is better to install a constant resistor selected during setup. Diodes VD 2 and HL 1 set the bias to the base of the transistor VT 2 and at the same time carry out temperature compensation of zerovoltage at the amplifier output due to the same thermal coefficients of the transistor VT 1 and diode VD 2 (it also sets the bias voltage along the circuit R 2, R 4, R 7).

Capacitor C4 corrects the input stage. From the collector VT 1 signal via VT 2 goes to the base of the emitter follower on the transistor VT 3. Its task is to increase the input resistance and thereby increase the overall gain, as well as speed up the turn-off of the transistor VT 5 and neutralizing the Miller effect. Zener diode VD 3 increases supply voltage for VT 3 and thus speeds up the turn-off of transistors VT 4, VT 5, increasing the speed of the leading edge.

From emitter VT 3 the signal goes to the base of the transistor VT 5. Chain L 1, R 13 carries out correction of the composite cascade on transistors VT 4 and VT 5. From the collector of the transistor VT 5 the signal goes to the gate of the low-side output transistor.From the collector of the transistor VT 4 a similar but antiphase current signal enters through the zener diode VD 7 to the gate of the high-side output transistor.

Chain R 11, C7 in VT 4 base performs inclusive correction of the output stage, increasing stabilityamplifier in limiting mode. Circuits C 10, R 22 and L 2, R 24 increase the stability of the amplifier when the load resistance changes and its capacitive nature.

Diode VD 8 reduces by half the thermal power dissipated by the resistor R 20, due to the fact that only the charging current of capacitor C8 flows through it. The quiescent current of the output stage, equal to 0.2 A, is set with an adjusted resistor R 17.

For thermal stabilization of the quiescent current UMZCH diodes VD 5 and VD 6 installed on the heat sink next to the output transistors. Transistors VT 4, VT 6 are equipped with small plate heat sinks, since the thermal power dissipated by them reaches 0.8 W. Light-emitting diode HL 2 used to set the bias of the current source on the transistor VT 6 and at the same time to indicate that the amplifier is turned on.

The output transistors must be installed on a radiator with an area of at least 3000 cm2. The use of a fan will dramatically reduce its size, which will significantly reduce the size and weight of the amplifier.

When you first turn on the amplifier, resistors are used to protect the output transistors R 19 and R 23 it is recommended to replace them with higher resistance ones (up to 3…10 Ohm), and only after checking the voltage on the gates can you set the corresponding circuit to 0.10m and set the quiescent current. Moreover, for IRG 4PC 30W voltage U ze = 5.7 V.

As can be seen from the complete circuit (Fig. 4), the amplifier uses a rather complex frequency response correction (four capacitors and a choke, not counting resistors). This is a small price to pay for an asymmetrical structure to behave no worse than a symmetrical one. th (with complementary devices) and obtain high amplifier stability in the limitation zone. We can say that the first problem of achieving low distortion after choosing a structural diagram is the problem of choosing an amplifier frequency response correction that creates the necessary amplifier stability margin with a large change in output currents and voltages, and at the same time ensuring a minimum phase delay in the operating frequency range. In most cases, it is the correction that becomes decisive, negating the advantages of many schemes.

The designer is always faced with a dilemma - whether to increase the depth of the overall feedback loop to improve the linearity of the amplifier or reduce its depth in order to increase the stability margin, which is necessary if the speaker impedancehas a complex character. And if amplifiers sound differently, this is largely due to the stability margin, which is very noticeable at high levels. That is why UMZCHs with “simple” circuits often show better results than those with a complex (often on microcircuits) structure. And each new cascade must be introduced after careful testing of the effectiveness of the new s x elements. Moreover, increasing the depth of environmental protection in most cases does not give the desired result, but only worsens the margin of stability. And here the correct assessment of the criteria for linearity and dynamic stability of the amplifier, which in turn depend on competent correction, comes to the fore. Moreover, competent correction should minimize the phase delay in the operating frequency range without compromising the overall stability. Often a good correction is much more effective than a new cascade.

Of course, the chosen method of zero balance at the amplifier output is far from the best, and it is attractive only for its simplicity. A “floating” bias of up to several tens of millivolts may occur at the output of the UMZCH, but it does not noticeably affect either the sound or the operating point of the output transistors. To reduce the drift of the “zero,” it is useful to introduce a tracking unit on a precision microcircuit, even if this complicates the amplifier.

Transistors used IRG 4PC 30W are inexpensive, but they have a noticeable nonlinearity in the initial section and a large input capacitance. If you check the entire series of episodes IGBT, offered by manufacturers, you can certainly find devices with greater linearity and lower input capacitance. The author did not have the opportunity to carry out such work. With the proposed transistors, linearity can be improved by doubling the quiescent current to 0.5 A, but this will require increasing the radiator area.

In conclusion, I would like to note that if there is no need for high amplifier power, then it is quite possible to use it at the output instead of IGBT transistors field effect transistors with insulated gate and channel n -type, the linearity of which is noticeably higher. The amplifier will receive higher linearity, and you only need to select resistors for a different supply voltage and zener diodes for a different gate voltage. A reduced-power analogue of the UMZCH on field-effect transistors, corresponding to the circuit shown here, has been successfully used by the author for six years, delivering a lot of pleasant moments when listening to various kinds of music programs at home.

Literature

1. Danilov A.A. Precision low frequency amplifiers M Hotline - Telecom, 2004.

2. Kozyrev "Amplifiers" Krell KAV -4-xi", "Audio Analogue Maestro", "Plinius 9200." - Audio Store, 2003, No. 6, pp. 71,72.

3. Shpak S.V. Patent RU No. 2316891 dated 04/10/2006.

4 Douglas Self on a previously unnoticed source of distortion in transistor UMZCHs with general environmental feedback - Radiohobby, 2003, No. 3, p. 10.11.

5. Vitushkin A., Telesnin V. Amplifier stability and natural sound. - Radio 1980, No. 7, from 36.37.

Sergey Shpak, Kazan, Tatarstan

P.S. The topic of editorial errors has already been raised on the site, here is another example of such an error: -

Vovk@

Hi-Fi amplifier based on complementary field-effect transistors. Hi-fi amplifier using complementary field-effect transistors Hi-fi amplifier using complementary field-effect transistors

Hi-Fi AMPLIFIER WITH COMPLEMENTARY FIELD TRANSISTORS

Related Posts