Umzch hi fi on field-effect transistors. Wideband UMZ with low distortion. Old but golden

Low-frequency amplifiers are very popular among radio electronics enthusiasts. Unlike the previous scheme, this power amplifier based on field-effect transistors consists mainly of transistors and uses an output stage that, with a bipolar supply voltage of 30 volts, can provide up to 70 W output power on speakers with a resistance of 4 Ohms.

Schematic diagram of an amplifier using field-effect transistors

The amplifier is assembled on the basis of an operational amplifier TL071 (IO1) or any similar one, which creates the main amplification of the differential signal. The amplified low-frequency signal from the output of the op-amp, most of which flows through R3 to the midpoint. The remainder of the signal is sufficient to be directly amplified by the IRF9530 (T4) and IRF530 (T6) MOSFETs.

Transistors T2, T3 and their surrounding components serve to stabilize the operating point of the variable resistor, since it must be correctly set in the symmetry of each half-wave across the amplifier load.

All parts are assembled on a single-sided printed circuit board. Please note that three jumpers must be installed on the board.

Amplifier settings

Setting up the amplifier is best done by applying a sinusoidal signal to its input and connecting a load resistor with a value of 4 ohms. After this, resistor R12 is installed in such a way that the signal at the output of the amplifier is symmetrical, i.e. the shape and size of the positive and negative half-waves were the same at maximum volume.

The figure shows the circuit of a 50 W amplifier with MOSFET output transistors.
The first stage of the amplifier is a differential amplifier using transistors VT1 VT2.
The second stage of the amplifier consists of transistors VT3 VT4. The final stage of the amplifier consists of MOSFETs IRF530 and IRF9530. The amplifier output is connected through coil L1 to an 8 Ohm load.
The chain consisting of R15 and C5 is designed to reduce noise levels. Capacitors C6 and C7 are power filters. Resistance R6 is designed to regulate the quiescent current.

Note:
Use bipolar power supply +/-35V
L1 consists of 12 turns of insulated copper wire with a diameter of 1 mm.
C6 and C7 should be rated at 50V, the remaining electrolytic capacitors at 16V.
A heatsink for the MOSFETs is required. Dimensions 20x10x10 cm made of aluminum.
Source - http://www.circuitstoday.com/mosfet-amplifier-circuits

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  To date, many versions of UMZCH with output stages based on field-effect transistors have been developed. The attractiveness of these transistors as powerful amplifying devices has been repeatedly noted by various authors. At audio frequencies, field-effect transistors (FETs) act as current amplifiers, so the load on the pre-stages is negligible and the insulated gate FET output stage can be directly connected to the pre-amplifier stage operating in class A linear mode.
  When using powerful PTs, the nature of nonlinear distortions changes (fewer higher harmonics than when using bipolar transistors), dynamic distortions are reduced, and the level of intermodulation distortions is significantly lower. However, due to a lower transconductance than that of bipolar transistors, the nonlinear distortion of the source follower turns out to be large, since the transconductance depends on the level of the input signal.
  The output stage on powerful PTs, where they can withstand a short circuit in the load circuit, has the property of thermal stabilization. Some disadvantage of such a cascade is the lower utilization of the supply voltage, and therefore it is necessary to use a more efficient heat sink.
  The main advantages of powerful PTs include the low order of nonlinearity of their pass-through characteristics, which brings the sound features of PT amplifiers and tube amplifiers closer together, as well as a high power gain for signals in the audio frequency range.
  Among the latest publications in the journal about UMZCH with powerful PTs, articles can be noted. The undoubted advantage of the amplifier is the low level of distortion, and the disadvantage is low power (15 W). The amplifier has more power, sufficient for residential use, and an acceptable level of distortion, but appears to be relatively complex to manufacture and configure. Hereinafter we are talking about UMZCHs intended for use with household speakers with a power of up to 100 W.
  UMZCH parameters, focused on compliance with international IEC recommendations, determine the minimum requirements for hi-fi equipment. They are quite justified both from the psychophysiological side of human perception of distortion, and from the actually achievable distortion of audio signals in acoustic systems (AS), on which the UMZCH actually works.
  In accordance with the requirements of IEC 581-7 for hi-fi speakers, the total harmonic distortion factor should not exceed 2% in the frequency range 250 ... 1000 Hz and 1% in the range above 2 kHz at a sound pressure level of 90 dB at a distance of 1 m. The characteristic sensitivity of household speakers is 86 dB/W/m, this corresponds to an UMZCH output power of only 2.5 W. Taking into account the peak factor of music programs, taken equal to three (as for Gaussian noise), the output power of the UMZCH should be about 20 W. In a stereophonic system, the sound pressure at the low frequencies approximately doubles, which allows the listener to move away from the speaker by 2 m. At a distance of 3 m, the power of a stereo amplifier of 2x45 W is quite sufficient.
  It has been repeatedly noted that distortions in UMZCH on field-effect transistors are caused mainly by the second and third harmonics (as in working speakers). If we assume that the causes of nonlinear distortions in the speakers and the UMZCH are independent, then the resulting harmonic coefficient for sound pressure is determined as the square root of the sum of the squares of the harmonic coefficients of the UMZCH and the speaker. In this case, if the total harmonic distortion coefficient in the UMZCH is three times lower than the distortion in the speakers (i.e., does not exceed 0.3%), then it can be neglected.
  The range of effectively reproduced frequencies of the UMZCH should be no longer audible to humans - 20...20,000 Hz. As for the rate of rise of the output voltage of the UMZCH, in accordance with the results obtained in the author’s work, a speed of 7 V/μs is sufficient for a power of 50 W when operating at a load of 4 Ohms and 10 V/μs when operating at a load of 8 Ohms.
  The basis for the proposed UMZCH was an amplifier in which a high-speed op-amp with tracking power was used to “drive” the output stage in the form of composite repeaters on bipolar transistors. Tracking power was also used for the output stage bias circuit.

  The following changes have been made to the amplifier: the output stage based on complementary pairs of bipolar transistors has been replaced by a cascade with a quasi-complementary structure using inexpensive IRFZ44 insulated gate PTs and the depth of the total SOS is limited to 18 dB. The circuit diagram of the amplifier is shown in Fig. 1.

  The KR544UD2A op-amp with high input impedance and increased performance was used as a pre-amplifier. It contains an input differential stage on a PT with a p-n junction and an output push-pull voltage follower. Internal frequency equalization elements provide stability in various feedback modes, including voltage follower.
  The input signal comes through the low-pass filter RnC 1 with a cutoff frequency of about 70 kHz (here the internal resistance of the signal source = 22 kOhm). which is used to limit the spectrum of the signal entering the power amplifier input. Circuit R1C1 ensures the stability of the UMZCH when the value of RM changes from zero to infinity. To the non-inverting input of op-amp DA1, the signal passes through a high-pass filter built on elements C2, R2 with a cutoff frequency of 0.7 Hz, which serves to separate the signal from the constant component. Local OOS for the operational amplifier is made on elements R5, R3, SZ and provides a gain of 43 dB.
  The voltage stabilizer for the bipolar supply of op-amp DA1 is made on elements R4, C4, VDI and R6, Sat. VD2 respectively. The stabilization voltage is chosen to be 16 V. Resistor R8 together with resistors R4, R6 form a divider of the output voltage of the UMZCH to supply “tracking” power to the op-amp, the swing of which should not exceed the limit values ​​of the common-mode input voltage of the op-amp, i.e. +/-10 V "Tracking" power supply allows you to significantly increase the range of the op-amp's output signal.
  As is known, for the operation of a field-effect transistor with an insulated gate, in contrast to a bipolar one, a bias of about 4 V is required. For this, in the circuit shown in Fig. 1, for transistor VT3, a signal level shift circuit is used on elements R10, R11 and УУЗ.У04 to 4.5 V. The signal from the output of the op-amp through the circuit VD3VD4C8 and resistor R15 is supplied to the gate of transistor VT3, the constant voltage on which relative to the common wire is +4, 5 V.
  The electronic analogue of the zener diode on elements VT1, VD5, VD6, Rl2o6ecne4H shifts the voltage by -1.5 V relative to the op-amp output to ensure the required operating mode of transistor VT2. The signal from the output of the op-amp through circuit VT1C9 also goes to the base of transistor VT2, which is connected according to a common emitter circuit, which inverts the signal.
  On R17 elements. VD7, C12, R18 an adjustable level shift circuit is assembled, which allows you to set the required bias for transistor VT4 and thereby set the quiescent current of the final stage. The capacitor SY provides “tracking power” to the level shift circuit by supplying the UMZCH output voltage to the connection point of resistors R10, R11 to stabilize the current in this circuit. The connection of transistors VT2 and VT4 forms a virtual field-effect transistor with a p-type channel. i.e., a quasi-complementary pair is formed with the output transistor VT3 (with an n-type channel).
  Circuit C11R16 increases the stability of the amplifier in the ultrasonic frequency range. Ceramic capacitors C13. C14. installed in close proximity to the output transistors serve the same purpose. Protection of the UMZCH from overloads during short circuits in the load is provided by fuses FU1-FU3. since IRFZ44 field-effect transistors have a maximum drain current of 42 A and can withstand overloads until the fuses blow.
  To reduce the DC voltage at the output of the UMZCH, as well as to reduce nonlinear distortions, a general OOS has been introduced on elements R7, C7. R3, NW. The AC OOS depth is limited to 18.8 dB, which stabilizes the harmonic distortion coefficient in the audio frequency range. For direct current, the op-amp, together with the output transistors, operates in the voltage follower mode, providing a constant component of the UMZCH output voltage of no more than a few millivolts.

  
  

  Old but golden

  Old but golden

  Amplifier circuitry has already gone through a spiral in its development and now we are witnessing a “tube renaissance”. In accordance with the laws of dialectics that were so persistently drummed into us, a “transistor renaissance” should follow. The very fact of this is inevitable, because lamps, for all their beauty, are very inconvenient. Even at home. But transistor amplifiers have their own shortcomings...
  The reason for the “transistor” sound was explained back in the mid-70s - deep feedback. It gives rise to two problems at once. The first is transient intermodulation distortion (TIM distortion) in the amplifier itself, caused by signal delay in the feedback loop. There is only one way to combat this - by increasing the speed and gain of the original amplifier (without feedback), which can seriously complicate the circuit. The result is difficult to predict: either it will happen or not.
  The second problem is that deep feedback greatly reduces the output impedance of the amplifier. And for most loudspeakers this is fraught with the occurrence of those same intermodulation distortions directly in the dynamic heads. The reason is that when the coil moves in the gap of the magnetic system, its inductance changes significantly, so the impedance of the head also changes. With a low output impedance of the amplifier, this leads to additional changes in the current through the coil, which gives rise to unpleasant overtones, mistakenly taken for distortion of the amplifier. This can also explain the paradoxical fact that with an arbitrary choice of speakers and amplifiers, one set “sounds” and the other “does not sound.”

  secret of tube sound =
  high output impedance amplifier
  + shallow feedback .
  However, similar results can be achieved with transistor amplifiers. All the circuits given below have one thing in common - an unconventional and now forgotten “asymmetrical” and “irregular” circuit design. However, is she as bad as she is made out to be? For example, a bass reflex with a transformer is a real Hi-End! (Fig. 1) And the phase inverter with a divided load (Fig. 2) is borrowed from tube circuitry...
  Fig.1

  
  Fig.2

  
  Fig.3

  These schemes are now undeservedly forgotten. But in vain. Based on them, using modern components, you can create simple amplifiers with very high sound quality. In any case, what I collected and listened to sounded decent - soft and “tasty”. The depth of feedback in all circuits is small, there is local feedback, and the output resistance is significant. There is no general environmental protection for direct current.

  However, the given diagrams work in the classroom B, therefore they are characterized by “switching” distortions. To eliminate them, it is necessary to operate the output stage in a “pure” class A. And such a scheme also appeared. The author of the scheme is J.L.Linsley Hood. The first mentions in domestic sources date back to the second half of the 70s.
  
  
  Fig.4

  The main disadvantage of class amplifiers A, limiting the scope of their application is a large quiescent current. However, there is another way to eliminate switching distortions - the use of germanium transistors. Their advantage is low distortion in mode B. (Someday I will write a saga dedicated to germanium.) Another question is that these transistors are not easy to find now, and the choice is limited. When repeating the following designs, you need to remember that the thermal stability of germanium transistors is low, so there is no need to skimp on radiators for the output stage.

  Fig.5

  This diagram shows an interesting symbiosis of germanium transistors with field effect transistors. The sound quality, despite the more than modest characteristics, is very good. To refresh the impressions from a quarter of a century ago, I took the time to assemble the structure on a mock-up, slightly modernizing it to suit modern part values. Transistor MP37 can be replaced with silicon KT315, since during setup you will still have to select the resistance of resistor R1. When operating with an 8 Ohm load, the power will increase to approximately 3.5 W, the capacitance of capacitor C3 will have to be increased to 1000 µF. And to operate with a 4 Ohm load, you will have to reduce the supply voltage to 15 volts so as not to exceed the maximum power dissipation of the output stage transistors. Since there is no overall DC OOS, the thermal stability is only sufficient for home use.

  The following two diagrams have an interesting feature. The AC output stage transistors are connected according to a common emitter circuit, and therefore require a low excitation voltage. There is no need for a traditional voltage boost. However, for direct current they are connected in a common collector circuit, so a “floating” power supply not connected to ground is used to power the output stage. Therefore, a separate power supply must be used for the output stage of each channel. In the case of using pulse voltage converters, this is not a problem. The power supply of the preliminary stages can be common. The DC and AC OOS circuits are separated, which, in combination with the quiescent current stabilization circuit, guarantees high thermal stability with a low AC OOS depth. For MF/HF channels this is an excellent circuit.
  
  Fig.6
  
  
  Fig.7 Author: A.I. Shikhatov (drafting and comments) 1999-2000
  Published: collection "Designs and diagrams for reading with a soldering iron" M. Solon-R, 2001, pp. 19-26.
  • Schemes 1,2,3,5 were published in the magazine "Radio".
  • Scheme 4 is borrowed from the collection
    V.A. Vasiliev "Foreign amateur radio designs" M. Radio and Communications, 1982, pp. 14...16
  • Schemes 6 and 7 are borrowed from the collection
    J. Bozdekh "Design of additional devices for tape recorders" (translated from Czech) M. Energoizdat 1981, p. 148,175
  • Details about the mechanism of intermodulation distortion: Should the UMZCH have a low output impedance?
  Table of contents
  

  UMZCH on field-effect transistors

  UMZCH on field-effect transistors

  The use of field-effect transistors in a power amplifier can significantly improve sound quality while simplifying the overall circuit. The transfer characteristic of field-effect transistors is close to linear or quadratic, therefore, there are practically no even harmonics in the spectrum of the output signal; in addition, the amplitude of higher harmonics quickly decreases (as in tube amplifiers). This makes it possible to use shallow negative feedback in field-effect transistor amplifiers or to abandon it altogether. After conquering the vastness of “home” Hi-Fi, field-effect transistors began their attack on car audio. The published diagrams were originally intended for home systems, but maybe someone will risk applying the ideas contained in them in a car...
  
  
  Fig.1
  This scheme is already considered classic. In it, the output stage, operating in AB mode, is made of MOS transistors, and the preliminary stages are of bipolar ones. The amplifier provides fairly high performance, but to further improve the sound quality, bipolar transistors should be completely excluded from the circuit (next picture).
  
  
  Fig.2
  After all reserves for improving sound quality have been exhausted, only one thing remains - a single-ended output stage in “pure” class A. The current consumed by the preliminary stages from a higher voltage source in both this and the previous circuit is minimal.
  
  
  Fig.3
  The output stage with a transformer is a complete analogue of tube circuits. This is for a snack... The integrated current source CR039 sets the operating mode of the output stage.
  
  
  Fig.4
  However, a wideband output transformer is a rather complex unit to manufacture. An elegant solution - a current source in the drain circuit - was proposed by the company

  Recently, more and more often, many companies and radio amateurs are using powerful field-effect transistors with an induced channel and an insulated gate in their designs. However, it is still not easy to purchase complementary pairs of field-effect transistors of sufficient power, so radio amateurs are looking for UMZCH circuits that use powerful transistors with channels of the same conductivity. The magazine “Radio” published several such designs. The author proposes another one, but with a structure slightly different from a number of circuits common in UMZCH designs.

  Technical Parameters:

  Rated output power into 8 ohm load: 24 W

  Rated output power into 16 ohm load: 18 W

  Harmonic distortion at rated power into 8 ohm load: 0.05%

  Harmonic distortion at rated power into 16 ohm load: 0.03%

  Sensitivity: 0.7V

  Gain: 26dB

  For the last three decades, the classic transistor UMZCH has used a differential stage. It is necessary to compare the input signal with the output signal returning through the OOS circuit, as well as to stabilize the “zero” at the amplifier output (in most cases, the power supply is bipolar, and the load is connected directly, without an isolating capacitor). The second is the voltage amplification stage - a driver that provides the full amplitude of the voltage required for the subsequent current amplifier on bipolar transistors. Since this cascade is relatively low-current, the current amplifier (voltage follower) consists of two or three pairs of composite complementary transistors. As a result, after the differential stage, the signal passes through another three, four, or even five amplification stages with corresponding distortion in each of them and a delay. This is one of the reasons for the occurrence of dynamic distortions.

  In the case of using powerful field-effect transistors, there is no need for multi-stage current amplification. However, to quickly recharge the gate-channel interelectrode capacitance of a field-effect transistor, a significant current is also required. To amplify audio signals, this current is usually much less, but in switching mode at high audio frequencies it turns out to be noticeable and amounts to tens of milliamps.

  The UMZCH described below implements the concept of minimizing the number of cascades. At the amplifier input there is a cascade version of a differential stage on transistors VT2, VT3 and VT4, VT5, the load for which is applied to an active current source with a current mirror on transistors VT6, VT7. The current generator on VT1 sets the mode of the differential stage for direct current. The use of sequential connection of transistors in a cascade allows the use of transistors with a very high base current transfer coefficient, which are characterized by a small maximum voltage value (usually UKEmax = 15 V).

  Between the negative power supply circuit of the amplifier (source VT14) and the bases of transistors VT4 and VT5, two zener diodes are connected, the role of which is played by the reverse-connected base-emitter transitions of transistors VT8, VT9. The sum of their stabilization voltages is slightly less than the maximum permissible gate-source voltage VT14, and this ensures protection of the powerful transistor.

  In the output stage, the drain of the field-effect transistor VT14 is connected to the load through the switching diode VD5. Half-cycles of the minus-polarity signal are supplied through the diode to the load; half-cycles of the positive polarity do not pass through it, but are supplied through the transistor VT11 to control the gate of the field-effect transistor VT13, which opens only during these half-cycles.

  Similar output stage circuits with a switching diode are known in the circuit design of bipolar transistor amplifiers as a stage with a dynamic load. These amplifiers operated in class B mode, i.e. without through quiescent current. In the described amplifier with field-effect transistors, there is also a transistor VT11, which performs several functions at once: a signal is received through it to control the gate VT13, and local feedback on the quiescent current is formed, stabilizing it. In addition, the thermal contact of transistors VT11 and VT13 stabilizes the temperature regime of the entire output stage. As a result, the output stage transistors operate in class AB mode, i.e. with a level of nonlinear distortion corresponding to most versions of push-pull stages. A voltage proportional to the quiescent current is removed from resistor R14 and diode VD5 and supplied to the base VT11. The VT10 transistor contains an active source of stable current, which is necessary for the operation of the output stage. It is a dynamic load for VT14 when it is active during the corresponding half-cycles of the signal. The composite zener diode formed by VD6 and VD7 limits the gate-source voltage of VT13, protecting the transistor from breakdown.

  Such a two-channel UMZCH was assembled in the housing of the ROTEL RX-820 receiver to replace the UMZCH existing there. The plate heat sink is reinforced with metal steel struts to increase the effective area to 500 cm 2 . The oxide capacitors in the power supply were replaced with new ones with a total capacity of 12000 μF for a voltage of 35 V. Differential stages with active current sources (VT1-VT3) from the previous UMZCH were also used. The breadboards contain cascode continuations of the differential stage with current mirrors for each channel (VT4-VT9, R5 and R6) and active current sources for the output stages (VT10 of both channels) on a common board with common elements R9, VD3 and VD4. The VT10 transistors are pressed to the metal chassis with their back sides to avoid the need for insulating spacers. The output field-effect transistors are fixed to a common heat sink with an area of ​​at least 500 cm2 through heat-conducting insulating pads with screws. Transistors VT11 of each channel are mounted directly on the terminals of transistors VT13 so as to ensure reliable thermal contact. The remaining parts of the output stages are mounted on the terminals of powerful transistors and mounting racks. Capacitors C5 and C6 are located in close proximity to the output transistors.

  About the parts used. Transistors VT8 and VT9 can be replaced with zener diodes for a voltage of 7-8 V, operable at a low current (1 mA), transistors VT1-VT5 can be replaced with any of the KT502 or KT3107A, KT3107B, KT3107I series, and it is advisable to select them close in current transfer coefficient bases in pairs, VT6 and VT7 can be replaced with KT342 or KT3102 with letter indices A, B, in place of VT11 there can be any of the KT503 series. It is not worth replacing the D814A zener diodes (VD6 and VD7) with others, since the dynamic load current is approximately 20 mA, and the maximum current through the D814A zener diodes is 35 mA, so they are quite suitable. The inductor winding L1 is wound on resistor R16 and contains 15-20 turns of PEL 1.2 wire.

  The establishment of each channel of the UMZCH begins with the drain outlet VT13 temporarily disconnected from the power circuit. Measure the emitter current of VT10 - it should be approximately 20 mA. Next, connect the drain of transistor VT13 to the power source through an ammeter to measure the quiescent current. It should not significantly exceed 120 mA, this indicates correct assembly and the serviceability of the parts. The quiescent current is regulated by selecting resistor R10. After turning it on, it should be immediately set to about 120 mA; after warming up for 20-30 minutes, it will decrease to 80-90 mA.

  Possible self-excitation is eliminated by selecting capacitor C8 with a capacity of up to 5-10 pF. In the author's version, self-excitation arose due to a defective transistor VT13 in one of the channels. For other supply voltages, the heat sink area should be recalculated based on changes in the maximum power in one direction or another and ensure that the permissible parameters for the semiconductor devices used are not exceeded.

  "Radio" No. 12, 2008