Bridge UMZCH with BSIT. Scheme, description

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Bridge UMZCH with BSIT. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers

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In the designs of radio amateurs, powerful transistors, called BSITs and intended mainly for switching devices (switching power supplies, horizontal scanning units, powerful switches), are not yet used so often.

We bring to the attention of readers a description of one of the options for a hundred-watt UMZCH using just such transistors. Referring to the metrological indicators, as well as the results of multiple expert auditions, the author highly appreciates his development.

The controversy among radio amateurs and developers discussing the issue of "better sounding" tube amplifiers continues to this day [1]. It should be recognized that in UMZCH on tube triodes, small distortions even when operating on a complex load are due, in particular, to the internal feedback inherent in these devices. But the advantages of a vacuum triode are also inherent in a field-effect transistor with a control transition, called a transistor with static induction SIT [1]. Another modification of such devices is a bipolar transistor with static induction BSIT. operating with a significant gate current and having an output characteristic similar to that of a generator lamp in the mode of operation with grid currents. Its input characteristics are almost the same. like bipolar.

The current decay time of a device with a vertical normally closed channel of n-type - powerful LSIT KP958A [2] - is Tsl = 60 ns, which is approximately the same order as that of conventional high-power bipolar transistors, however, the saturation voltage (depends on the degree of saturation transistor) is several times smaller. The use of BSIT turned out to be quite effective in UMZCH. made according to the bridge scheme (3).

The scheme proposed here for a hundred-watt bridge UMZCH at BSIT is shown in the figure.

Bridge UMZCH with BSIT

Main Specifications

Rated input voltage, V......1
Rated output power, W, at Rн = 8 Ohm......100
Frequency response unevenness in the frequency range of 10 ... 100 Hz, dB, no more ...... 000
Harmonic coefficient at a frequency of 1000 Hz, %, no more......0.02
Quiescent current, mA, no more ...... 500
Relative noise level, dB, not more than......80
Output voltage slew rate, V/µs, not less than......50

UMZCH contains two almost identical amplifiers, one of which is inverting with respect to the input signal. The load is connected between the outputs of the amplifiers. Due to the fact that the output voltage of the amplifiers is applied to the load in antiphase. the output voltage is doubled. The op-amp is a voltage amplifier, and for the upper cascade with OOS, the gain is Ku = R3 / R1 + 1, and for the lower one it is determined by the ratio R18 / R16. With the specified resistor values, the gain of both amplifiers in the bridge circuit is the same.

Except OU. the amplifier contains phase-inverted cascades on transistors of different structures VT4, VT5. Together with them, transistors VT6 - VT9 perform the function of current amplifiers operating in class AB. The upper amplifier according to the scheme is covered by a serial OOS: it is fed from the output to the inverting input of the op-amp (pin 2 of the DA1 chip) through the divider R3R1; the lower amplifier in the circuit is covered by a parallel OOS through the resistor R18.

The output voltage of the bridge amplifier is determined by the sum of the output voltage of the two op-amps (the voltage drop at the gate-source junctions can be neglected). To increase the output voltage, the "tracking" power supply of the op-amp is used. changing synchronously with its output signal. In the absence of an input signal, the voltage at the junction point of the zener diodes VD1 and VD2 is zero. At the same time, at pins 7 and 4 of the DA1 microcircuit, the supply voltage is maintained equal to 15 V due to stabilizers made on transistors VT1, VT2. The appearance at the output of the signal amplifier leads to a common-mode bias of the supply voltages of the op-amp. in connection with which the limitation of the signal of the op-amp does not occur. Thus, the output voltage of the op-amp approximately doubles.

Resistor R7 connects the zener diodes VD1, VD2 to the amplifier output, its resistance, if necessary, is selected during tuning, replacing it with a variable one (resistance 3 - 5 kOhm). By applying a signal from the audio frequency generator to the UMZCH input, the maximum possible undistorted voltage is set at the outputs of both arms of the amplifier (oscilloscope control). It must be remembered that an excessive decrease in resistance can disrupt the stability of the amplifier.

Resistors R6, R8, R9 and transistor VT3 create an initial bias at the gates of the current amplifier transistors. By changing the position of the trimmer resistor R8. you can adjust the value of the quiescent current of the output stage over a wide range. Transistor VT3 serves for thermal stabilization of the mode, it needs to provide thermal contact with the heat sinks of transistors.

To achieve low distortion, it is desirable to select transistors VT4 - VT9 in pairs, for which they remove the dependence of the drain current at different values of the gate current at Us = 10 V. It is advisable to measure at several points. In one cascade, transistors with similar characteristics are installed.

Resistors R1, R3, R16, R18 - C2-29V with a tolerance of 1% (up to 2% is possible). The remaining resistors are of any type with a tolerance of no more than 10%. The K140UD11 microcircuit can be replaced by the K154UDZA without any changes in the circuit, but an appropriate high-frequency correction may be required. OA K154UDZA is more preferable in terms of its frequency properties, although it is possible that the maximum output power of the UMZCH will decrease.

The output transistors must be installed on heat sinks with a surface area of at least 600 cm2 for each transistor. The author used needle radiators with forced cooling from a small fan in his design. In this case, it turned out to be sufficient to use a small radiator.

A correctly assembled amplifier starts working immediately, you just need to set the quiescent currents of both arms of the amplifier to about 200 ... 250 mA and the maximum possible output signal amplitude by selecting resistor R7.

Quite simply, you can double the power of the amplifier - up to 200 W - by doubling the number of output stage transistors (VT8. VT9). For uniform distribution of currents in the emitter circuits of these transistors, it is necessary to include equalizing resistors with a resistance of 0.1 ... 0,2 Ohm.

Literature

Circuitry of devices on powerful field-effect transistors. Directory. Ed. V. P. Dyakonova. - M: Radio and communication, 1994.
chip-dip.ru/catalog/index.html
Korzinin M. High fidelity audio frequency power amplifier circuitry. - Radio. 1997. No. 3. p. 15 - 17.

Author: N. Rekunov, Togliatti, Samara region

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