UMZCH on field-effect transistors. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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UMZCH on field-effect transistors. 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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The described UMZCH with powerful field-effect transistors is characterized by high temperature stability, has a low quiescent current, is not afraid of short circuits in the load, is quite stable and reliable. The features of the proposed design include limited output current and, therefore, the need to use loudspeakers with a nominal impedance of 8 or 16 ohms.

Amplifier Specifications

Rated output power at a load of 8 ohms, W......32
Sensitivity, V ...... 1
Input impedance, kOhm......20
OOS depth at a frequency of 20 kHz, dB......36
Harmonic distortion factor at Pout = 8 W at a frequency of 20 kHz, %, not more than ...... 0,015

The signal-to-noise ratio of the amplifier was not measured, but near the speakers, noticeable noise is not heard when the UMZCH is turned on.

The main feature of the device is the use of high-frequency generator field-effect transistors with a horizontal channel structure (KP904A). As is known from [1], this type of MIS transistors is distinguished by a relatively linear transfer characteristic and high speed. However, the relatively low slope of the characteristic and increased on-state resistance limit the maximum current of the transistor.

As it turned out from experiments with KP904A transistors, the curvature of the initial section of their throughput characteristic is insignificant, and at a quiescent current of about 30 mA, the throughput characteristic of the output stage is already quite linear, so switching distortions turn out to be very low. The relatively small values of the capacitances of these transistors make it possible to refuse their forced recharging.

Transistors of the KP904 series are also promising as a voltage amplifier, as they provide significant linear gain and speed in the absence of a saturation effect. Due to their fairly linear characteristics, the distortion in such an amplifier does not have the wide spectrum of harmonics that occurs with bipolar transistors.

The amplifier itself is covered by a general OOS of medium depth, which practically does not decrease at all audio frequencies. Corrections "forward" or "back", causing an overload on the impulse signal or reducing speed characteristics, are not used in it.

The UMZCH scheme is shown in fig. 1. The input signal after the LPF R2C1 comes to one of the inputs of a differential amplifier made on transistors VT1 - VT4. The use of compound transistors increases the linearity of the input stage and its input impedance. The cascade current generator is made on VT5; diodes VD2, VD3 and resistor R11 set its current, and resistor R12 improves the symmetry of the cascade arms at high frequencies. This generator itself is powered by a voltage determined by the zener diode VD1. A differential amplifier with a quiescent current of 3 mA has a gain drop of 1 dB at about 360 kHz (the input capacitance of the subsequent stage is about 300 pF).

(click to enlarge)

From the output of the first stage, antiphase signals are connected to the gates of powerful field-effect transistors VT6, VT7 of the second differential stage - the main voltage amplifier. Powerful KP904A transistors are used here because at a VT7 drain current of 20 mA they have a high slope of the characteristic and a large gain: at a frequency of 20 kHz - about 170. The cascade develops a voltage of up to 25 Veff. The quiescent current is selected to provide high slew rate and linearity.

From the output of the voltage amplifier, the signal enters the gate of a powerful transistor VT11 through an emitter follower on VT9, and it arrives at the gate of the lower transistor VT12 of the output stage through a phase-inverse stage made on VT10. Resistor R23 is selected in such a way that the transfer coefficient of both arms of the output stage is strictly the same. Elements R29-R31, C3 set the depth of the OOS UMZCH for direct and alternating current, and the capacitor C4 is used for phase correction of the OOS loop. Elements L3, C23, R27, R28 ensure the normal operation of the amplifier with a complex load at high frequencies.

This UMZCH at a given depth of the total environmental protection is quite stable. As an experiment, the FOS depth in it was temporarily increased to 54 dB and the gain was reduced to 2 with C4 soldered out - and in this case, no instability was found.

The power supply circuit is shown in fig. 2. As you can see, it is extremely simple. It should be noted that the power filter capacitors are located on the boards of each UMZCH channel. Thus, each channel has its own filter located near the output stage. Resistors R2 - R5 (0,5 Ohm) limit the inrush current during power-up and provide some additional decoupling of the amplifiers. This method is recommended in [2].

UMZCH on field-effect transistors

The protection device for the UMZCH was not developed, and the relay at the UMZCH output is not used due to the fact that the click of the transient when turned on is barely audible.

It should be borne in mind that the more expensive transistors of the 2P904A series, which have a smaller spread of parameters, should be used in the described amplifier in the second differential stage. The scheme of the attachment for measuring the initial drain current is shown in fig. 3. Transistors with a large initial current, as a rule, have a large steepness.

UMZCH on field-effect transistors

A little about the installation of the amplifier. The printed circuit board for this amplifier has not been developed, only a two-channel layout with volumetric mounting has been made. When installing or self-wiring a printed circuit board, you should pay attention to a number of important points.

The common wire of the power circuits (shown in the diagram with a thick line) and the common wire of the signal circuits (thin line) are separated by a 10 ohm resistor (R33).

In the diagram, the source circuit VT12 includes a diode VD8, shunted by a tantalum capacitor C22. These elements should be installed only if a particular instance of VT12 KP904A will have an initial drain current above 5 mA; in this case, this "stand" will be simply necessary. But still, it would be much better to install an instance with an initial drain current of less than 12 mA in place of VT5, and install transistors with a large current in the upper shoulder or a differential amplifier.

It would be useful to recall that during installation, all the conclusions of the elements and conductors should be tried to be made as short as possible, and the power ones should be thicker. It is important that the drain VT11 and the source VT12 (or diode "stand") be connected directly to the terminals of the filter capacitors, the length of the conductors here should be minimal.

The output transistors VT11, VT12 are located on separate ribbed heat sinks with dimensions of 90x65x50 mm, which were used in the MS-3 line scan units of TVs. The thickness of the heat sink plate is 5 mm, and to mount the transistor case, you only need to drill a hole with a diameter of 8,5 mm.

The VT8 transistor must also be placed on a heat sink, which in the author's version consists of two duralumin plates measuring 40x25x2 mm, placed on both sides of the circuit board and fastened with a screw. During installation, these plates are connected to the VT8 collector, on which a high-amplitude voltage of the amplified signal acts. Therefore, such a heat sink should be placed away from the input circuits of the amplifier. The plates can be isolated from the transistor, but you should not connect them to a common wire or case, as a significant parasitic load capacitance is formed, which can significantly reduce the slew rate of the stage output voltage.

MLT-0,125 resistors can be used in the amplifier, but in positions R6 - R9 it is better to use precision resistors C2-14, C2-29 with a tolerance of no more than 1% or ordinary ones, selected with an ohmmeter.

Capacitors C1, C4 - KT-1; C2, C3, C6, C9, C18-C21 - K73-17; C7, C22 - K53-4; C23 - K73-9. Oxide capacitors C5, C8 for a voltage of 63 V - JAMICON. Capacitors C10-C17 are small-sized imported NRZ, but larger ones - JAMICON - are also suitable.

Inductors L1, L2 - D1-0,1 from the DPM series or similar with an inductance of 200 ... 500 μH for a current of 100 mA. Coil L3 is wound on a resistor MLT-2 (R27) turn to turn and contains 20 turns of PEV-20,8 mm wire.

About setting up an amplifier. After applying power, you should check whether the DC modes correspond to those indicated in the diagram. The current of the second differential stage (40 mA) in case of a noticeable deviation can be changed by selecting the resistor R11. If the voltage across resistors R8, R9 is very different (more than 20%), this indicates a significant difference in the parameters of transistors VT6, VT7; it is desirable to select them more accurately. By choosing the resistor R17, the quiescent current of the output transistors is set to 30 ... 40 mA.

Next, the UMZCH is loaded onto a load equivalent with a resistance of 8 ohms and, having applied a signal with a frequency of 3 kHz and an amplitude of 1 V, to the input from the 1H generator, they check for the presence of a sinusoidal signal with an amplitude of about 16 V at the output. The reason for a significant deviation from this value or distortion of the waveform is usually an error installation or use of faulty elements.

Further, temporarily turning off the capacitor C1, a "meander" signal with a swing of about 73 V and a frequency of 17 kHz is fed to the UMZCH input through the K1,5-0,25 capacitor with a capacity of 100 μF; the selection of capacitor C4 achieves the minimum amplitude and duration of the transient oscillatory process. After this check, the capacitor C1 is installed in place. It may turn out that the capacitor is not needed at all. On this setting can be considered complete.

The amplifier is characterized by a natural, open and light sound of musical instruments, and low distortion contributes to a detailed reproduction of the spatial scene and microdynamics of sound images. AC S-90D was used as amplified loudspeakers.

Literature

Dyakonov V. P. et al. Circuitry of devices on powerful field transistors. - M.: Radio and communication, 1994.
Ataev D. I., Bolotnikov V. A. Practical schemes for high-quality sound reproduction. MRB. - M.: Radio and communication, 1986.

Author: Ya.Tokarev, Moscow

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