UMZCH on MIS transistors. Scheme, description

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UMZCH on MIS 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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There are two main types of powerful field-effect transistors (FETs) with a control pn-junction: conventional ones with a "pentode" current-voltage characteristic (CVC), and with static induction (SIT) - with a "triode" one. SIT transistors are normally (i.e., at Uzi = 0) open. When a negative bias is applied to the gate, they work like a vacuum triode and therefore, without any feedback, they have a low output resistance (Rout ~ 1/So). Usually it is a fraction of an ohm. The quadratic transfer characteristic of such transistors with fairly long linear sections leads to the almost complete disappearance of even harmonics, and the use of push-pull circuits ensures the suppression of odd harmonics as well. The harmonic coefficient is very low even in the absence of external feedback, there is a rapid decrease in the amplitude of higher harmonics, which is inherent in tube circuits. Another advantage of SIT is excellent temperature stabilization.

With a positive bias on the control electrode, the SIT actually turns into a bipolar transistor. The bipolar mode of operation makes it possible to obtain a smaller Ri in the initial section of the CVC, but leads to a sharp decrease in performance due to the accumulation of excess charges of minority carriers in the structure.

The industry of the CIS countries produces SIT only with an n-type channel. The choice of foreign transistors of this class is also very limited. In addition, these transistors require special bias circuits to ensure that they are turned off before the supply voltage is applied or the drain power is delayed.

Currently, conventional MIS transistors are more common. The proposed amplifier is designed on the basis of just such transistors and is a modernized version of the UMZCH from [1]. Thanks to the integrator in the OOS, the amplifier has a low output impedance at infra-low frequencies and at direct current. Due to the shallow OOS covering the output stage, the influence of the loudspeaker on the UMZCH output is minimal. Overload distortion is monotonous.

Main characteristics of UMZCH:

Output power, W .......... 100;
Reproducible frequency range, Hz .......... 5...150000
Harmonic coefficient (in the band 20...20000 Hz), %, not more than .......... 0,5
Input voltage, V .......... 1,0

The amplifier circuit is shown in Fig. 1. The amplifier is inverting, and consists of two stages covered by local environmental protection. For DC voltage, the amplifier is covered by the OOS with the help of an integrator on DA2. The first stage is made on a high-speed operational amplifier of the K140UD11 type (KR140UD11, KR140UD1101) according to a non-inverting circuit. The stage gain depends on the ratings of R3 and R19. On transistors VT1, VT2, VT5, VT6, a parallel amplifier is made with a bias circuit for R5, R6, VD1, VD2 and current generators for VT3, VT4. By selecting R9, you can achieve the so-called "Non-switching amp" mode, i.e. mode without cutoff of the output transistors. But there is a danger of large through currents.

Fig.1. Schematic diagram of the amplifier (click to enlarge)

The output stage is made on transistors VT7, VT8, covered by two OOS loops: parallel OOS for voltage - through R10 ... R13 and serial - for current - through R14, R15. Voltage feedback is calculated in such a way that the output transistors operate with virtually no current cutoff. Figures 2 and 3 show the voltage waveforms at the gates of the output transistors.

UMZCH on MIS transistors
Ris.2
Ris.3
The power supply of the preliminary stage is ±15 V, so there is no need to protect the gate-source junctions of field-effect transistors (the maximum allowable voltage is ±20 V). Thermal stabilization of the cascade is provided by diodes VD1, VD2, which are elements of the level shift circuit.

Details and design. It is desirable to make the power supply transformer on a toroidal magnetic circuit (for a stereo amplifier - on two transformers). Between the primary and secondary windings, a screen winding is wound in one layer with PEL wire d0,4 mm, one of its outputs is grounded. It is advisable to place the diode bridge and filter capacitors (at least 10000 uF) at a minimum distance from the UMZCH board (you can directly on it). The wires of the secondary winding are led to the board in the screen.

To minimize the amplitude modulation of the audio signal by power supply ripples, it is desirable to use L-shaped LC filters in it. Filter chokes can be made on a ShLM25x32 core or similar with a 1mm gap. They are wound with PEL wire d0,69 mm until the frame is filled. The inductor L1 is wound with a wire d0,69 mm, turn to turn (before filling), on a resistor R18 (MLT-2).

Diodes VD1, VD2 are fixed on a heat-conducting paste on the radiators of the output transistors (it is possible - under the washer for fastening the output transistors). As VD3, VD4, you can use any red LEDs, for example, AL307A (B). Transistors VT5, VT6 are desirable to be equipped with heat sink flags. Field-effect transistors - Minsk software "Integral", it is desirable to choose them with a spread in slope of no more than 20%. BSIT transistors such as KP959, KP960 are also suitable. You can use any foreign complementary transistors of suitable power and acceptable voltage (for example, IRF540 and IRF9540). Resistors R14, R15 - homemade, wire, from manganin or constantan d0,4 ... 0,5 mm. In order to minimize their parasitic inductance, a piece of wire (about 10 cm) is folded in half and wound in 1,5 mm increments on a d4 mm mandrel.

Establishment. First, the quiescent current is set and the arms of the output stage are symmetrical for direct current. To do this, break the connection between the DA1 output and the bases of transistors VT1, VT2 (it is worth providing a technological jumper in the board) and temporarily connect the transistor bases and the UMZCH output to the "common wire". The sliders of resistors R5 and R6 are brought to the position corresponding to the minimum resistance. The drains of the output transistors temporarily include wire resistors of 10 ohms with a power of 10 ... 25 watts. By measuring the voltage drop across them, set the required quiescent current.

Restore the connection of the bases VT1, VT2 with the output DA1, remove the "short" from the output and make sure that at the output DA1 in the absence of an input signal, the constant level is close to zero. If necessary, it is carefully adjusted with one of the resistors R5, R6. The voltage drop across the drain resistors finally regulates the quiescent current of the output transistors. After that, the drain resistors are removed.

If necessary, distortion can be minimized by selecting resistors R12, R13.

Literature

A. Petrov. Economical UMZCH with "pentode" sound. - Radiomir, 2002, N9, S.Z.

Author: A. Petrov, Mogilev; Publication: radioradar.net

See other articles Section Transistor power amplifiers.

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