UMZCH with a voltage amplifier according to a common base circuit. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers

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Until recently, the classic power amplifier structure [1] was popular among radio amateurs, in which the differential stage at the UMZCH input is loaded with a voltage amplification stage with a common-emitter transistor, followed by a power amplification stage, usually consisting of two or three-stage current amplifier. Such a structure is now the basis for UMZCH integrated circuits.

Over the past four decades, this circuit has changed little, its variants have multiplied due to the spread of powerful field-effect transistors. It provides the necessary parameters, measured by low values ​​of non-linear distortion, easily amenable to calculation of the output power and gain. The expediency of using an input differential stage, which ensures high stability of the entire device in a static mode, is quite understandable. The output stage, which is a two- or three-stage emitter follower, introduces minimal harmonic distortion with an almost complete voltage swing at the output of the device, commensurate with the supply voltage (more correctly, with half of it). Things are more complicated with a voltage amplifier - a driver. Over the 60-year period of the existence of a bipolar transistor, its inclusion in a circuit with a common emitter (CE) has been well studied, all its strengths and weaknesses have been identified, which served as its use in all analog and digital devices in a wide frequency range, as well as in DC amplifiers. current.

The disadvantages of the transistor cascade according to the OE circuit include low temperature stability and far from the most linear amplification mode. Both that, and another in the majority of devices is eliminated by various kinds of negative feedbacks, which reduce the dynamic characteristics of the cascade and its gain. In addition, the listener's ear has become accustomed to the sound of a classic transistor amplifier over the years, and most listeners do not make new demands.

Fig. 1

In the described UMZCH (its diagram in the figure), the driver stage is assembled on bipolar transistors VT6, VT7, connected according to a common base (OB) circuit. Such a cascade has a better frequency response and allows you to get a large output signal amplitude, since the saturation voltage of a transistor connected according to the OB circuit is lower than that of a similar cascade with the transistor connected according to the OE circuit.

Of course, the cascade according to the OB circuit is also not without drawbacks. It does not provide current amplification, so the current must be amplified in the differential stage that precedes it, which can be assembled on composite transistors.

At the input of the device there is a filter R1C3 that does not pass signals with a frequency above 100 kHz, from which the signal is fed to the inverting input of the UMZCH through an analogue of a non-polar oxide capacitor in the form of C1, C2. A polarizing bias voltage is applied to the connection point of these capacitors through resistor R2. The same input receives the OOC signal from the output of the device through the resistor R14. The current through each arm of the differential stage, as well as the collector current of the voltage amplification stage, is 3 mA. For all its shortcomings, an inverting amplifier is known for being more stable than a non-phase-inverting one.

The output stage, consisting of two stages of an emitter follower, has a somewhat non-standard unit for stabilizing the quiescent current and temperature conditions on transistors VT8 and VT9. It provides stabilization of the quiescent current of the first stage of the output stage, and hence the voltage across the resistor R15. It. accordingly, it leads to stabilization of the quiescent current of transistors VT12 and VT13, in the emitter circuits of which there are wire resistors R16 and R17. As many years of author's practice has shown, such a stabilization circuit can significantly reduce switching distortions, leading to the appearance of high-order harmonics, characteristic of "transistor sound". The author has been using this technical solution in his design and repair practice for more than ten years [2], and it fully justifies itself. A smooth "step" is well tracked by the OOS circuits, bringing the operation of the output stage closer to the so-called economy class A mode, which makes the subjective perception of the reproduction of an audio signal easier and more transparent. The dashed lines show the circuit when using a mains transformer without outputting a midpoint in the secondary winding. R20 and R21 are mandatory in the power circuit, resistor R22 should be replaced with a wire jumper, and fuse FU3 should be excluded.

Briefly about the parameters of the amplifier. With a sensitivity of 2 V, the described UMZCH provides a sinusoidal power of 8 W at a load with a resistance of 120 Ohms. When using a load with a resistance of 4 Ohms, the number of output transistors should be doubled along with resistors in their emitter circuits, then it will be possible to obtain an output sinusoidal power up to 180 ... 200 W. Oscillographic observation through an active notch filter, which suppresses the fundamental harmonic of a sinusoidal signal by 40 dB, showed that the level of harmonic distortion is approximately 0,03 ° O. With the values ​​​​of the resistor R14 of the OOS circuit and the resistor at the input R3 indicated in the diagram, the gain is 26 dB.

To mount the amplifier, a breadboard was used, on which a differential stage and a voltage amplifier for two channels were assembled. Their power supply circuits of positive and negative polarity are connected by a "star" at the terminals of capacitors C5, C6, respectively
The entire output stage was assembled on a common heat sink together with the output stage current stabilization unit. It is important that the powerful transistors of each channel (VT10-VT13) be installed on a common heat sink, and the current and thermal stabilization circuit transistors VT8 and can be mounted on the terminals of transistors VT9 and VT10) The power supply circuits of the output stage are also connected by a "star" at the terminals of capacitors C11, C7 and C9, SU, respectively.

The mains transformer T1 must have an overall power of at least 250 W with a secondary winding rated for a voltage of 70 V at a current of at least 3,5 A with or without a midpoint output (subject to the above changes).

All transistors of the output stage must be installed on a heat sink with an area of ​​at least 1200 cm2 (per channel).

Instead of oxide capacitors C1, C2, you can use one film (polyethylene terephthalate) capacitor with a capacity of 1 ... 2,2 μF for a voltage of 63 V (K73-16, K73-17), excluding, of course, the polarization resistor R2. The capacitance of blocking capacitors C7, C8 can be increased to 1 ... 2,2 uF.

Setting up the amplifier should begin with checking the correct installation and its compliance with the circuit diagram. In the author's version, the differential stage and the voltage amplifier are assembled on a separate board, so this particular node was first checked without connecting it to the output stage. To do this, the collectors of transistors VT6 and VT7 and the output of the resistor R14, right according to the scheme, were temporarily connected together. After applying power to the amplifier at this connection point, the voltage should not exceed 1 ... 15mV. It is also useful to check the currents of the shoulders of the differential stage and the voltage amplifier for compliance with the values ​​\uXNUMXb\uXNUMXbspecified in the diagram.

After checking, you should connect the voltage amplifier to the output stage by turning on a milliammeter instead of one of the fuses (FU2 or FU3), and by applying the supply voltage, make sure that the current consumption of the entire device is not more than 150 ... 200 mA (as a rule, it does not over 100 mA). You also need to make sure that the output voltage of the device is close to zero.

Then, by connecting an 8 ohm resistor and an oscilloscope to the UMZCH output, it is necessary to apply a rectangular signal to the UMZCH input in order to use the oscilloscope at different signal levels to make sure that there is no self-excitation or significant surges due to voltage drops. If this is still present, then it is necessary to increase the capacitance of the capacitor C4 (in the author's version, the amplifier is stable even without it).

It should be borne in mind that immediately after switching on, the quiescent current of the output transistors must be within 70 ... 90 mA. However, after a half-hour warm-up, it should rise to 120 ... 150 mA and stabilize.

Literature

1. Danilov A. A. Precision amplifiers of low frequency. - M.: Hotline - Telecom, 2004, p. 56, 57.
2. Sapozhnikov M. Refinement of UMZCH with non-standard inclusion of an op-amp. - Radio 2000, No. 8, p. 17.

Author: M. Sapozhnikov, Ganei Aviv, Israel; Publication: radioradar.net

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