An amplifier with an output stage that amplifies the signal voltage. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Audio equipment

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The device uses symmetrical output stages on transistors of different structures, connected according to a common emitter circuit. The advantage of such cascades compared to traditional ones (current amplifying) is the possibility of obtaining voltage amplification from them, which makes it possible to reduce the signal voltage at the output of the previous cascade.

As a result, at the maximum signal amplitude, the transistors of the final stage do not enter regions close to cutoff and saturation, it becomes possible to introduce local OOS, decoupling filters in the power circuits. As a result, it is possible to significantly reduce the harmonic distortion even with a shallow FOS covering the entire amplifier.

Another advantage of the output stage, which amplifies the signal voltage, is the high utilization of the supply voltage, which is very important for portable radio equipment with an independent power source.

Ris.1

The main parameters:

• Rated frequency range, Hz.......60...20 000
• Rated output power, W at a load with a resistance of 8 ohms at a harmonic coefficient of up to 1% and supply voltage. B: 9.......0,8
• 12......1,8
• Rated input voltage, mV.......7
• Tone control limits (at frequencies of 100 Hz and 10 kHz). DB......±12
• Input impedance, kOhm.......15
• The interval of supply voltages in which the operability of the amplifier is maintained, V ....... 3 ... 12

The amplifier for a portable radio receiver consists of an input stage on a transistor V1, a passive tone control, a differential stage on transistors V2, V3, a pre-terminal stage on a transistor V5 with a current source on a transistor V7 and an end (output) stage on transistors V8-V11.

The use of a current source on transistor V5 as a load of the pre-terminal stage (V7) made it possible to reduce the influence of the nonlinearity of the input characteristics of transistors V8, V9 and the quiescent current of transistors V10, V11. Rigid stabilization of the DC component at the output of the amplifier is achieved by introducing a deep DC feedback covering all stages except the first one.

To reduce non-linear distortion, transistors V10, V11 must be selected with close values ​​of the static current transfer coefficient h21e. Diode V6 is placed on the heat sink of one of the output transistors (V10 or V11). The quiescent current (2,5 ... 3 mA) of these transistors is set by selecting the resistor R19 *.

It is advisable to use a symmetrical output stage with voltage gain in devices with an op-amp. A schematic diagram of one of these devices is shown in Fig.2.

Ris.2

The main parameters:

• Rated frequency range, Hz.......10...60000
• Maximum output power, W, (with a harmonic coefficient of not more than 2% and a frequency of 1000 Hz) at a load with resistance, Ohm: 8 ....... 12
• 4......24
• Rated input voltage, V ....... 1
• Input impedance, kOhm.......1,3

The amplifier contains a pre-amplification stage on the op-amp A1, a current source on the transistor V2 and an output stage on the transistors V3-V6. The quiescent current of transistors V5, V6 (about 5 mA) is set when adjusting the selection of resistor R6 *. For temperature stabilization of the operating mode of these transistors, the zener diode V1 must be installed on their heat sink.

In addition to the elements of the amplifying path, the device contains a stereo balance indicator, consisting of signal rectifiers for the left (V7) and right (V8) channels, op-amp A2 and LEDs V9, V10. With the appearance of a signal at the input of the amplifier, capacitors C5 and C6 are charged to voltages corresponding to the average signal levels of the left and right channels, respectively. With equal signals in both channels, the output current of op-amp A2 is absent, and the LEDs V9, V10 do not light up. Stereo imbalance causes the voltages at the inputs of op-amp A2 to become different, and its output current causes one of the LEDs to light up (the one that corresponds to the channel with a high signal level) The difference in the output signals of the power amplifier, to which the indicator reacts, is about 200 mV .

A stabilized source is required to power the amplifier.

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