Audio amplification

The circuits you have seen so far have been general, not application specific. With the capacitors of several microfarads, and when is biased for class A, these circuits are representative of audio amplifiers. As with RF amplifiers, there is not enough room to go into great depth about the audio amplifiers here.

Frequency response
High-fidelity audio amplifiers, of the kind used in music systems, should have more or less constant gain from 20 Hz to 20 kHz. It is the frequency range of 1000: 1. Audio amplifiers  for voice communications should work from 300 Hz to 3 kHz, a 10: 1 span of frequencies. In the digital communications, the audio amplifiers are designed to work over the narrow range of frequencies, sometimes below 100 Hz wide.

Hi-fi amplifiers are usually equipped with resistor-capacitor (RC) networks that tailor the frequency response. These are tone controls, called as bass and treble controls. The simple hi-fi amplifiers use the single knob to control tone. The more sophisticated amps have different controls, one for bass and other for treble. The advanced hi-fi systems make use of graphic equalizers, which are having controls that affect amplifier gain over several different frequency spans.

Gain-versus-frequency curves for 3 hypothetical audio amplifiers are shown in figure given below. At point A, a wideband, flat curve is illustrated. This is characteristic of hi-fi system amplifiers. At point B, a voice communications response is shown. At point C, a narrowband response curve, characteristic of audio amplifiers in Morse code or low speed digital-signal receivers, is shown.

Volume control

Audio amplifier systems consist of 2 or more stages. A stage is one bipolar transistor, plus peripheral resistors and capacitors. Stages are cascaded one after the other to obtain high gain.In one of the stages in an audio system, the volume control is used. This control is a potentiometer which allows the gain of a stage to be adjusted without affecting the linearity of it. An example of a simple volume control is shown in the Figure given below. In this amplifier, gain through the transistor is constant. The alternating current output signal passes through C1 and appears across R1, a potentiometer. The wiper of the potentiometer picks off more or less of the alternating current output signal, depending on position of control shaft. When the shaft is fully counterclockwise, arrow is at bottom of the zig-zag line, and none of the signal passes to output. When shaft is fully clockwise, arrow is at the top of zig-zag line, and the entire signal passes to output. At intermediate positions of control shaft, various proportions of full output signal will appear at the output. Capacitor C2 isolates potentiometer from direct current bias of the following stage.

Volume control is normally done in a stage where audio power level is quite low. This permits the use of small potentiometer, rated perhaps 1 W. If volume control were done at the high audio power levels, the potentiometer would be required to be able to dissipate large amounts of power, and would be unnecessarily expensive.