The circuits you’ve seen so far have been general, not application-specific. With capacitors of several microfarads, and when biased for class A, these circuits are representative of audio amplifiers.
High-fidelity audio amplifiers, of the kind used in music systems, must have more or less constant gain from 20 Hz to 20 kHz. This is a frequency range of 1000:1. Audio amplifiers for voice communications must work from 300 Hz to 3 kHz, a 10:1 span of frequencies. In digital communications, audio amplifiers are designed to work over a narrow range of frequencies, sometimes less than 100 Hz wide. Hi-fi amplifiers are usually equipped with resistor-capacitor (RC) networks that tailor the frequency response. These are tone controls, also called bass and treble controls. The simplest hi-fi amplifiers use a single knob to control the tone. More sophisticated amplifiers have separate controls, one for bass and the other for treble. The most advanced hi-fi systems make use of graphic equalizers, having controls that affect the amplifier gain over several different frequency spans.
A basic volume control (potentiometer R1) can be used to vary the gain in a low-power audio amplifier.
Audio amplifier systems usually consist of two or more stages. A stage is one bipolar transistor or FET (or a push-pull combination), plus peripheral resistors and capacitors. Stages can be cascaded one after the other to get high gain. In one of the stages in an audio system, a volume control is used. This control can be as simple as a potentiometer that allows the gain of a stage to be adjusted without affecting its linearity.
An example of a basic volume control is shown in above figure. In this amplifier, the gain through the transistor is constant. The ac output signal passes through C1 and appears across R1, a potentiometer. The wiper (indicated by the arrow) of the potentiometer picks off more or less of the ac output signal, depending on the position of the control shaft. Capacitor C2 isolates the potentiometer from the dc bias of the following stage. A volume control should normally be placed in a stage where the audio power level is low. This allows the use of a low-wattage, low-cost potentiometer.
An example of transformer coupling between amplifier stages. Component designators are discussed in the text.
Transformers can be used to transfer (or couple) signals from one stage to the next in a cascaded amplifier system (also known as an amplifier chain). An example of transformer coupling is shown in above figure. Capacitors C1 and C2 keep one end of the transformer primary and secondary at signal ground. Resistor R1 limits the current through the first transistor, Q1. Resistors R2 and R3 provide the proper base bias for transistor Q2.
The main disadvantage of this scheme is that it costs more than capacitive coupling. But transformer coupling can provide an optimum signal transfer between amplifier stages. By selecting a transformer with the correct turns ratio, the output impedance of Q1 can be perfectly matched to the input impedance of Q2.
In some amplifier systems that employ transformer coupling, capacitors are added across the primary and/or secondary of the transformer. This results in resonance at a frequency determined by the capacitance and the transformer winding inductance. If the set of amplifiers is intended for use at only one frequency (and this is often the case in RF systems), this method of coupling, called tuned-circuit coupling, enhances the system efficiency. But care must be taken to be sure that the amplifier chain doesn’t oscillate at the resonant frequency of the tuned circuits!