An amplifier is a circuit designed to impose a specified voltage waveform, V(t), or, sometimes, a specified current waveform, I(t), upon the terminals of a device known as the “load.” The specified waveform is often supplied in the form of an analog “input signal” such as the millivolt-level signal from a dynamic microphone. In a public address system, an audio amplifier produces a scaled-up copy of the microphone voltage (the input signal) and this amplified voltage (the output signal) is connected to a loudspeaker (the load). An audio amplifier generally supplies more than a watt to the loudspeaker. The microphone cannot supply more than milliwatts, so the audio amplifier is a power amplifier as well as a voltage amplifier. The ability to amplify power is really the defining characteristic of an amplifier. Of course energy must be conserved; amplifiers contain or are connected to power supplies, usually batteries or power line-driven ac-to-dc converter circuits, originally known as “battery eliminators” but long since simply called “power supplies” (see Chapter 29). The amplifiers discussed in this chapter are the basic “resistance-controlled” circuits in which transistors (or vacuum tubes) are used as electronically variable resistors to control the current through the load. Such circuits span the range from monolithic op-amps to the output amplifiers in high-power microwave transmitters.

Single-loop amplifier

Figure 3.1 shows the simplest resistance-controlled amplifier. This circuit is just a resistive voltage divider. The manually variable resistor (rheostat) in (a) represents the electronically variable resistor (transistor) in (b).