Power Amplifiers

A small-signal amplifier is essentially a voltage amplifier — it turns a small input voltage into a much larger output voltage but can deliver very little power. When an amplifier must instead drive a motor or feed a loudspeaker, where large switching currents are needed, a power amplifier is required.

A power (large-signal) amplifier boosts the power level of the input signal and delivers that amplified power to the load. It works by converting DC power drawn from the supply into an AC voltage signal at the load — in effect it is a DC-to-AC power converter whose action is controlled by the input signal. This conversion is never perfect: some power is always lost as heat and the amplifier itself consumes power, so the efficiency is usually poor even though an ideal amplifier would reach 100%. Efficiency is the ratio of the AC power delivered to the load to the DC power taken from the supply.

Voltage amplifier versus power amplifier

A voltage amplifier is designed for the largest possible voltage gain and draws little output power. A power amplifier instead feeds a large amount of power to the load, and to obtain that output power its input-signal voltage must itself be large — which is why, in a practical electronic system, a voltage amplifier always precedes the power amplifier.

Classification of power amplifiers

Power amplifiers are classified into four groups by the portion of the input cycle over which output (collector) current flows, which in turn is set by the position of the operating (Q) point:

• Class A — Q-point at the centre of the linear amplifying region; output current flows for the full 360° cycle.
• Class B — Q-point at the cut-off region; output current flows for only half (180°) of the cycle.
• Class AB — Q-point between the x-axis and the centre of the load line; output current flows for more than half but less than the full cycle.
• Class C — Q-point beyond cut-off (below the x-axis); output current flows for less than half of the cycle.

The input and output waveforms of the Class A, Class B and Class AB stages are shown in Figures 3.5.1, 3.5.2 and 3.5.3: the Class A output reproduces the whole cycle about the bias current level, the Class B output shows only the positive half-cycle humps, and the Class AB output conducts for a little more than a half-cycle with a small bias current.

Class B push–pull amplifier

The transformer-coupled Class B push–pull amplifier (Figure 3.5.4) uses two centre-tapped transformers, T1 and T2, and two identical transistors Q1 and Q2. The input transformer T1 acts as a phase splitter, producing two drive voltages V1 and V2 that are 180° out of phase; the output transformer T2 couples the amplified signal from the collectors to the loudspeaker. Both transistors are biased at cut-off by giving them zero bias (base and emitter terminals joined); for balance, the two emitters connect to the centre tap of T1's secondary and Vcc connects to the centre tap of T2's secondary.

Working: on the positive half-cycle the upper transistor Q1 conducts while Q2 is cut off, and Q1's collector current is coupled through T2 to the loudspeaker as an amplified, inverted signal. On the negative half-cycle the polarities reverse, so Q2 turns on and Q1 turns off, and Q2 amplifies the other half. Because each transistor amplifies one half of the input, the loudspeaker receives a complete amplified cycle. The power efficiency is the AC output power divided by the DC input power.

Cross-over distortion

A transistor does not begin to conduct until the input signal voltage exceeds about 0.7 V for silicon or 0.3 V for germanium. In a push–pull stage this leaves a short interval near each zero crossing, between the positive and negative alternations, when neither transistor is conducting. The flattening of the output during this dead interval is called cross-over distortion (Figure 3.5.5).

Class C collector-tuned power amplifier

A single-tuned Class C power amplifier uses a parallel LC (tuned) circuit as its collector load (Figure 3.5.6). The inductance and capacitance are chosen so the circuit resonates at the frequency to be amplified, and the output is taken through a coupling capacitor. …