Efficiency in Power Amplifiers

In power amplification, efficiency is important. It not only provides optimum output power with minimum heat generation and minimum strain on the transistors, but it conserves energy as well. This translates into reduced cost, reduced size and weight, and longer equipment life compared with inefficient power amplifiers.

Determining dc Power Input

Suppose you connect an ammeter or milliammeter in series with the collector or drain of an amplifier and the power supply. While the amplifier is in operation, this meter will have a certain reading. The reading might appear constant, or it might fluctuate with changes in the input signal level. The dc collector power input to a bipolar-transistor amplifier circuit is the product of the collector current (IC) and the collector voltage (EC). Similarly, for an FET, the dc drain power input is the product of the drain current (ID) and the drain voltage (ED). These power figures can be further categorized as average or peak values. This discussion involves only average power.
 
The dc collector or drain power input can be high even when there is no input signal applied to an amplifier. A class A circuit operates this way. In fact, when a signal is applied to a class A amplifier, the meter reading, and therefore the dc collector or drain power input, does not change compared to the value under conditions of no input signal. In class AB1 or class AB2, there is low current (and therefore low dc collector or drain power input) with zero input signal, and a higher current (and therefore a higher dc power input) with an input signal. In class B and class C, there is no current (and therefore zero dc collector or drain power input) when there is no input signal. The current, and therefore the dc power input, increases with increasing signal input. The dc collector or drain power input is usually measured in watts, the product of amperes and volts. It can be indicated in milliwatts for low-power amplifiers, or kilowatts for high power amplifiers.

Power Output

The power output of an amplifier must be measured by means of a specialized ac wattmeter. The design of AF and RF wattmeters is a sophisticated specialty in engineering. When there is no signal input to an amplifier, there is no signal output, and therefore the power output is zero. This is true no matter what the class of amplification. The greater the signal input, in general, the greater the power output of a power amplifier, up to a certain point. Power output, like dc input, is measured in watts. For very low power circuits, it can be in milliwatts; for high-power circuits, it is sometimes given in kilowatts.

Definition of Efficiency

The efficiency of a power amplifier is the ratio of the ac power output to the dc collector or drain power input. In a bipolar-transistor amplifier, let PC be the dc collector power input, and let Pout be the ac power output. For an FET amplifier, let PD be the dc drain power input, and let Pout be the ac power output. Then the efficiency, eff, of the bipolar transistor amplifier is given by:
eff = Pout/PC
For the FET circuit, the efficiency is:
eff = Pout/PD
These are ratios, and they are always between 0 and 1. Efficiency is often expressed as a percentage instead of a ratio, so the preceding formulas are modified as follows:
eff% = 100 Pout/PC
and
eff% = 100 Pout/PD

Problem 1

Suppose a bipolar-transistor amplifier has a dc collector input of 115 W and an ac power output of 65.0 W. What is the efficiency in percent?
Use the formula for the efficiency of a bipolar transistor amplifier expressed as a percentage:
eff% = 100 Pout/PC = 100 × 65/115 = 100 × 0.565 = 56.5%.

Problem 2

Suppose an FET amplifier is 60 percent efficient. If the power output is 3.5 W, what is the dc drain power input?
Plug in values to the formula for the efficiency of an FET amplifier expressed as a percentage.
The resulting equation is solved as follows:
60 = 100 × 3.5/PD
60 = 350/PD
60/350 = 1/PD
PD = 350/60
= 5.8 W

Efficiency versus Class

Class A amplifiers have efficiency figures from 25 percent to 40 percent, depending on the nature of the input signal and the type of transistor used. A good class AB1 amplifier is 35 percent to 45 percent efficient. A class AB2 amplifier, if well designed and properly operated, can be up to about 50 percent efficient. Class B amplifiers are typically 50 percent to 65 percent efficient. Class C amplifiers can have efficiency levels as high as 75 percent.