Figure 2-3. Example of distortion as a result of overdriving an amplifier stage

TM 11-490-5

distorts an applied signal. An example of nonlinear

operation of a transistor is shown in figure 2-3. This

figure shows the manner in which the output voltage

waveform becomes distorted as a result of operation

over the nonlinear portion of a transistor's base current

versus collector current operating curve. This distorted

output contains frequencies which were not present in

the sine wave input signal.

These are actual

frequencies which comprise the distorted output signal

in that they respond to such actions as filtering,

coupling, and frequency selection as though they had

been input to the circuit. In the case of two or more RF

signals being presented to an RF amplifier under such

nonlinear

conditions,

sum

and

difference

intermodulation product frequencies can be produced.

Many nonlinear devices are used to perform useful

operations, such as within a mixer stage where the

nonlinearity is used to heterodyne or beat the incoming

signal frequency with the local oscillator frequency to

produce an intermediate frequency. A diode is a

nonlinear device which may be placed in a circuit so as

to pass only the positive going voltage peaks, while

shunting the negative going peaks to ground. However,

when the RF amplifier of a receiver is nonlinear, or is

operated over a nonlinear portion of its characteristic

curve (overdriven), it has the potential for producing

cross modulation and interlmodulation products. Either

of these types of distortion is objectionable.

Figure 2-3. Example of distortion as a result of

overdriving an amplifier stage.

2-5