5.6 Summary

Complex amplifier circuits can be modeled with a functional block. The ideal model includes the input and output impedances along with a controlled source. This source would exhibit a signal gain or amplification factor. Usually, G stands for power gain while Av and Ai represent voltage and current amplification, respectively. This amplification factor may be negative which indicates that the amplifier inverts the phase of the input, that is, the waveform is flipped upside down. The impedances allow calculation of loading effects while the gain determines the the size of the output signal.

If the input signal is too large, the output signal may be limited in amplitude or clipped. The maximum output amplitude is referred to as the compliance. Clipping is a gross form of distortion but more subtle forms exist as well. In general, distortion creates new frequency components. If these new components are integer multiples of the original input frequency, which they are typically, they are referred to as harmonics. One method of quantifying distortion performance is to sum all of the harmonics and compare that to the original signal. This is called THD or total harmonic distortion. Along with distortion, the amplifier might also add undesirable noise to the output signal. Noise is a random signal that contains many different frequencies. Typically, this is measured via a signal-to-noise ratio, or S/N, at the output.

An amplifier also operates over a given range of frequencies, from a lower limit, f1, to a high limit, f2. Some amplifiers are able to amplify down to 0 Hz (DC) and effectively do not have an f1 but all amplifiers do have an upper limit.

Finally, Miller’s Theorem is an analysis technique that allows an impedance that bridges from the input of an inverting voltage amplifier to its output to be split into equivalent input and output parallel impedances. These impedances will be smaller than the original bridging impedance and are a function of the gain of the amplifier.