AMPLIFIER GAINS

AV = Voutput / Vinput

AV = 30 V / 2 V

AV = 15

Correspondingly, if we know the gain of an Amplifier and the magnitude of the input signal, we can calculate the magnitude of the output. For example, if an Amplifier with an AC current gain of 3.5 is given an AC input signal of 28 mA RMS, the output will be 3.5 times 28 mA, or 98 mA:

Ioutput = (AV)(Vinput)

Ioutput = (3.5)(28 mA)

Ioutput = 98 mA

In the last two examples I specifically identified the gains and signal magnitudes in terms of “AC.” This was intentional, and illustrates an important concept: electronic Amplifiers often respond differently to AC and DC input signals, and may amplify them to different extents. Another way of saying this is that Amplifiers often amplify changes or variations in input signal magnitude (AC) at a different ratio than steady input signal magnitudes (DC). The specifc reasons for this are too complex to explain at this time, but the fact of the matter is worth mentioning. If gain calculations are to be carried out, it must first be understood what type of signals and gains are being dealt with, AC or DC.

Electrical Amplifier gains may be expressed in terms of voltage, current, and/or power, in both AC and DC. A summary of gain defnitions is as follows. The triangle-shaped “delta” symbol (¢) represents change in mathematics, so “¢Voutput / ¢Vinput” means “change in output voltage divided by change in input voltage,” or more simply, “AC output voltage divided by AC input voltage”:

DC gains AC gains

If multiple Amplifiers are staged, their respective gains form an overall gain equal to the product (multiplication) of the individual gains:

Input signal Amplifier Output signal   => Amplifier gain = 3 => gain = 5 Overall gain = (3)(5) = 15