Amplifier gain
Because amplifiers have the ability to
increase the magnitude of an input signal, it is useful to be able to
rate an amplifier's amplifying ability in terms of an output/input
ratio. The technical term for an amplifier's output/input magnitude
ratio is gain. As a ratio of equal units (power out / power in,
voltage out / voltage in, or current out / current in), gain is
naturally a unitless measurement. Mathematically, gain is symbolized by
the capital letter "A".
For example, if an amplifier takes in an
AC voltage signal measuring 2 volts RMS and outputs an AC voltage of 30
volts RMS, it has an AC voltage gain of 30 divided by 2, or 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:
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 specific 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 definitions is as follows. The triangle-shaped "delta"
symbol (Δ) represents change in mathematics, so "ΔVoutput
/ ΔVinput" means "change in output voltage divided by change
in input voltage," or more simply, "AC output voltage divided by AC
input voltage":
If multiple amplifiers are staged, their
respective gains form an overall gain equal to the product
(multiplication) of the individual gains:
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