Amplifier Gain | Decibel or dB Gain
An amplifier is such an electronic device which controls a bigger electrical signal by means of a far smaller electrical signal. The output signal of this device may be voltage controlled or current controlled. An amplifier just creates a copy of input signal (control signal) but in larger magnitude with the help of external DC source. Hence, there must be an external power source in all amplifier devices. Each amplifier device has its own ability to amplify an input signal to its highest possible output magnitude. The gain of an amplifier is defined as the measure this ability of amplifying a signal. Gain is a ratio so it does not have any unit.
The ratio of output current to input current of an amplifier is defined as current gain, the ratio of output voltage to input voltage is defined as voltage gain and similarly the ratio of output power to input power of an amplifier is known as power gain of amplifier. For example if the input voltage of an amplifier is 2.5 VRMS and output voltage is 50 VRMS, the voltage gain would be, An amplifier does always not amplify in same extend as its input. It may response differently for AC and DC. Hence, another way of representing gain is the ratio of change of output signal in respect of change in input signal. That means how much change occurred in output signal for certain change in input or control signal. The gain of amplifier can be calculated easily by taking ratio of rms value of output and input signals but it is not justified technique for AC signal.
Decibel Gain or dB GainAlthough amplifier gain does not have any unit as it is ratio of same unit signals, but if any one tries to represent the gain in logarithmic scale then it will have a unit. Generally the gain expressed in logarithmic scale will have a unit called decibel which is one tenth fraction of bel. The name of this unit that is “bel” comes from the name Alexander Graham Bell, the famous Scottish inventor who is well known for his invention of telephone systems. Now, decibel gain is defined in terms of the common (base 10) logarithm of a power ratio that means if the conventional gain of an amplifier is Poutput / Pinput, then this gain is expressed in decibel scale as 10log(Poutput / Pinput).
If we show the table below, it will be more clearly understood.
|Gain as ratio||Gain as decibel|
|0.1||- 10 dB|
|0.01||- 20 dB|
|0.001||- 30 dB|
|0.0001||- 40 dB|