Noise
Noise can be defined as any kind of unwanted signal not derived from or related to the input signal. Noise assumes great importance in the case of small signals. The signal to noise ratios (S/N) should be high. High quality amplifiers are those where input and output S/N ratios match each other. The performance of the amplifier is measured as
F = input (S/N) / output(S/N) (10.42)
i.e.,
F = (SI / NI) / (So / No) (10.43)
F is greater than unity. When expressed in decibels the noise factor F is called the noise figure.
NF = 10 log10F dB (10.44)
An ideal amplifier would thus have,
SI / Ni = So / No, F = 1 & NF = 0dB. (10.45)
If G is the power gain of the amplifier then
G = So / Si i.e. So = GSI(10.46)
No = Gni + NA. (10.47)
The Decibel system
This is widely used in audio radio & TV circuits. An individual s response to seeing or learning is non linear. Changes in power and audio levels are related logarithmically. If audio level changes by a factor of three then the relationship is written as,
Log 464 = 3 (10.48)
Power level = 10 log10 ( P2/P1 ) bel.(10.49)
1 bel = 10 decibels. (10.50)
Voltage and current levels
Decibels though defined in association with power, they can be used for voltage and current levels.
Consider an amplifier with an input voltage of Vi and output voltage of VO.
AV or amplification = VO/VI (10.51)
Its decibel voltage gain
GV = 20log10 Av dB = 20 log10 VO/Vi dB (10.52)