Q. Explain about Amplifier noise?

Amplifier noise arises from both thermal sources (resistances) and nonthermal sources (semiconductor devices). Although nonthermal noise is not related to physical temperature and does not necessarily have a uniform spectrum like that of thermal noise, one still refers to an amplifier's noise temperature Ta, for convenience, as a measure of noisiness referred to the input. The model of a noisy amplifier is shown in Figure (a) with input noise N = ηB = kTB from a source at temperature T, and the output power given by

N_out = G_N + N_a = G_N + GkT_aB = Gk(T + T_a)B

where N_a = GkT_aB is the output noise power caused only by the amplifier, G is the power gain of the amplifier, and B is the bandwidth of the amplifier. Note that the amplifier noise N_a is added to the amplified source noise to yield the output power in Equation. If T = T₀ (i.e., room temperature), then N_out ≅ N_a, and the amplifier noise dominates the source noise, which is a common occurrence. When T_a >> T₀, the amplifier is very noisy, although not physically hot.

Figure (b) depicts the variation of noise temperature with frequency for a nonthermal source. Several phenomena lumped together under the term one-over-f (1/f ) noise lead to the pronounced low-frequency rise in Figure (b). Such 1/f noise is produced by transistors and certain transducers, such as photodiodes and optical sensors.