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### Noise in BJT

The objective is to determine , and for a bipolar transistor.

The hybrid- model that includes the noise sources is shown below

Fig 5-3

The feedback elements . This is OK for frequency < . At frequencies above that value the noise mechanisms are partially correlated.

is caused by the thermal noise of the base-spreading resistance. the noise current generator is the shot noise of the total base current, and is the shot noise of the collector current.

The 1/f noise contribution is represented by a single noise current generator which flows through the entire base resistance. The spectral density of 1/f noise is given by

Equivalent Input Noise

- The exponent ranges between 1 and 2. The experimental value of K varies over a wide range of values. An alternative expression for 1/f noise is
- where fL varies from 3.7MHz to 7 MHz. An expression for the 1/f noise voltage generator is
- where . 1/f noise results from trapping and detrapping of carriers in surface and bulk defect states. Transistors with high at very low collector currents seem to have little 1/f noise as these traps are also recombination centers.

- To derive the equivalent input noise we first calculate the total noise at the transistor output, the gain from the source to output, and then divide the output noise by the gain.
- The output is shorted then the output noise current is

Noise in Field Effect Transistors

- For an input signal Vs, the output short-circuit current is
- The transfer gain is
- The equivalent input noise is therefore
- For a zero source resistance case

- Three main types of noise are found in FETs -- Shot noise, flicker noise and thermal noise
- The small-signal noise equivalent circuit for an FET is shown below
- Fig 6-1

- There are three principle sources of noise in a MOSFET identified as
- where KF is the flicker noise coefficient
- IDQ is the quiescent drain current, AF is a constant, f is the frequency.
- The total noise current at the output drain-current channel is
- We reflect this noise current to the gate as an equivalent input noise voltage using the Ktr reflection coefficient defined as

- The first term of the above equation is equivalent to a single resistor of value

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