Internal Noise

• Electrical interference generated within a device or circuit itself.
• This is due to the functioning of active and passive component inside the device itself.
• Internal noise can be reduced by proper receiver design.
• Internal noise can be further classified into three types as follows.

1. Shot Noise
2. Transit Time Noise
3. Thermal Noise

a.Shot Noise :

• The other source of internal noise in communication receiver is shot noise.
• Active devices like diode,transistor,etc… has shot noise due to electron and holes coming randomly at the output electrodes.
• It is occur in bipolar transistor.
• The root-mean-square value of the shot noise current i_n is given by the Schottky formula

i_n=√2I_q∆B

where , I – DC current.

q – charge of electron.

∆B – bandwidth in hertz.

b.Transit Time Noise :

• This noise occurs in the semiconductor devices, where the transit time of the carrier crossing the junction is comparable with periodic time of the signal.
• In this case some of the carriers may defuse back to the source and thus transit time noise occurs.
• Transit time noise occurs at high frequency only.
• Based on which equipment is taking more time to transmit carrier signal time delay can be divided into three types.

1. Base time delay
2. Emitter time delay
3. Collector time delay

c.Thermal Noise :

• It is sometimes called as Johnson or Nyquist noise.
• It is generated by the random thermal motion of charge carriers, inside an electrical conductor, which happens regardless of any applied voltage.
• Thermal noise is approximately white, meaning that its power spectral density is nearly equal throughout the frequency spectrum.
• The root mean square(RMS) voltage due to thermal noise v_n, generated in a resistance R(ohms) over bandwidth ∆f(hertz), gven by

V_{n =} √4k_BTR∆f

where,                         k_B  is Boltzmann’s constant (J/K).

T is the resistor’s absolute temperature (kelvin).

• An example of such a noise source may be a cable or transmission line.

• The noise power is given by:

P_n α TB

P_n=kTB

Where,             k = Boltzman’s constant (1.38 x 10^-23 J/K)

T = temperature in degrees Kelvin

B = bandwidth in Hz