* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Noise in BJT
Survey
Document related concepts
Electromagnetic compatibility wikipedia , lookup
Alternating current wikipedia , lookup
Current source wikipedia , lookup
Quantization (signal processing) wikipedia , lookup
Switched-mode power supply wikipedia , lookup
Ground loop (electricity) wikipedia , lookup
Multidimensional empirical mode decomposition wikipedia , lookup
Buck converter wikipedia , lookup
Resistive opto-isolator wikipedia , lookup
Opto-isolator wikipedia , lookup
Rectiverter wikipedia , lookup
Transcript
Noise in BJT The objective is to determine Eni , En and I n 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 C and r are removed fT < . At frequencies above that value the noise mechanisms are 01 / 2 partially correlated. Eniis caused by the thermal noise of the base-spreading resistance. the noise current generator is Ithe shot noise of the total base current, and nb I nc is the shot noise of the collector current. 2 E x2 4kTrx , I nb 2qI B , I nc2 2qI C 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 I 2f KI B f • 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 2qf L I B 2 is I f f • where fL varies from 3.7MHz to 7 MHz. An expression for the 1/f noise '2 2qf L I B rx voltage generator is E 2f f • where rx' rx / 2. 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. Equivalent Input Noise • To derive the equivalent input noise Eni 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 2 I no I nc2 ( g m E ) 2 or 2 ( E x2 Es2 ) Z2 ( I nb I 2f ) Z2 (rx Rs ) 2 I I g 2 (rx Rs Z ) 2 (rx Rs Z ) 2 no 2 nc 2 m g VZ • For an input signal Vs, the output short-circuit current is I o g mV m s rx Rs Z I g Z m • The transfer gain is K o t rx Rs Z Vs • The equivalent input noise is therefore 2 I no E 2 Kt 2 ni I nc2 (rx Rs Z ) E E ( I I )( rx Rs ) g m2 Z2 2 x 2 s 2 nb 2 f 2 • For a zero source resistance case 2qI C r 2qf I r 2 f En2 4kTrx 2qI B rx2 2 qI r C x 2 0 f fT 2 '2 L B x 2 Noise in Field Effect Transistors • 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 2 I ng 2qI dc 2 I nd I 2f • • • • • 8kTgm 3 AF K f I DQ f Cox L2eff 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 2 2 I no I nd I 2f We reflect this noise current to the gate as an equivalent input noise voltage using the Ktr reflection coefficient defined as K tr id ( signal) v gs ( signal) gm 2 I 2f I nd E 2 2 En2 gm gm 2 ni 8kT KF 3 g m 2 K p fCoxWLeff • The first term of the above equation is equivalent to a single resistor of value 2 Rn 3g m