9 BJT DC Biasing - npn / pnp transistor Basic Principle DC...

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npn / pnp transistor DC Biasing Circuits Basic Principle
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DC Biasing Circuits :- The ac operation of an amplifier depends on the initial dc values of I B , I C , and V CE . By varying I B around an initial dc value, I C and V CE are made to vary around their initial dc values. DC biasing is a static operation since it deals with setting a fixed (steady) level of current (through the device) with a desired fixed voltage drop across the device. The purpose of the dc biasing circuit :- to turn the device “ON” to place it in operation in the region of its characteristic where the device operates most linearly, i.e. to set up the initial dc values of I B , I C , and V CE Typical amplifier circuit. ** ONLY single power supply in the circuit ** R C R B +V CC i c v ce i b v in v out V BE I B Q V CE I C Q B C E n n p I C I E
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Defining the linear (undistorted) region of operation for a transistor :-
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Transistor Saturation :- Saturation maximum collector current I Cmax or I Csat V BC no longer reverse – biased output amplified signal will be distorted ??? WHY ??? I Csat maximum possible collector current for the chosen design and the level to stay below if we expect linear amplification ex. Fixed – Bias Circuit :- assume V CE ~ 0 , i.e. R CE ~ 0 Ω C CC Csat R V I =
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Emitter – Stabilized Bias Circuit :- ?? I B , V CE , I Csat ?? R E provides the improved stability , that is, the DC bias circuits and voltages remain closer to where they were set by the circuit even when outside conditions, such as supply voltage, temperature and even transistor beta β , change. let fix I B :- β I C ?? ( Δ temp Δβ ) I C ( I C = I B ) I E V E I B I C - Ve feedback !!!! R C R B +V CC v in v out C 2 C 1 R E I C I E I B V E n n p
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!! same as before !! Kirchoff’s voltage law :- 0 = - - - E E BE B B CC R I V R I V E B BE CC B R R V V I ) 1 ( + + - = β C I ?? E B BE CC E B BE CC E B BE CC B C R R V V R R V V R R V V I I + - + + - = + + - = = ) 1 ( ) 1 ( if E B R R < E BE CC C R V V I - ) ( fn I C in practice R E > 10 R B without R E ???
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?? E
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This note was uploaded on 01/11/2011 for the course EE 2106 taught by Professor Ytchow during the Spring '07 term at City University of Hong Kong.

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9 BJT DC Biasing - npn / pnp transistor Basic Principle DC...

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