240ln06 - MULTIPLE COMPONENT CIRCUITS Common Emitter...

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1 MULTIPLE COMPONENT CIRCUITS LN #6 Common Emitter Amplifier with Emitter Resistance (this is called emitter degeneration) V CC R c v i R E v o g m v 1 r π + v i i b + - v 1 i c R c v o + - R E i b + i c R in = r π + ( β + 1) R E High Input Resistance Find the circuit G m + + = + + = E c E c b b i R r i R i i i r v ) 1 1 ( ) ( β π E m m i c m R g g v i G + = 1 and thus we see the effective transconductance has gone down this means lower gain as an amplifier Calculate the R out remember the procedure for calculating R out 1.short the input 2.apply v t at the output 3. determine i t g m v 1 r π i b + - v 1 r o R E i b + i t + - v t i t i 1 v E Write down voltage and current relations for the circuit v E + v 1 = 0 v e = -v 1 i b r π + R E (i b + i t ) = 0 1 1 1 1 1 1 1 v r i v r i v R r R r g i v g i i i R r R r r i v i r R R i o E o t E E m t m t t E E b t E E b = + = + + = = + = = + =
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2 R o = v t /i t = r π ||R e + r o [ 1 + g m (r π ||R E ) ] small R o ? r o [1 + g m ( r π || R E ) ] Output resistance increases with emitter resistance **NB** MULTIPLE STAGE CIRCUITS Let us look at a simple 2 stage amplifier R c v o R L V cc V cc It is highly idealized v i r π i b1 + - v 1 v i g m v 1 r π + - v 2 g m v 2 i c1 i b2 R c R L v o + - i c2 R in2 SSEC Take note of this result – in many multi transistor circuits you will need to use this again and again As in the single device amplifiers we need to find R i , R o , A v and G m . Using the SSEC, work backwards from the second stage, i.e. we can determine A v and R i for the second stage v 2 = i b2 r π and i c2 = g m v 2 v o = -i c2 R L = - g m v 2 R L L m o R g v v = 2 R in2 = r π Now consider the 1st stage driving the second stage result as in the diagram
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3 r π i b1 + - v 1 v i g m v 1 + - v 2 i c1 R c v 2 = -i c1 (R in2 ||R c ) = -g m v 1 (R in2 || R c ) v 1 = v i v 2 = -g m (R in2 || R c ) v i ( ) π r R and R R r R r g A THUS R r R r g R g v v v v v v A R r R r g v v in L c c m v L L m L m i o i o v c c m i = + = + = = = + = 2 2 2 2 R in2 What about R out ? r
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This note was uploaded on 03/29/2011 for the course ECE 4201 taught by Professor N/a during the Spring '11 term at UConn.

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240ln06 - MULTIPLE COMPONENT CIRCUITS Common Emitter...

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