240ln04 - 1 AMPLIFIERS AS TWO-PORT NETWORKS LN #4 v 1 +- v...

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Unformatted text preview: 1 AMPLIFIERS AS TWO-PORT NETWORKS LN #4 v 1 +- v 2 +- y 11 y 22 y 12 v 2 y 21 v 1 I 1 = y 11 v 1 + y 12 v 2 I 2 = y 21 v 1 + y 22 v 2 y 11 = i 1 /v 1 v2=0 Input admittance with output short y 12 = i 1 /v 2 v1=0 Reverse g m with input short y 21 = i 2 /v 1 v2=0 Forward g m with output short y 22 = i 2 /v 2 v1=0 Input admittance with output short i 1 i 2 Unilateral Amplifier y 12 =0 short circuit transconductance The result is a) y 11 is independent of output circuit b) y 22 is independent of input circuit Impedances are also used v 2 +- Z o G m v 1 Z i i 1 i 2 v 1 +- v 2 +- Z o a v v 1 Z i i 1 i 2 v 1 +- OR NORTON THEVENIN v 2 +- R o G m v 1 R i i 1 i 2 v in +- +- v 1 R s R L When you include LOADING EFFECTS 2 To characterize the amplifier you need 1. R in 2. Any 2 of G m , Z o , a V This is because a v = v 2 = -G m Z o v 1 i 2 =0 SINGLE STAGE AMPLIFIERS v i R c V o I c I b For V i 0, the transistor is cut off and V o = V cc With increasing V i , the transistor enters the forward active mode and then V o = V cc I c R c with I c = I s exp(V i /V T ) In the forward active region, I b = I c / F With Increasing V i , I c increases resulting in V o 0. When that happens, the device enters saturation & V o is fixed at V ce , sat (0.1V). The voltage transfer characteristic (V TC ) looks like Common Emitter V o V i V CE,sat cutoff Forward Active Saturation For a device operating in the (FA) mode the voltage gain is determined from the small signal equivalent circuit (SSEC) B E r C v 1 +- g m v 1 r o C r c C r R s r b Remember the rule: For the SSEC, all power supplies are grounded Look at some typical values 1. r is in the range M 2. Source resistance R s (~50) is much less than r 3. r is typically is in the range of K 4. r b is typically ranges from 50~500 is < r 5. r c is typically 50 ~ 500 3 Now if we are analyzing mid-band frequencies, the capacitors with an impedance 1/ C can be considered open circuits v o +- r o g m v 1 r i 1 i c +- v 1 R c v i At the input : v 1 = v i o m c o o m R v L c o o c m o v L i c o c o m c o m o r g R r r g A R gain voltage circuit open R r r R g v v A with R v R r R r g R r v g v c = + = > + = = >...
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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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240ln04 - 1 AMPLIFIERS AS TWO-PORT NETWORKS LN #4 v 1 +- v...

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