240ln10 - ACTIVE LOADS LN#10 BIASING OF A SIMPLE TRANSISTOR...

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1 ACTIVE LOADS LN #10 BIASING OF A SIMPLE TRANSISTOR STAGE means supplying a constant current value to the transistor stage. This may be done with a current mirror I I v o v o v o + - + - + - v i v i v i This is a single BJT common emitter stage: R o = r o1 || r o2 , A v = -g m1 (r o1 || r o2 ) Q1 Q2 2 1 1 A T A T v V V V V A + = Note: A v is independent of I c I v o + - v i M1 M2 Same applies to the Common Source MOS stage. R o = r o1 || r o2 A v = g m1 (r o1 || r o2 )
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2 v o + - v i Q1 Q2 v o Therefore there are other ways to bias the CS stage using NMOS devices which saves real estate A: Gate to drain connection of NMOS V DD -V T V DD v i v o + - v i Q1 Q2 B: Gate to Source connection of the NMOS depletion device (-ve V th ) V DD v i Reduced voltage swing Full voltage swing Now the gain depends on the bias point Now for the depletion device load (and also the diode connected load), the gain becomes 2 1 2 2 1 1 1 || || mb m mb o o m v g g g r r g A and we see that the body effect is somewhat of a limitation since then ( ) ( ) 2 1 / / 1 L W L W A v χ For this reason, the depletion load has been primarily a logic circuit rather than a small signal amplifier. For amplifiers and logic, CMOS is a preferred solution since then the body effect does not pose a problem as shown below.
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3 v i v o CMOS A v = - g m1 (r o1 || r o2 ) Because g mb2 = 0 g mb1 =0 APPLICATION OF CURRENT MIRRORS TO DIFFERENTIAL AMPLIFIERS v i1 + - V cc + - v i2 V EE + - v i1 + - v i2 V EE V cc BJT v o + _ T3 T4 T1 T2
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4 APPLICATION OF CURRENT MIRRORS TO DIFFERENTIAL AMPLIFIERS v i1 + - V DD + - v i2 V SS + - v i1 + - v i2 V SS V DD MOS v o + - T3 T4 I Current Mirrors have two functions in Differential Amplifiers 1.Replace the passive (resistor) loads with active transistor loads which has the benefits of
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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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240ln10 - ACTIVE LOADS LN#10 BIASING OF A SIMPLE TRANSISTOR...

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