9. Discrete Transistor Amplifiers Lecture notes: Sec. 6 Sedra & - PowerPoint PPT Presentation

9. Discrete Transistor Amplifiers Lecture notes: Sec. 6 Sedra & Smith (6 th Ed): Sec. 5.6, 5.8, 6.6 & 6.8 Sedra & Smith (5 th Ed): Sec. 4.6, 4.8, 5.6 & 5.8 ECE 65, Winter2013, F. Najmabadi

How to add signal to the bias Bias & Signal Bias & Signal v GS = V GS + v gs v DS = V DS + v ds 1. Direct Coupling 2. Capacitive Coupling  Use bias with 2 voltage supplies o For the first stage, bias such that  Use a capacitor to separate bias V GS = 0 voltage from the signal. o For follow-up stages, match bias  Simplified biasing problem. voltages between stages  Used in discrete circuits  Difficult biasing problem  Only amplifies “AC” signals  Used in ICs  Amplifies “DC” signals! F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (2/42)

Capacitive coupling is based on the fact that capacitors appear as open circuit in bias (DC) Real Circuit Bias Circuit Signal Circuit  At a high enough frequency, Z c = 1/ ( ω C ) , becomes small (effectively, capacitors become short circuit). o Mid-band parameters of an Amplifier.*  At low frequencies, Z c cannot be ignored. As Z c depends on frequency, amplifier is NOT linear (for an arbitrary signal) for these low frequencies. (We do NOT want to operate the amplifier in these frequencies!) o Capacitors introduce a lower cut-off frequency for an amplifier (i.e., amplifier should be operated above this frequency). In ECE102, you will see that transistor amplifiers also have an “upper” cut-off frequency F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (3/42)

How to Solve Amplifier Circuits 1. Find Bias and Signal Circuits. 2. Bias circuit (signal = 0): o Capacitors are open circuit. o Use transistor large-signal model to find the bias point. o Use bias parameters to find small-signal parameters ( r π , g m , r o ). 3. Signal Circuit (IVS becomes short, ICS becomes open circuit): o Assume capacitors are short to find mid-band amplifier parameters. o Replace diodes and/or transistors with their small-signal model. o Solve for mid-band amplifier parameters ( A v , R i , R o ). For almost all circuits, we can use fundamental amplifier configurations, • instead of solving signal circuits. o Include impedance of capacitors to find the lower cut-off frequency of the amplifier. F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (4/42)

Emitter-degeneration bias circuits have similar signal circuits Bias with one power supply Bias with two power supplies (voltage divider) Signal Circuits The same circuit for R = 1 || We will solve this R R 2 B B B configuration F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (5/42)

By-pass capacitors Basic CE Configuration  There is no R E in the basic Common-Emitter configuration.  However, R E is necessary for bias in discrete circuits.  Use a by-pass capacitor Real Circuit Bias Circuit: Signal Circuit: Cap is open, R E stabilizes bias Capacitor shorts R E F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (6/42)

Discrete Common-Emitter Amplifier Real Circuit Standard Bias Circuit:* Caps are open circuit CE amplifier: Input at the base Output at the collector * Bias calculations are NOT done here as we have done them before. F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (7/42)

Signal circuit of the discrete CE Amplifier Real Circuit Short caps Zero bias supplies Rearrange F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (8/42)

Discrete CE Amplifier (Gain)  Signal input at the base  Signal output at the collector  No R E ′ = || R R R L C L Fundamental CE configuration v = − o ( || || ) g r R R m o C L v i v ′ = − o ( || ) g r R m o L v i F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (9/42)

Discrete CE Amplifier ( R i ) R = r = = | π R R r π i CE Fundamental CE configuration i = || R R r π B F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (10/42)

Discrete CE Amplifier ( R o ) 1) Set v sig = 0 Controlled current source becomes open circuit because g m v π = 0 2) Replace transistor with its SSM = 0 v π R = || R r o C o F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (11/42)

Discrete CE and CS Amplifiers v = − o ( || || ) g r R R m o C L v i = || R R r π i B = || R R r o C o → ∞ π r v = − o ( || || ) g r R R m o D L v i = R R i G = || R R r o D o v R v = × o i o + v R R v sig i sig i F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (12/42)

Discrete CS Amplifier with R S Real Circuit Bias Circuit Caps open CS amplifier with R S Input at the gate Output at the drain Signal Circuit Short caps Zero bias supplies F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (13/42)

Discrete CS Amplifier with R S (Gain)  Signal input at the gate  Signal output at the drain  R S ! ′ = || R R R L D L Fundamental CS configuration with R S ( || ) v g R R = − o m D L + + 1 ( || ) / v g R R R r i m S D L o ′ v g R = − o m L ′ + + 1 / v g R R r i m S L o F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (14/42)

Discrete CS Amplifier with R S ( R i ) = ∞ R = = ∞ | R R / i CS RS Fundamental CS configuration with R S R = R i G F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (15/42)

Discrete CS Amplifier with R S ( R o ) 1) Set v sig = 0 2) Replace transistor with its SSM  Since i g = 0 , v g = 0 and gate is grounded F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (16/42)

Discrete CS Amplifier with R S ( R o )  Attach v x , compute i x ( R o = v x /i x ) = − v i R gs y S = − + KVL: ( ) v i g v r i R x y m gs o y S = − − + ( ) v i r g r i R i R x y o m o y S y S = + + [ ( 1 ) ] v i r g r R x y o m o S v = x i + + y ( 1 ) r g r R By KCL o m o S v v v = + = + x x x KCL: i i + + x y ( 1 ) R R r g r R D D o m o S v = x i + + x || [ ( 1 ) ] R r g r R D o m o S v v [ ] ≈ = = + x x i || ( 1 ) R R r g R + + x || [ ] || [ ( 1 )] R r g r R R r g R o D o m S D o m o S D o m S F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (17/42)

Discrete CE and CS Amplifiers with R E / R S ( || ) v g R R = − o m C L + + + 1 [( || ) / ]( 1 / ) v g R R R r R r π i m E C L o E   β R = + + E ||   R R r R π + i B E  1 [( || ) / ]  R R r C L o     β R   = +   E || 1 R R r   + + o C o   || r R R R     π E B sig ( || ) v g R R = − o m D L + + 1 ( || ) / v g R R R r i m S D L o = R R i G [ ] = + || ( 1 ) R R r g R o D o m S v R v = × o i o + v R R v sig i sig i F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (18/42)

Discrete CB Amplifier Real Circuit Bias Circuit Caps open Signal Circuit CB amplifier Short caps Input at the gate Zero bias supplies Output at the drain Capacitor C B is necessary. Otherwise, Amp gain drops substantially. F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (19/42)

Discrete CB Amplifier (Gain)  Signal input at the emitter  Signal output at the collector ′ = || R R R L C L Fundamental CB Configuration v v = + ′ = + o ( || || ) g r R R o ( || ) g r R m o C L m o L v v i i F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (20/42)

Discrete CB Amplifier ( R i ) ′ + r R = || o L R r ′ + π + r R 1 g r = = o L | || m o R R r π + Fundamental i CB 1 g r m o CB configuration   + ( || ) r R R = || ||  o C L  R R r π + i E  1  g r m o F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (21/42)

Discrete CB Amplifier ( R o ) 1) Set v sig = 0 2) Replace transistor with its SSM F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (22/42)

Discrete CB Amplifier ( R o )  Attach v x , compute i x ( R o = v x /i x ) By KCL = || || R R R r π 1 E sig = − v i R π y 1 = − + KVL: ( ) v i g v r i R π 1 x y m o y = − − + ( ) v i r g r i R i R x y o m o y 1 y 1 = + + [ ( 1 ) ] v i r g r R 1 x y o m o v = x i + + y ( 1 ) r g r R 1 o m o v v v KCL: = + = + x x x i i + + x y { } ( 1 ) R R r g r R C C o m o 1 = + || [ 1 ( || || )] R R r g r R R π v o C o m E sig = x i + + x || [ ( 1 ) ] R r g r R 1 C o m o v v ≈ = x x i + + x || [ ] || [ ( 1 )] R r g r R R r g R 1 1 C o m o C o m F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (23/42)