# Common-Collector Amplifier Section 13.1-13.4. Topics Emitter Follower as a power amplifier Push and Pull Output Stage.

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Common-Collector Amplifier Section 13.1-13.4

Topics Emitter Follower as a power amplifier Push and Pull Output Stage

Design Guideline Specs: Av, rsp, v_re gm=Av/(1-Av)/rsp Calculate IC from gm. Determine re from IC and V_RE Calculate VBE. Calculate VB. Calculate IB from IC. Assume that I1=40 IB Calculate R2 from VB. Calcualte R1

Common-Collector Amplifier

V in, m =1 mV V in,pp =2 mVV out,pp =1.47 mV Av(ADS)=0.735 Av(matlab)=0.783

Large Signal Behavior of Common-Collector Amplifier The output does not track the input during the negative swing.

V in, m =1.71 V V BE

Push-Pull Stage Q1 pushes current into RL. Q1 pulls current from RL. Q1 and Q2 are off. Assume that VCC is +5V and VEE is -5V.

Vout versus Vin

DC Sweep

V out Versus V in

Transient Simulation

Vout Versus Vin Crossover Distortion

Improved Push-Pull Stage Q1 and Q2 can not be on at the same time! Keep Q1 and Q2 on V1=Vout+VBE1 V2=Vout-|VBE2| V1-V2=VBE1+|VBE2|=VB

Example 13.5

Implementation of VB Add I1 to provide current For D1 and D2.

Current flowing through the Source

Improved Push-Pull Stage Use Q4 to drive the base of Q2

DC Bias

Voltage Gain Full circuit

Voltage Gain from Vin to VN Full circuit To compute VN/Vin, Calculate RN and multiply RN by gm4.

Voltage Gain from N to Out Full circuitGeneral formula: RL/(1/gm+RL)

Small Signal Model of BJT Transistors (NPN)(PNP)

Sanity Check Intuition: gm1=gm2, rpi1=rpi2 Vout/VN=(1+2gm rpi/2)/(rpi/2/RL+1+2gm rpi/2) =(1+beta)/(rpi/2/RL+1+beta) ~=1/(rpi/2/RL/beta+1) =RL/(/(2gm)+RL) Compare Vout /Vn to the generic formula: RL/(1/gm+RL) we see that this formula makes sense!

Voltage Gain

Output Impedance

Sanity Check! R S is transformed to a smaller output resistance.