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

Vin, m=1 mV

Vin,pp=2 mV Vout,pp=1.47 mV

Av(ADS)=0.735Av(matlab)=0.783

Large Signal Behavior of Common-Collector Amplifier

The output does not trackthe input during the negative swing.

Vin, m=1.71 V

VBE

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

Vout Versus Vin

Transient Simulation

Vout Versus Vin

CrossoverDistortion

Improved Push-Pull Stage

Q1 and Q2 can not beon at the same time!

Keep Q1 and Q2 on V1=Vout+VBE1V2=Vout-|VBE2|

V1-V2=VBE1+|VBE2|=VB

Example 13.5

Implementation of VB

Add I1 to provide currentFor 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 circuit General formula: RL/(1/gm+RL)

Small Signal Model of BJT Transistors

(NPN) (PNP)

Sanity Check

Intuition: gm1=gm2, rpi1=rpi2Vout/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 thatthis formula makes sense!

Voltage Gain

Output Impedance

Sanity Check!

RS is transformed to a smaller output resistance.

Cadence Simulation

Gain Calculated Using Matlab

Change R to change Gain.

Tweak R to change gmand the gain at the expenseof burning more power.