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.