common-collector amplifier section 13.1-13.4. topics emitter follower as a power amplifier push and...
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Common-Collector Amplifier
Section 13.1-13.4
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Topics
• Emitter Follower as a power amplifier• Push and Pull Output Stage
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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
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Common-Collector Amplifier
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Vin, m=1 mV
Vin,pp=2 mV Vout,pp=1.47 mV
Av(ADS)=0.735Av(matlab)=0.783
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Large Signal Behavior of Common-Collector Amplifier
The output does not trackthe input during the negative swing.
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Vin, m=1.71 V
VBE
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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.
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Vout versus Vin
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DC Sweep
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Vout Versus Vin
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Transient Simulation
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Vout Versus Vin
CrossoverDistortion
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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
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Example 13.5
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Implementation of VB
Add I1 to provide currentFor D1 and D2.
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Current flowing through the Source
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Improved Push-Pull Stage
Use Q4 to drive the base of Q2
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DC Bias
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Voltage Gain
Full circuit
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Voltage Gain from Vin to VN
Full circuit
To compute VN/Vin, Calculate RN and multiply RN by gm4.
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Voltage Gain from N to Out
Full circuit General formula: RL/(1/gm+RL)
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Small Signal Model of BJT Transistors
(NPN) (PNP)
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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!
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Voltage Gain
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Output Impedance
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Sanity Check!
RS is transformed to a smaller output resistance.
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Cadence Simulation
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Gain Calculated Using Matlab
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Change R to change Gain.
Tweak R to change gmand the gain at the expenseof burning more power.