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DR. PHẠM NGUYỄN THANH LOAN
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CHAPTER 6: FEEDBACK
Hà Nội, 9/24/2012
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Introduction
Classification
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series feedback (Kiểu điện áp nối tiếp)
Voltage shunt feedback (Kiểu điện áp song song)
Current series feedback(Kiểu dòng điện nối tiếp)
Current shunt feedback (Kiểu dòng điện song song)
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Chapter 18 (ref.1)
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Introduction
Feedback: a part of
output is used to feed
the input
2 kinds of feedback:
negative and positive
Applications:
Positive feedback
oscillator
Negative feedback
amflifier
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Negative feedback?
Advantage of negative feedback
gain
Increase input impedance and decrease output
impedance
Increase bandwidth
Increase linear region
Reduce noise
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Stable
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Classification
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Base on the input connection to determine series or
shunt (nối tiếp/song song) and output signal to
derermine voltage/current (điện áp/dòng điện)
series feedback (Kiểu điện áp nối tiếp)
voltage shunt feedback (Kiểu điện áp song song)
current series feedback (Kiểu dòng điện nối tiếp)
current shunt feedback (Kiểu dòng điện song song)
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Classification
Base on the input connection to determine series or shunt
and output signal to deterrmine voltage/current
Kiểu điện áp nối tiếp (voltage series feedback)
Input:
dòng điện nối tiếp (current series feedback))
Input:
Kiểu
current; Output: voltage
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Kiểu
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điện áp song song (voltage shunt feedback)
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Kiểu
voltage; output: voltage
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Input:
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voltage; Output: current
dòng điện song song (current shunt feedback)
Input:current;
Output: current
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Voltage series feedback (1/3)
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A=Vo/Vi (withoutVf)
β=Vf/Vo
Af = Vo/Vs (overall gain)
Zi = Vi/Ii;
Zif=Vs/Ii=Zi(1+βA)
Zof=Zo/(1+βA)
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Voltage series feedback(2/3)
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Vi =Vs - Vf
Af = Vo/Vs = A/(1+βA)
Zi = Vi/Ii; Zif=Vs/Ii Zif = Zi(1+βA)
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Voltage series feedback (3/3)
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A = - gm(RD//Ro//(R1+R2)
= - gm.RL
β=Vf/Vo=- R2/(R1+R2)
Af=A/(1+βA)
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R
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R R /( R R ) g
2 L
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Zif=Zi(1+βA)
Zof=Zo/(1+βA)
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Voltage shunt feedback (1/2)
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A=Vo/Ii
β=If/Vo
Af=Vo/Is=A/(1+βA)
Zif=Zi/(1+βA)
Zof=Zo/(1+βA)
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Voltage shunt feedback (2/2)
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Current series feedback (1/2)
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A=Io/Vi
β=Vf/Io
Af=Io/Vs=A/(1+βA)
Zif=Zi(1+βA)
Zof=Zo(1+βA)
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Current series feedback (2/2)
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Current shunt feedback (1/2)
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A=Io/Ii
β=If/Io
Af=Io/Is=A/(1+βA)
Zif=Zi/(1+βA)
Zof=Zo(1+βA)
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Current shunt feedback(2/2)
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A=Io/Ii
β=If/Ie2=RE/(re+RE+Rf)
Af=Io/Is=A/(1+βA)
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Summary
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Impedances with feedback
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Impact on Bandwidth
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Homework
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Chapter 18: 1, 2, 3, 4, 5
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