1. Chapter 6: FEEDBACK Dr. PHẠM NGUYỄN THANH LOAN
Hà Nội, 9/24/2012

2. Content Introduction Classification Chapter 18 (ref.1)
Voltage 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)
Chapter 18 (ref.1)
In the list above, voltage refers to connecting the output voltage as input to the
feedback network; current refers to tapping off some output current through the feedback
network. Series refers to connecting the feedback signal in series with the input
signal voltage; shunt refers to connecting the feedback signal in shunt (parallel) with
an input current source.
Series feedback connections tend to increase the input resistance, while shunt
feedback connections tend to decrease the input resistance. Voltage feedback tends to
decrease the output impedance, while current feedback tends to increase the output
impedance. Typically, higher input and lower output impedances are desired for most

3. 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

4. Negative feedback? Advantage of negative feedback Stable gain
Increase input impedance and decrease output impedance
Increase bandwidth
Increase linear region
Reduce noise
A typical feedback connection is shown in Fig The input signal, Vs, is apllied to a mixer network, where it is combined with a feedback signal, Vf. The dif-rence of these signals, Vi, is then the input voltage to the amplifier. A portion of the
mplifier output, Vo, is connected to the feedback network (), which provides a re-
ced portion of the output as feedback signal to the input mixer network.
If the feedback signal is of opposite polarity to the input signal, as shown in Fig.1, negative feedback results. While negative feedback results in reduced overall voltage gain, a number of improvements are obtained, among them being:

5. Classification
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)
voltage 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)

6. 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: voltage; output: voltage
Kiểu điện áp song song (voltage shunt feedback)
Input: current; Output: voltage
Kiểu dòng điện nối tiếp (current series feedback))
Input: voltage; Output: current
Kiểu dòng điện song song (current shunt feedback)
Input:current; Output: current

7. Voltage series feedback (1/3)
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)
p.753; p.754, 755

8. Voltage series feedback(2/3)
Vi =Vs - Vf
Af = Vo/Vs = A/(1+βA)
Zi = Vi/Ii; Zif=Vs/Ii  Zif = Zi(1+βA)

9. Voltage series feedback (3/3)
A = - gm(RD//Ro//(R1+R2)
= - gm.RL
β=Vf/Vo=- R2/(R1+R2)
Af=A/(1+βA)
Zif=Zi(1+βA)
Zof=Zo/(1+βA)
p.758; 759
Làm ví dụ 18.3

10. Voltage shunt feedback (1/2)
A=Vo/Ii
β=If/Vo
Af=Vo/Is=A/(1+βA)
Zif=Zi/(1+βA)
Zof=Zo/(1+βA)

11. Voltage shunt feedback (2/2)

12. Current series feedback (1/2)
A=Io/Vi
β=Vf/Io
Af=Io/Vs=A/(1+βA)
Zif=Zi(1+βA)
Zof=Zo(1+βA)

13. Current series feedback (2/2)

14. Current shunt feedback (1/2)
A=Io/Ii
β=If/Io
Af=Io/Is=A/(1+βA)
Zif=Zi/(1+βA)
Zof=Zo(1+βA)

15. Current shunt feedback(2/2)
A=Io/Ii
β=If/Ie2=RE/(re+RE+Rf)
Af=Io/Is=A/(1+βA)

16. Summary

17. Impedances with feedback

18. Impact on Bandwidth

19. Homework
Chapter 18: 1, 2, 3, 4, 5