Feedback
• Feedback: Taking a portion of the output and
feeding it back to the input
• Positive feedback: Feedback signal is in phase with
the input signal
– Oscillations (good and bad)
– Increase circuit gain
• Negative feedback: Feedback signal is out of phase
with the input signal
– Used in almost all practical amplifiers
– Pros: Increased circuit stability against fluctuations
(including temp. changes), increased Zin, decreased Zout,
decreased signal distortion, higher frequency range for
constant gain
– Cons: Decreased circuit gain (feedback reduces input)
Negative Voltage Feedback: Flashback
(see Lab 5)
(most like what you
will see in Lab 8)
Q1
Q1
Q2
Q2
(Student Manual for The Art of Electronics, Hayes and Horowitz, 2nd Ed.)
Negative Voltage Feedback
vout
A0 
v
open  loop gain 
vout
A0
1
Af 


vin 1  A0 B B
–Bvout
vout
closed  loop gain 
(Introductory Electronics, Simpson, 2nd Ed.)
Negative Voltage Feedback
• Negative feedback through a voltage divider
1 R1  R2
R2
Af  
 1
B
R1
R1
(Introductory Electronics, Simpson, 2nd Ed.)
Operational Amplifiers (Op Amps)
• Basic prototype: Very high-gain DC-coupled
differential amplifier
non-inverting (+)
inverting (–)
Circuit symbol:
input 1
output
input 2
(The Art of Electronics, Horowitz and Hill, 2nd Ed.)
LF411 Op Amp
(The Art of Electronics, Horowitz and Hill, 2nd Ed.)
(Student Manual for The Art of Electronics, Hayes and Horowitz, 2nd Ed.)
LF411 Op Amp
(Student Manual for The Art of Electronics, Hayes and Horowitz, 2nd Ed.)
Op Amp “Golden Rules”
I.
The output attempts to do whatever is
necessary to make the voltage difference
between the two inputs zero (consequence
of very high voltage gain).
II.
The inputs draw “no” current (consequence
of very high input impedance).
(Note that these rules hold only if there is external negative
feedback.)
Inverting Amplifier
(Lab 8–2)
(The Art of Electronics, Horowitz and Hill, 2nd Ed.)
Non-Inverting Amplifier
(Lab 8–3)
(The Art of Electronics, Horowitz and Hill, 2nd Ed.)
Follower
(Lab 8–4)
(The Art of Electronics, Horowitz and Hill, 2nd Ed.)
Restrictions on the Golden Rules
•
Golden rules I and II are followed only if:
1. The op amp is in the active region (not saturated)
2. There is negative feedback
out
1k
(c)
(b)
(a)
1N914
out
out
out
(d)
(Lab 8–1)
1k
1N914
out
out
(e)
(f)
(Student Manual for The Art of Electronics, Hayes and Horowitz, 2nd Ed.)
Circuit (e)
Current Source
(Lab 8–5)
(Student Manual for The Art of Electronics, Hayes and Horowitz, 2nd Ed.)
Improved Current Source
(Lab 8–5)
OR
(BJT for Q1)
(The Art of Electronics, Horowitz and Hill, 2nd Ed.)
(MOSFET for Q1)
(Student Manual for The Art of Electronics, Hayes and Horowitz, 2nd Ed.)
Improved Current Source
(Exercise 4.1)
(The Art of Electronics, Horowitz and Hill, 2nd Ed.)
Current-to-Voltage Converter
(Lab 8–6a)
• Photodiode as input current source
I
I
VTT 1113
(The Art of Electronics, Horowitz and Hill, 2nd Ed.)
(Student Manual for The Art of Electronics, Hayes and Horowitz, 2nd Ed.)
Photodiode
(Introductory Electronics, Simpson, 2nd Ed.)
Current-to-Voltage Converter
(Lab 8–6b)
• Phototransistor as input current source
I
I
VTT 1113
(Student Manual for The Art of Electronics, Hayes and Horowitz, 2nd Ed.)
Application of Photometer Circuit
(Lab 8–6c)
(Scope sweep rate of  0.5 s/div works best)
(Student Manual for The Art of Electronics, Hayes and Horowitz, 2nd Ed.)
“Ideal” Current Meter
(Exercise 4.4)
Need current-limiting
resistor here
Feedback resistor
Iin
0V
Imeter
500 W
(Student Manual for The Art of Electronics, Hayes and Horowitz, 2nd Ed.)
Summing Amplifier
(Lab 8–7 and Exercise 4.5)
V1
V1
V2
V2
V3
V3
(Student Manual for The Art of Electronics, Hayes and Horowitz, 2nd Ed.)
Push-Pull Buffer
(Lab 8–8)
 Crossover distortion
evident – see Lab 5–6
(Student Manual for The
Art of Electronics, Hayes
and Horowitz, 2nd Ed.)
 Crossover distortion eliminated
(The Art of Electronics, Horowitz and Hill, 2nd Ed.)