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

(Student Manual for The Art of Electronics, Hayes and Horowitz, 2nd Ed.)

(see Lab 5)

(most like what you will see in Lab 8)

Q1 Q2

Q1Q2

Negative Voltage Feedback

(Introductory Electronics, Simpson, 2nd Ed.)

gainloopopen

out0

v

vA

gainloopclosed

1

1 0

0

in

out

BBA

A

v

vAf

vout–Bvout

Negative Voltage Feedback• Negative feedback through a voltage divider

(Introductory Electronics, Simpson, 2nd Ed.)

1

2

1

21 11

R

R

R

RR

BAf

Operational Amplifiers (Op Amps)• Basic prototype: Very high-gain DC-coupled

differential amplifier

(The Art of Electronics, Horowitz and Hill, 2nd Ed.)

non-inverting (+) inverting (–)

Circuit symbol:

input 1

input 2

output

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”Op Amp “Golden Rules”

I.I. The output The output attemptsattempts to do whatever is to do whatever is necessary to make the voltage difference necessary to make the voltage difference between the two inputs zero (consequence between the two inputs zero (consequence of very high voltage gain).of very high voltage gain).

II.II. The inputs draw “no” current (consequence The inputs draw “no” current (consequence of very high input impedance).of very high input impedance).

(Note that these rules hold only if there is external negative feedback.)

Inverting Amplifier

(The Art of Electronics, Horowitz and Hill, 2nd Ed.)

(Lab 8–2)

Non-Inverting Amplifier

(The Art of Electronics, Horowitz and Hill, 2nd Ed.)

(Lab 8–3)

Follower

(The Art of Electronics, Horowitz and Hill, 2nd Ed.)

(Lab 8–4)

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

(Student Manual for The Art of Electronics, Hayes and Horowitz, 2nd Ed.)

(Lab 8–1)

(a) (b) (c) (d)

(e) (f)

1k

1k1N914

1N914

out out out

out

out out

Circuit (e)

Current Source(Lab 8–5)

(Student Manual for The Art of Electronics, Hayes and Horowitz, 2nd Ed.)

Improved Current Source

OR

(Student Manual for The Art of Electronics, Hayes and Horowitz, 2nd Ed.)(The Art of Electronics, Horowitz and Hill, 2nd Ed.)

(Lab 8–5)

(BJT for Q1) (MOSFET for Q1)

Improved Current Source(Exercise 4.1)

(The Art of Electronics, Horowitz and Hill, 2nd Ed.)

Current-to-Voltage Converter

• Photodiode as input current source

(Lab 8–6a)

(Student Manual for The Art of Electronics, Hayes and Horowitz, 2nd Ed.)(The Art of Electronics, Horowitz and Hill, 2nd Ed.)

VTT 1113

I

I

Photodiode

(Introductory Electronics, Simpson, 2nd Ed.)

Current-to-Voltage Converter

• Phototransistor as input current source

(Lab 8–6b)

VTT 1113

I I

(Student Manual for The Art of Electronics, Hayes and Horowitz, 2nd Ed.)

Application of Photometer Circuit(Lab 8–6c)

(Student Manual for The Art of Electronics, Hayes and Horowitz, 2nd Ed.)

(Scope sweep rate of 0.5 s/div works best)

“Ideal” Current Meter

(Student Manual for The Art of Electronics, Hayes and Horowitz, 2nd Ed.)

(Exercise 4.4)

Iin

Imeter

Need current-limiting resistor here

500

Feedback resistor

0 V

Summing Amplifier(Lab 8–7 and Exercise 4.5)

(Student Manual for The Art of Electronics, Hayes and Horowitz, 2nd Ed.)

V1

V2

V3

V1

V2

V3

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.)