1. MALVINO
Electronic
PRINCIPLES
SIXTH EDITION

2. Operational Amplifiers
Chapter 18

3. The typical op amp has a differential input and a single-ended output.
+VCC
Class B
push-pull
emitter
follower
Diff
amp
More
stages
of gain
-VEE

4. Op amp symbol and equivalent circuit
Noninverting
input
+VCC
Output
Inverting
input
-VEE
Equivalent circuit
AOL(v1-v2)
Rout
Rin v1 v2
vout

5. The 741 op amp is an industry standard.
Rout v1
vout
Rin
AOL(v1-v2) v2
Rin = 2 MW
Rout = 75 W
AOL = 100,000
Iin(bias) = 80 nA
Iin(off) = 20 nA
Vin(off) = 2 mV
funity = 1 MHz
CMRR = 90 dB

6. Bode plot of the 741 op amp funity 100 dB 80 dB 20 dB/decade rolloff
10 Hz
100 Hz
1 kHz
10 kHz
100 kHz
1 MHz
funity

7. 741C pinout and offset nulling
+VCC RB 7 2 6
Adjust
for null 5 3 1 RB 4
10 kW
-VEE

8. The internal frequency compensation capacitor found in most
op amps also limits the rate at which the output can change.
SR = 0.5 V/ms (for the 741)
Slew rate
distortion
Slope > SR
When a signal exceeds the slew-rate of an op amp, the
output becomes distorted and amplitude limited.

9. change (slope) is directly related to both amplitudedv dt dv dt
The rate of voltage
change (slope) is directly
related to both amplitude
and frequency:
> v t
SS = 2pfVp
The power bandwidth
of an op amp is given by: dv dt dv dt v
> t
fmax =
2pfVp SR

10. The inverting amplifier
The negative feedback produces a
virtual ground at the inverting terminal.
A virtual ground is a short for voltage but an open for current.

11. Analyzing the inverting amplifier
iin
iin
vin
vout
vin = iinR1 and vout = iinR2
ACL = = R1 R2
vout
vin
zin(CL) = R1

12. Negative feedback increases the closed-loop bandwidth.
funity
ACL
80 dB
60 dB
40 dB
20 dB
0 dB
10 Hz
100 Hz
1 kHz
10 kHz
100 kHz
1 MHz

13. Negative feedback reduces error
V1err = (RB1 - RB2)Iin(bias)
V2err = (RB1 + RB2)Iin(off)/2
V3err = Vin(off)
Verror = ± ACL(± V1err ± V2err ± V3err)
V1err eliminated with resistor compensation
Use offset nulling in demanding applications

14. Resistor compensation for V1err
vin
vout
RB2 = R1 R2
RB2 has no effect on the virtual-ground approximation
since no signal current flows through it.

15. produces a virtual short.
The noninverting amplifier
The negative feedback
produces a virtual short. R2 R1
A virtual short is a short for voltage but an open for current.

16. Analyzing the noninverting amplifier
vout
vin i1 R2
vin = i1R1 and vout = i1(R2+R1) i1 R1
ACL = = R1
R2+R1
vout
vin R2
+ 1
zin(CL)  

17. The summing amplifier R1 RF v1 R2 v2
vout R2 RF v2 R1 v1 +
vout =

18. The voltage follower Rhigh vout vin
The virtual short tells us vout = vin
Rlow
ACL = 1
zin(CL)  
zout(CL)  0
f2(CL) = funity

19. Other than the 741 BIFET op amps offer extremely low input currents.
High-power op amps supply amperes of output current.
High-speed op amps slew at tens or hundreds of volts/ms and some have hundreds of MHz of bandwidth.
Precision op amps boast small offset errors and low temperature drift.

20. Other linear ICs
Audio amps in the mW range optimized for low noise (preamplifiers)
Audio amps in the watt range for driving loudspeakers
Video amps with wide bandwidths
RF and IF amps for receiver applications
Voltage regulators