1. Section 8.1 (Razavi) Chapter 12 (Sedra Smith)
741 Op-Amp Circuit
Section 8.1 (Razavi)
Chapter 12 (Sedra Smith)

2. History
The first uA741 Op-Amp was designed by Bob Widlar in 1963 at Fairchild Semidconductor.
uA702 sold for 300 a piece.
uA709 was sold for 70 a piece.
Bob asked for a raise, didn’t get it, so he quit. Bob sent to National Semiconductor.
LM101

3. History David Fullagar (Fairchild) Scrutinize LM 101
Incorporated a 30 pF Cap.
uA741 was introduced.
Gordon Moore

4. BJT Op-Amp

5. BJT Op-Amp Reference Input-Stage Bias Bias Current Second-Stage
Output-Stage

6. Reference Bias Current
VCC=15 V
VEE=-15 V
VBE11=0.7 V
VEB12=0.7 V
IREF=0.73 mA

7. Input-Stage Bias IREF=0.73 mA R4=5 KΩ (IS10=IS11, assumption)
Solve through trial and error
IC10=19 uA

8. Input Stage (1) From previous Calculation: IC10=19 uA (from symmetry)
(β is high )
(~0)

9. Negative Feedback
(2)
(3)
(1)
(4)
(constant)

10. Input Stage (2)

11. Input Stage (3) (IB1) (IB2) (Input Bias Current)
(Input Offset Current)

12. Input Resistance (1)
(AC Ground)

13. Input Resistance (AC Ground) R into the emitter of Q3: 1/gm3
R into the base of Q1: rπ1+β1/gm3
Differential resistance 2(rπ1+β1/gm3)
gm=IC/VT
Large input resistance since IC is kept small!
Typical input impedance is in excess of 2 MOhms

14. Second-Stage Bias 0.75IS12=IS13B IC12=0.73 mA IC13B=0.55 mA IC17=IC13B
(0)

15. Class AB Output Stage (Review)

16. Output Stage Bias (0) IS13,A=0.25IS12 IC23=0.25IREF Assume
VBE18=VBE19=0.6 V
Βp=50
Βn=200
ICE23=180 uA
IR10=15 uA
IE18=IE23-IR10
IB18=IC18/200=0.8 uA
IE19=IB18+IR10
VBB=VBE18+VBE19
(Output resistance is small)
(0)

17. Op-Amp

18. Equivalent Circuit of an Op-Amp
Characteristics:
Negligible input current
Large input AC resistance
Small AC output resistance Large Voltage Gain

19. Op-Amp as an Amplifier (non-inverting input) (inverting input)
Op-Amp is a circuit that amplifies the difference
between the inputs.

20. Inverting/Non-Inverting Configuration

21. Consequence of Large Gain
As Ao approaches infinity, Vin1-Vin2 approaches 0.

22. Unity Gain Buffer
(If A0 is 1000, Vout/Vin=0.999)