Transcript 741 Op-Amp Circuit Section 8.1 (Razavi) Chapter 12 (Sedra Smith) History • The first uA741 Op-Amp was designed by Bob Widlar in 1963 at.

741 Op-Amp Circuit
Section 8.1 (Razavi)
Chapter 12 (Sedra Smith)
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. http://en.wikipedia.org/wiki/Bob_Widlar
– LM101
History
• David Fullagar (Fairchild)
– Scrutinize LM 101
– Incorporated a 30 pF Cap.
– uA741 was introduced.
– Gordon Moore
BJT Op-Amp
BJT Op-Amp
Reference
Bias
Current
Input-Stage
Bias
Input-Stage
Second-Stage
Output-Stage
Reference Bias Current
VCC=15 V
VEE=-15 V
VBE11=0.7 V
VEB12=0.7 V
IREF=0.73 mA
Input-Stage Bias
IREF=0.73 mA
R4=5 KΩ
(IS10=IS11, assumption)
Solve through trial and error
IC10=19 uA
Input Stage (1)
From previous
Calculation:
IC10=19 uA
(from symmetry)
(β is high )
(~0)
Negative Feedback
(2)
(3)
(1)
(4)
(constant)
Input Stage (2)
Input Stage (3)
(IB1)
(Input Bias Current)
(Input Offset Current)
(IB2)
Input Resistance (1)
(AC Ground)
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
Second-Stage Bias
0.75IS12=IS13B
IC12=0.73 mA
IC13B=0.55 mA
IC17=IC13B
(0)
Class AB Output Stage (Review)
Output Stage Bias
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
(0)
(Output resistance is small)
(0)
Op-Amp
Equivalent Circuit of an Op-Amp
Characteristics:
Negligible input current
Large input AC resistance
Small AC output resistance
Large Voltage Gain
Op-Amp as an Amplifier
(non-inverting input)
(inverting input)
Op-Amp is a circuit that amplifies the difference
between the inputs.
Inverting/Non-Inverting
Configuration
Consequence of Large Gain
As Ao approaches infinity, Vin1-Vin2 approaches 0.
Unity Gain Buffer
(If A0 is 1000, Vout/Vin=0.999)