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The 741 Operational-Amplifier. 1. Reference Bias Current : The 741 op-amp circuit. . Reference Bias Current. V CC. -V EE. DC Analysis of the 741 Reference Bias Current. Bias for input stage The 741 op-amp circuit. . Bias for input stage. I C10. Input Stage Bias. +. +. V BE11. V BE10. - PowerPoint PPT Presentation
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1
The 741 Operational-Amplifier
Reference Bias Current : The 741 op-amp circuit.
Reference Bias Current
DC Analysis of the 741Reference Bias Current
VCC
-VEE
Bias for input stage The 741 op-amp circuit.
Bias for input stage
IC10
Input Stage Bias
IC10 can be determined by knowing IREF and R4
+
-
+
-
VBE11 VBE10
+
-
+
IC10
1010
10
111011
11111111
10
10101010
lnlnln
ln
ln
11
10
C
REFT
S
CT
S
REFTBEBE
S
REFTBE
VV
SC
S
CTBE
VV
SC
IIV
IIV
IIVVV
IIVVeII
IIVVeII
TBE
TBE
Biasing Input Stage : The 741 op-amp circuit.
IC3 IC4
IC1 IC2
The dc analysis of the 741 input stage.
npn β very high
Base current IE/1+βP
Relationship IREF & IO
Base currents add together
IIAs CP 2 1 10
8
A Simple BJT Current Source
Negative Feed-back Loop : 741 input stage.
Negative Feed back Loop
For some reason I in Q1 & Q2 increasesCauses current pulled from Q8 to increase
Output current of Q8 Q9 correspondingly increase
Since Ic10 remains constant, it forcescombined current of Q3 & Q4 to decrease
Input Stage : The 741 op-amp circuit.
IC7
The dc analysis of the 741 input stage, continued.
Ic5=I
Bias Current of Q7
25
1
3
6
7 4
The dc analysis of the 741 input stage, continued.
50 KΩ
The 741 op-amp circuit : SECOND STAGE
IC16 IC17
DC Analysis : Second Stage
Neglect Base Current of Q23
IC17=IC13B
Q13 is lateral pnp transistorQ13B has a scale of 0.75 times that of Q12
IC13B=0.75 IREFIREF = 0.73 mA & βP>>1IC13B = 550 µA & IC17 = 550 µA
Output Stage Bias : The 741 op-amp circuit.
The 741 output stage without the short-circuit protection devices.
Q13 is lateral pnp transistorIS of Q13A is 0.25 times IS of Q12
Neglect Base current of Q14 & Q20
Base Current of Q23 is 180/50=36 μ ANegligible as assumed
Output Stage Bias
Voltage VBE18 ≈ 0.6V Current Thru R10=0.6/40k=15 µ A
VBB
+
-
14 20
Summary Collector Currents : 741 Op Amp
Small-signal analysis of the 741 input stage.
Collectors Q1 & Q2 connected to dc voltage so are groundedQ3 & Q3 are biased by constant current source so are open cct
Small-signal analysis of the 741 input stage.
Input appears across four input resistors
ee
i
b
iid r
iv
ivR )1(4)1(
The 741 op-amp circuit.
Small signal model :
The load circuit of the input stage.
The load circuit of the input stage
Q5 & Q6 are identical and their bases are tied togetherSo their collector currents are equal
The load circuit of the input stage
Output Resistance : 741 Op AmpRo1 is parallel equivalent of Ro4 & Ro6
Output Resistance Ro1
Assume that common bases of Q3 & Q4
are at virtual ground
MRI
VrrR
equationrRgrRCircuitBaseCommon
o
AoeE
Emoo
5.10
,,
118.6 ...... ||1
4
4
Output Resistance : 741 Op Amp
Output Resistance Ro1
MRI
VrRR
equationrRgrRCircuitBaseCommon
o
AoE
Emoo
2.18
,,
118.6 ...... ||1
6
2
6
Assume that the base of Q6 is at virtual groundBecause signal is very small
Output Resistance : 741 Op Amp
Ro1 is parallel equivalent of Ro4 & Ro6
Ro1=Ro4||Ro6
Ro1=6.7 MΩ
Output Resistance Ro1
Figure 9.22 Small-signal equivalent circuit for the input stage of the 741 op amp.
Second Stage :The 741 op-amp circuit.
Figure 9.24 The 741 second stage prepared for small-signal analysis.
Input Resistance : Second Stage
Transconductance : Second Stage
Thus current through the output resistance of Q13B is zero
Output Resistance R02 : Second Stage
R02
Output Resistance R02 : Second Stage
R02
Since the resistance between the base of Q17 and ground is relatively small, The base is grounded and circuit is CB
Output Resistance R02 : Second Stage
R02
Output Resistance R017
Since the resistance between the base of Q17 and ground is relatively small, The base is grounded and circuit is CB
Figure 9.25 Small-signal equivalent circuit model of the second stage.
Figure 9.27 Thévenin form of the small-signal model of the second stage.
Open Circuit Voltage Gain =
Output Stage :The 741 op-amp circuit.
The 741 output stage.
The 741 output stage.
• Input from second stage Q17
• Loaded with 2 kΩ resistor
• Q18 & Q19 and R10 provide Class AB bias to output stage.
• Q14 & Q20 are output transistors
• Output stage is driven by emitter follower Q23 acts as buffer
Output Voltage LimitsMaximum positive output voltage vomax is limited by input circuitSaturation of Q13A
Minimum negative output voltage vomin is limited by input circuit Saturation of Q17
1413max BEAECCCo VVVv
Small Signal Model for the 741 output stage.
vo2=-Gm2R02vi2 Gm2 = 6.5mA/V & RO2 = 81kΩ
Rin3 is input resistance of the output stage with load RL
Input resistance Rin3 of output stage
Rin
Rin3
inineb
bin RRr
ivR 232323
23
233 1
Input resistance Rin
Rin20
Rin
Rout18
Rin=Rin20||Rout18
Suppose Q20 is conducting and Q14 is cut-off
Input resistance Rin20
LLeb
bin RRr
ivR 202020
20
2020 1 Rin20
Input resistance Rout18
Rout18
Aout rR 01318
Rout18 is ro13A in series with output resistance of Q18 & Q19
ro13A >>Output resistance of Q18 & Q19
Output resistance of Q18 & Q19
= 163 Ω
Input resistance Rin3 of the output stage
Rin20
Rin
Rout18
Rin=Rin20||Rout18
Rin3
inin RR 233
Aout rR 01318
Lin RR 2020
β20 = β23 = 50 , RL= 2kΩ, ro13A= 280kΩRin3 = 3.7 MΩ
Small Signal Model for the 741 output stage.
Rin3 = 3.7 MΩ Ro2 = 81 kΩ
Rin3 >> Ro2
So Rin3 will have little effect On the performance of the op
amp = -515 V/V
Open Circuit Overall Voltage Gain Gvo
Small Signal Model for the 741 output stage.
LR
o
ovo v
vG2
3
Open Circuit Overall Voltage Gain Gvo
LR
o
ovo v
vG2
3
Q14 , Q20 & Q23 are common collector circuits, So gain is unity
13 voG
Circuit for finding the output resistance Rout.
Exact Value of Rout will depend upon which transistor (Q14 or Q20) is conductingSuppose Q20 Is conducting and Q14 is cut-off.
Input source feeding the output stage is grounded
Circuit for finding the output resistance Rout.
2
1
23
23
e
eout i
vR
Output Short Circuit Protection Stage :The 741 op-amp circuit.
Output Short-Circuit Protection• If any terminal of the IC is
short circuited to one of the power supplies, IC will burnout.
• Protection Circuit limits the current in the output transistors in the event of short circuit.
Output Short-Circuit ProtectionAgainst maximum current the op amp can source
• In normal case– Current thru the emitter of
Q14 is 20mA, voltage drop across R6 is approx 540mV and Q15 is off
• In the event of short circuit,– if current in the emitter of
Q14 exceeds 20mA, voltage drop across R6 will increase above 540mV and Q15 will conduct.
• Robs some of the current supplied by Q13A, thus reducing the base current of Q14.
• This limits the current that the op amp supplies from the output terminal in the outward direction to 20mA.
Output Short-Circuit ProtectionAgainst maximum current the op amp can source
• In normal case– Current thru the emitter of
Q20 is 20mA, voltage drop across R7 is approx 540mV and Q21 is off
• In the event of short circuit,– if current in the emitter of
Q20 exceeds 20mA, voltage drop across R7 will increase above 540mV and Q21 will conduct.
• Robs some of the current supplied by Q24, thus reducing the base current of Q20.
• This limits the current that the op amp supplies from the output terminal in the inward direction to 20mA.
Small Signal Gain
Gain is found from the cascade of the equivalent circuits of the op amp
Frequency Response
Cc
Frequency Response
Frequency Response
Cc introduces a dominant low-frequency pole
Using Miller’s theorem, the effective capacitance due to Cc between the base of Q16 and ground is
The total resistance between base of Q16 and ground is
Cc
Figure 9.32 Bode plot for the 741 gain, neglecting nondominant poles.
The convenience of use of internally compensated 741 is achieved at the expense of a great reduction in open loop gain--- externally compensated op amp.
Slew Rate
Slew rate
em rG
21
IVr T
e
TTC
t VSR
VCI
42
tTVSR 4