Transcript Input Stage : The 741 op
The 741 Operational-Amplifier
1
Reference Bias Current : The 741 op-amp circuit. Reference Bias Current
DC Analysis of the 741 Reference Bias Current
V CC -V EE
Bias for input stage The 741 op-amp circuit. Bias for input stage I C10
Input Stage Bias
I C
10
I S
10
e V BE
1 0
V T
V BE
10
V T
ln
I C
10
I S
10
I C
11
I S
11
e V BE
1 1
V T
V BE
11
V T
ln
I REF I S
11
V BE
11
V BE
10
V T
ln
I REF I S
11
V T
ln
I C
10
I S
10
V T
ln
I REF I C
10 I C10 can be determined by knowing I REF and R 4 + V BE11 + V BE10 + + I C10
Biasing Input Stage : The 741 op-amp circuit. I C1 I C3 I C4 I C2
The dc analysis of the 741 input stage.
npn β very high Relationship I REF & I O Base current I E /1+β P Base currents add together
As
P
1
I C
10 2
I
A Simple BJT Current Source
8
Negative Feed-back Loop : 741 input stage.
Causes current pulled from Q8 to increase For some reason I in Q 1 & Q 2 increases Output current of Q8 Q9 correspondingly increase Negative Feed back Loop Since I c10 remains constant, it forces combined current of Q3 & Q4 to decrease
Input Stage : The 741 op-amp circuit.
I C7
The dc analysis of the 741 input stage, continued.
I c5 =I 6 5 Bias Current of Q 7 1 3 2 7 4
The dc analysis of the 741 input stage, continued.
50 KΩ
The 741 op-amp circuit : SECOND STAGE I C16 I C17
DC Analysis : Second Stage
Q 13 is lateral pnp Q 13B transistor has a scale of 0.75 times that of Q 12 I REF = 0.73 mA & β P >>1 I C13B = 550 µA & I C17 = 550 µA I C13B =0.75 I REF Neglect Base Current of Q 23 I C17 =I C13B
Output Stage Bias :
The 741 op-amp circuit.
The 741 output stage without the short-circuit protection devices.
Q 13 is lateral pnp I S of Q 13A transistor is 0.25 times I S of Q 12 Neglect Base current of Q 14 & Q 20 Base Current of Q 23 is 180/50=36 μ A Negligible as assumed
Output Stage Bias
Voltage V BE18 ≈ 0.6V Current Thru R 10 =0.6/40k=15 µ A + V BB 14 20
Summary Collector Currents : 741 Op Amp
Small-signal analysis of the 741 input stage.
Collectors Q 1 & Q 2 Q 3 & Q 3 connected to dc voltage so are grounded are biased by constant current source so are open cct
Small-signal analysis of the 741 input stage.
Input appears across four input resistors
R id
v i i b
v i i e
( 1 ) 4 ( 1 )
r e
The 741 op-amp circuit.
Small signal model : The load circuit of the input stage.
The load circuit of the input stage
Q 5 & Q 6 are identical and their bases are tied together So their collector currents are equal
The load circuit of the input stage
Output Resistance : 741 Op Amp
R o1 is parallel equivalent of R o4 & R o6 Assume that common bases of Q 3 are at virtual ground & Q 4
Common R o
4
r o
1
Base Circuit g m
R E
||
r
......
equation
6 .
118
R E R o
4
r e
,
r o
V A I
10 .
5
M
, Output Resistance R o1
Output Resistance : 741 Op Amp
Assume that the base of Q 6 is at virtual ground Because signal is very small
Common R o
6
r o
1
Base Circuit g m
R E
||
r
......
equation
6 .
118
R E R o
6
R
2 ,
r o
V A I
18 .
2
M
, Output Resistance R o1
Output Resistance : 741 Op Amp
R o1 is parallel equivalent of R o4 & R o6 R o1 =R o4 ||R o6 R o1 =6.7 MΩ Output Resistance R o1
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 Q 13B is zero
Output Resistance R 02 : Second Stage
R 02
Output Resistance R 02 : Second Stage
Since the resistance between the base of Q17 and ground is relatively small, The base is grounded and circuit is CB R 02
Output Resistance R 02 : Second Stage
R 02
Output Resistance R 017
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 Q 17 • Loaded with 2 kΩ resistor • Q 18 & Q 19 and R 10 provide Class AB bias to output stage.
• Q 14 & Q 20 transistors are output • Output stage is driven by emitter follower Q 23 acts as buffer
Output Voltage Limits
Maximum positive output voltage v omax is limited by input circuit Saturation of Q 13 A
v o
max
V CC
V EC
13
A
V BE
14 Minimum negative output voltage v omin is limited by input circuit Saturation of Q 17
Small Signal Model for the 741 output stage.
v o2 =-G m2 R 02 v i2 G m2 = 6.5mA/V & R O2 = 81kΩ R in3 is input resistance of the output stage with load R L
R in3 R in
Input resistance R
in3
of output stage
R in
3
v b
23
i b
23 23 1
r e
23
R in
23
R in
R out18
Input resistance R
in
R in =R in20 ||R out18
Suppose Q 20 Q 14 is cut-off is conducting and
R in R in20
Input resistance R
in20
R in20
R in
20
v b
20
i b
20 20 1
r e
20
R L
20
R L
R out18
Input resistance R
out18 R out18 & Q 19 is r o13A in series with output resistance of Q 18
Output resistance of Q 18 = 163 Ω & Q 19 r o13A >>Output resistance of Q 18 & Q 19
R out
18
r
013
A
R in3
Input resistance R
in3
of the output stage
R in R out18 R in20
R in
3 23
R in
R in =R in20 ||R out18
R in
20 20
R L R out
18
r
013
A
β 20 = β 23 = 50 , R L = 2kΩ, r o13A = 280kΩ R in3 = 3.7 MΩ
Small Signal Model for the 741 output stage.
R in3 = 3.7 MΩ R o2 = 81 kΩ R in3 >> R o2 So R in3 will have little effect On the performance of the op amp
= -515 V/V
Open Circuit Overall Voltage Gain G vo Small Signal Model for the 741 output stage.
G vo
3
v o v o
2
R L
Open Circuit Overall Voltage Gain G vo
G vo
3
v o v o
2
R L
Q 14 , Q 20 & Q 23 are common collector circuits, So gain is unity
G vo
3 1
Circuit for finding the output resistance R out .
Exact Value of R out Suppose Q 20 will depend upon which transistor (Q Is conducting and Q 14 is cut-off.
14 or Q 20 ) is conducting Input source feeding the output stage is grounded
Circuit for finding the output resistance R out .
1
R out
v e
23
i e
23 2
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 Protection Against maximum current the op amp can source • In normal case – Current thru the emitter of Q 14 is 20mA, voltage drop across R 6 and Q 15 is approx 540mV is off • In the event of short circuit, – if current in the emitter of Q 14 exceeds 20mA , voltage drop across R 6 will increase above 540mV and Q 15 will conduct.
• Robs some of the current supplied by Q Q 14 .
13A , thus reducing the base current of • This limits the current that the op amp supplies from the output terminal in the outward direction to 20mA.
Output Short-Circuit Protection Against maximum current the op amp can source • In normal case – Current thru the emitter of Q 20 is 20mA, voltage drop across R 7 and Q 21 is approx 540mV is off • In the event of short circuit, – if current in the emitter of Q 20 exceeds 20mA , voltage drop across R 7 will increase above 540mV and Q 21 will conduct.
• Robs some of the current supplied by Q 24 , thus reducing the base current of Q 20 .
• 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
Frequency Response
C c
Frequency Response
C c C c
introduces a dominant low-frequency pole Using Miller’s theorem, the effective capacitance due to Cc between the base of Q 16 and ground is The total resistance between base of Q 16 and ground is
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
G m
1 2
r e r e
V T I
Slew rate
t
I
2
C C V T
SR
4
V T SR
4
V T
t