the 741 operational-amplifier

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


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


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


R02

Since the resistance between the base of Q17 and ground is relatively small, The base is grounded and circuit is CB


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


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


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

the 741 operational-amplifier

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