The document discusses the internal components and operation of the 741 operational amplifier integrated circuit. It describes the bias circuitry that generates reference currents for the entire circuit. It then explains the input, output, and compensation stages. The input stage contains transistors that create complementary current signals. The output stage and protection circuitry limit current to prevent overheating. The 741 is compensated internally using a resistor-capacitor network to maintain stability at high frequencies and gains. The document also covers topics like frequency response, slew rate, and the gain-bandwidth relationship of the 741 op-amp.

1. 741 Op-Amp IC
By :
Mr. Himanshu Diwakar
Assistant Professor
JETGI
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2. Introduction
• With respect to operational amplifiers, a typical configuration is
shown in block diagram form
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3. Bias Circuitry
• The reference bias current for the entire 741 circuit is generated in the
branch containing the diode-connected transistors Q11 and Q12 and the
resistor R5.
• The bias current for the first stage is generated in the collector of Q10 from
the Widlar current source composed of Q10, Q11 and R4.
• The current mirror formed by transistors Q8 and Q9 provide the bias current
(IEE) of the differential amplifier formed by Q1, Q2, Q3 and Q4.
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5. The 741 Op-Amp
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6. Cont’d
• The 741 is internally compensated by means of an on-chip resistor-
capacitor (RC) network.
• We’re going to talk about frequency response, compensation and
stability in future sections, but for now we’re just going to define the
internal compensation of the 741 as the means by which the op-amp
maintains stability while still having a high gain and large operational
frequency range (i.e., bandwidth).
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7. Short Circuit Protection Circuitry
• The 741 circuit contains several transistors that are normally off and
that turn on (conduct) only when a large current exists at the output. In
the circuit above, the short circuit protection is provided by Q15, Q21,
Q22, Q24, R6 and R7.
• The function of this network is to limit the current in the output
transistors – all to prevent overheating and possible burnout of the IC
(i.e., it keeps from letting the smoke out).
• Refer to the figure above for the following analysis of the short circuit
protection circuitry.
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8. Cont’d
• The transistor Q15 is the normally off state. If the current in the
emitter leg of Q14 becomes too large, the voltage drop across R6 will
become large enough to turn Q15 on.
• Once Q15 is turned on, its collector will bleed off some of the current
supplied by Q13, thereby reducing the base current to Q14 and the
emitter current of Q14.
• This limits the maximum current that the op-amp can source, or supply
from the output terminal in the outward direction.
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9. Cont’d
• The current through Q20 is limited in a similar fashion, where the
relevant components are Q21, Q22, Q24, and R7.
• If the current through Q20 becomes too large, the voltage drop across
R7 turns Q21 on. Once Q21 is turned on, its collector will bleed off
some of the current supplied by the current mirror formed by Q22 and
Q24, reducing the base current (and therefore the emitter current) of
Q20.
• This mechanism limits the maximum current that the op-amp can sink,
or draw from the output terminal in the inward direction.
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10. Input stage
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12. The load circuit of the input stage fed by the two complementary current signals generated by Q1 through
Q4 in Fig. shown on previous slide Circled numbers indicate the order of the analysis steps.
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14. Figure Simplified circuits for finding the two components of the output resistance
Ro1 of the first stage.
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15. • Output resistance (R01) of the input stage is the resistance
seen looking back onto the collector of transistor Q6 .
• From the figure shown on slide 64, we can say that, R01 is
equal to the parallel combination of the output resistance of
the current source ie and the output resistance of Q6 .
• Assume that the base of Q4 is virtual ground.
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16. Second stage
Figure The 741 second stage prepared for small-signal analysis
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17. • This is used to determine the values of the parameters of the
equivalent circuit.
• Input Resistance: The resistance Ri2 is given by
• Trans conductance: from the small-signal equivalent circuit model
of the second stage, we can observe that,
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19. OUTPUT STAGE
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20. FREQUENCY RESPONSE
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21. Cont’d
• Frequency response of the op-amp is the plot of its open loop gain versus
frequency.
• The open loop gain changes with frequency.
• To plot the frequency response we need to refer high frequency model of
OP-AMP.
• After a certain frequency the rolloff decreases after certain frequency.
• The capacitor is due to BJT used in the 741.
• The BJTs has parasitic capacitances so the capacitances is too small.
• So in order to reduces the effect of this parasitic capacitances the
compensated capacitor is used in 741.
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23. SLEW RATE
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24. • Consider the unity-gain follower shown on previous slide.
• 10 volt step is applied at the input.
• The entire value of the step signal will appear as a differential
signal between the two input terminals.
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25. Cont’d
• From the circuit shown on previous slide, wee see that, output
voltage ramp with a slope of 2I/CC .
• So the slew rate is given by:
For 741 SR = 0.63 V/µs.
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26. ALL THE BEST FOR EXAMS
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