Download The 741 circuit consists of three stages: The input stage It consist of

*the 741 op-amp circuit.
The 741 circuit consists of three stages:
The input stage
It consist of transistors Q1 to Q7, with biasing performed by Q8,Q9,Q10. Transistors Q5,Q6, Q7
and resistors R1,R2,R3 form the load circuit of the input stage and on the other hand this is a
current mirror circuit as well. The output of the input stage is taken single-endedly at the collector
of Q6.
In the 741, the level shifting is done in the first stage using the lateral pnp transistors Q3,Q4. Their
usage in the first stage results in added advantage: protection of the input stage transistors Q1, Q2
against emitter-base junction break down.
*level shifting: to shift the dc level of the signal so that the signal at the op-amp output can swing positive
and negative.
The second stage
The intermediate stage is composed of Q16,Q17,Q13 and the two resistors R8,R9. Transistor Q16
acts as emitter follower. Transistor Q17 acts as CE amplifier.
The output of the second stage is taken at the collector of Q17.
The output stage
The purpose of the output stage is to provide the amplifier with a low output resistance. In
addition, the output stage should be able to supply relatively large load currents. the 741 uses an
effective output circuit known as class AB output stage.
In class-AB operation, each device operates the same way as in class B over half the waveform, but also
conducts a small amount on the other half. As a result, the region where both devices simultaneously are
nearly off is reduced. The result is that when the waveforms from the two devices are combined, the
crossover is greatly minimized or eliminated altogether.
Class AB sacrifices some efficiency over class B in favor of linearity; it is typically much more efficient than
class A.
*class B: Class-B amplifiers only amplify half of the input wave cycle, thus creating a large amount of
distortion.
This stage is driven by the second stage transistor Q17 and loaded by R12. As said before the
circuit is of the AB class, with the network composed of Q18,Q19 and R10 providing the bias of the
output transistors Q14 and Q20.
Another feature at the output stage which could be good to know is that the stage is driven by an
emitter follower Q23.
*Biasing is the method of establishing predetermined voltages or currents at various points of an electronic
circuit to set an appropriate operating point.