Design of Multistage Operational Amplifier

DESIGN OF MULTISTAGE OPERATIONAL AMPLIFIER
SNO CONTENT PG_NO
1 Multistage Amplifier 2
2 Configuration 2
3 Coupling types 6
4 Design Specifications 8
5 Frequency response of the Amplifiers 13
6 Result obtained 14
References 14

1. Multistage Amplifier:
The purpose of the Multistage Amplifier is due to the practical insufficiency of the single stage amplifier
although it could be a power or voltage amplifier. In the process of multistage amplifier, ac coupling
device couples the output of the first stage to the input of the succeeding stage. This coupling device
could be anything such as the transformer or the capacitor. This process of cascading several coupling
devices is known as Cascading. The one generic structure that is sufficiently used for the broad class of
application is by the operational Amplifier (op-amp) according to Neamen (156). The op-amp is capable
of performing several operation based on the external component configuration such as the addition,
subtraction, integration, differentiation and inverter performance. The absolute value could be
performed based on the non-linear components in the feedback network by estimating the exponent or
logarithmic value of input current or voltage and perform hysteresis or limiting functions. These
complicated operations seem to be very easy with the op-amp functional building block.
There are three fundamental stages of the amplifier are considered that are as follows (Sedra and Smith,
56):
 Differential amplifier input stage: This could amplify the difference among the two input signal
rather than considering single input signal.
 High voltage gain state: This is said to be the Common-Emitter amplifier.
 Low output impedance stage: This has the high current gain value and the voltage gain is less
than unity which is obviously a common-collector configuration (Salivahanan, 28).
In single stage amplifier, there is a limit in achieving the gain of the amplifier. The input and the out
impedance could also be limited in the single stage amplifier. A better control of input and output
impedance with the better gain performance could be made by the multistage amplifiers. The main
advantage of multistage amplifier is that they have a flexible input and output impedance and also
promotes larger gain value (Siddique et al. 67). Open loop and negative feedback are the two classes of
the multistage amplifier. The design process of the open loop is considered to be tedious and they are
very sensitive to environment s that the component variations could affect the accuracy. This drawback
is eradicated in the negative feedback and promotes better efficiency but they are very difficult to
understand. The Operation Amplifier (OP-AMP) is said to be the fundamental building block of the
negative feedback amplifier that has high voltage gain multistage open-loop amplifier (Samadi et al. 8)
PNP and NPN transistors are used in the design of the Multistage amplifier. In certain circuit, the
combination of both the circuit could highly reduce the impact of sensitivity caused due to the
temperature by cancelling the voltage drop of the base-emitter terminal. The voltage drop could be
caused due to the temperature drift that seems to be cancelled from the net result (both should be
subtracted). The optimum bias condition could be achieved by involving the PNP transistor followed by
the NPN transistor.
2. Configuration:
Common-emitter – common-emitter:

If the sufficient voltage gain needs to be very high then the CE – CE configuration could be adapted.
Figure 1: The multistage amplifier that involves the common CE – CE configuration
Common-emitter – common-collector
The output impedance could be sufficiently decreased in the case of CE – CC configuration
Figure 2: The multistage amplifier that involves the common CE – CE configuration

Common-collector – common-emitter:
This configuration could sufficiently increase the input impedance due to the CC configuration circuit. An
important thing to be noted is that the right side of the figure could consist of the PNP transistor
followed by the NPN transistor (Goswami, et al. 5). In this case the temperature effect could be
significantly cancelled that gives a better performance of analysis (MacLean, 15).
(A) (B)
Figure 3: The multistage amplifiers that involves the common CC – CE configuration (A) NPN
followed by a NPN transistor (B) PNP followed by NPN transistor
Common-collector – common-collector:
In this configuration, the current gain could be very gain but the voltage gain could be less that unity due
to the Common Collector configuration (Ciofi, 40). As motioned in the previous case, there could be
better temperature insensitivity, which could enhance the performance. Moreover, the output
impedance could be very low followed by higher input impedance.