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  Multistage Amplifiers, Physics tutorial

Classification of multistage amplifiers:

Multistage amplifiers are categorized as open-loop, and negative feedback amplifiers. Open-loop amplifiers will be more straight forward and simpler to understand and to design; though they have drawback of being sensitive to environment and component variations. Negative feedback amplifiers conversely are harder to comprehend and more challenging to design. Closed loop amplifiers are just open loop amplifiers with the feedback loop and design of closed loop amplifier always starts with the open loop amplifier.

For several transistor amplifier applications it is desirable for input impedance to be very high. Therefore, it is common for first amplifier stage to be either common-collector (emitter follower) bipolar junction transistor stage or common-drain (source follower) or even common source field effect transistor stage. At times high input impedance is not significant and first stage may be a common-emitter. Field effect transistors are usually utilized only for input stage and for specific application of very high input impedance.

For amplifier stages in-between it is common to use common-emitter circuits as they attain high voltage gain. Multistage amplifiers are analyzed a stage at a time starting with input stage and progressing to output stage. Analysis methods are identical to that of single stage amplifiers. One area of common contention, and which you must take serious note of in estimating direct coupled amplifiers is collector resistor of the preceding stage is base resistor for next stage to it.

When design multistage amplifiers, begin with output stage and progresses towards input stage and originally the number of stages might not be known. Add stages incrementally until desired needs are fulfilled and it might entail the number of iteration in design which number of stages need might vary with every iteration.

Multistage amplifiers allow you to get greater input impedance and lower output impedance compared to single stage amplifier. There are also advantages of higher gain and enhanced power handling capacity, mainly when implemented as integrated circuits that include large numbers of optimally matched transistors.

Open loop multistage amplifiers:

As performance obtainable from the single stage amplifier is generally insufficient, numerous stages may be combined to form the multistage amplifier in which stages are joined in cascade. This signifies that output of first stage is joined to input of second stage, whose output becomes the input for third stage. This is replicated until you get to last stage; that very frequently is the power stage.

The open loop gain of the amplifier is its gain when no feedback is utilized in its circuit and value of the open loop gain is generally very high indeed for such circuits as operational amplifiers as the ideal operational amplifier is supposed to have infinite open loop gain.

Overall gain of the multistage amplifier is product of individual gains of cascaded stages. This can be stated as follows where An is gain related with stage n :

Gain (A) = A1 A2 A3 A4 ... An.

Extremely high gain figures are feasible with open loop amplifiers, though the major drawback of open loop amplifiers is that overall open-loop gain falls very quickly with increasing frequency that grossly limits bandwidth.

Closed loop multistage amplifiers:

Increased stability in amplification decreases distortion, increased bandwidth and more precise determination of input and output impedances are may be the most thoughtful advantages derived from closed loop multistage amplifiers. Such benefits are though achieved at expense of overall gain that is less than that of open loop multistage amplifier.

Stability, nonlinear distortion, bandwidth needs and impedance matching are very significant in amplifiers and form part of the long list of problems in telecommunications. They are handled through application of negative feedback.

Negative feedback can be applied to the open loop amplifier through one of the four basic methods explained as follows:

Part of the output current is applied as voltage to input in series with source signal. This is expressed as series-series negative feedback.

Negative feedback being applied to input as the current in parallel to input signal current. This method is known as parallel-parallel negative feedback and results from the sampling of output voltage to derive feedback signal.

Third method is parallel-series negative feedback and involves that the portion of output current be fed through input as the current.

When output voltage is utilized to derive negative feedback voltage at input of amplifier, then amplifier is said to operate in series parallel feedback mode.

Advantages of multistage amplifiers:

Benefits of multistage amplifiers over single stage amplifiers are given below:

  • Compared to single stage amplifier, multistage amplifiers give increased input resistance, decreased output resistance, increased gain and increased power handling capability
  • Multistage amplifiers usually implemented on integrated circuits where large numbers of transistors with common (matched) parameters are obtainable
  • Typical inverter (Common Emitter) has fairly large gain and has input and output resistances in Kilohm range
  • Follower configuration has much higher input resistance, lower output resistance but has only unity gain
  • Amplifier needs desirable features of both configurations
  • There is increased stability in amplification through application of negative feedback.
  • Overall gain is less dependent on parameters of amplifier elements when negative feedback is applied
  • Ability to apply negative feedback really decreases waveform signal distortion

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