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Why do people use multi stage amplifiers instead of just one amplifier. Can't we build a single amplifier that can instantly boost a signal by applying a higher Vcc so that the output voltage will occupy the most of the peak to peak supply Vcc.

What are the negatives / downsides of a multistage amplifiers? Also, for multistage amplifiers, to have a much higher gain, Should we use different +Vcc at each stage because if we didn't, then there will come a point where distortion happens due to clipping at either saturation or cutoff

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I recommend this book. See if your library has it. –  Brian Drummond Nov 24 '12 at 15:57
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Why are people voting to close this question? It seems legitimate to me. –  Olin Lathrop Nov 24 '12 at 19:02
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@OlinLathrop, probably because it's "overly broad" and would really require a complete textbook on amplifier design to answer completely. –  The Photon Nov 24 '12 at 19:43
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@TheP: A basic answer doesn't need to be that broad. –  Olin Lathrop Nov 24 '12 at 21:32
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Can't we build a single amplifier that can instantly boost a signal by applying a higher Vcc so that the output voltage will occupy the most of the peak to peak supply Vcc.

A well-designed amplifier should have more characteristics than just high gain. For example, either very high or input-matched input impedance, low output impedance, low distortion and low power consumption are a few characteristics that are likely to be important in most applications.

The input and output impedance requirements in particular are ones that could drive a design to use multiple stages. Even if the first stage is simply a buffer, it allows the input stage to be optimized for high input impedance while the output stage is optimized for low distortion at low output impedance, while minimizing power consumption.

Similarly, high gain and low output impedance require different optimizations. If you consider the typical common-emitter amplifier

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then high (voltage) gain requires a high value of RC, while low output impedance requires the opposite.

If use a common emitter stage for gain, you can follow it with an emitter follower (or a classic class-AB output stage) for output impedance matching and meet both requirements.

Should we use different +Vcc at each stage because if we didn't, then there will come a point where distortion happens due to clipping at either saturation or cutoff

No matter what Vcc you use, there's some point where the final stage of the amplifier starts to distort too much for your application (unless your application is a clipping circuit, maybe).

Then the only question is whether the earlier stages should be run on a lower Vcc? The only benefit I can see is maybe reduced power consumption. But this is likely to be inconsequential because the output stage normally dominates the power consumption anyway. On the other hand, using lower Vcc for earlier stages means having to have multiple regulators (either provided by the user or built in to the amplifier circuit), adding to the cost of the system. So this idea has high cost but minimal benefit, which explains why its rarely done.

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Isn't it that higher gain is to lower value of Rc because gain is from ic / in and so if you lower collector resistor, you allow more Ic and so gain increases... –  IvanMatala Nov 25 '12 at 0:44
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@vvavepacket, I've edited to clarify I was referring to voltage gain. If you wanted a current gain amplifier, you would likely either use an emitter follower (aka common-collector circuit), or omit Rc entirely, putting the load in its place, since current "gain" that isn't delivered to the load wouldn't be useful. –  The Photon Nov 25 '12 at 18:09
    
Earlier stages may have to run at lower Vcc, simply because the devices used do not handle the Vcc of the output power stage. If the power section has +/- 80V power rails, that may not work for small-power transistors used in the earlier stages, or other components like op-amp IC's. –  Kaz Nov 25 '12 at 18:24
    
@Kaz, good point. But not really in line with OP's suggestion that different power rails will (in itself) increase gain or reduce clipping. –  The Photon Nov 25 '12 at 18:27
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Why do people use multi stage amplifiers instead of just one amplifier.

The short answer is that there isn't a single stage amplifier that remotely approaches the ideal voltage amplifier.

An approximation of the ideal voltage amplifier is nearly linear for large signals and has high input impedance, low output impedance, and wide bandwidth.

There are three configurations for single stage amplifiers: common-emitter, common-collector, and common-base.

The common-collector is quite linear, has high input impedance, low input impedance and wide bandwidth. It also has less than unity voltage gain so it simply isn't useful as a voltage amplifier.

The common-base has high voltage gain and high bandwidth but very low input impedance and moderately high output impedance so it's not a good approximation either.

Finally, the common-emitter has high voltage gain, moderate input impedance, moderately high output impedance, and moderate bandwidth. Overall, it's the best choice for voltage amplification.

But, importantly, it's far from ideal. Multi-stage amplifiers can get much closer to approximating the ideal voltage amplifier.

For example, to get low output impedance, a common-collector stage can follow the common-emitter. To get high input impedance, a common-collector can precede the common-emitter.

A more sophisticated approach would be to cascade two common-emitter stages to get enormous voltage gain and then use negative feedback to get the voltage gain down to the desired level. Remarkably, the negative feedback also lowers the output impedance and increases the input impedance all the while improving the linearity for large signals and extending the bandwidth.

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