multistage amplifiers

Question

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

Answer 1

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

then high (voltage) gain requires a high value of R_C, 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.

Answer 2

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.