Multi-stage Transistor Amplifiers !
As you experienced in the previous experiments, there is a limit to the gain that can be achieved from a single transistor. Single-stage amplifiers have a limit on their input and on their output impedances. Multi-stage amplifiers are used to achieve higher gains and to provide for a better control of input and output impedances. Multiple amplifiers can be connected in a cascaded arrangement with the output of one amplifier driving the input of the next. Each amplifier in the cascaded arrangement is known as a stage.
The overall gain of cascaded amplifiers is the product of individual gains:
Amplifier voltage gain is often expressed in decibels (dB) as follows, which is particularly useful in multi-stage systems because the overall dB voltage gain is then the sum of the individual voltage gains:
For many applications, it is desirable for the input impedance to be very high. In this case, it is common for the first amplifying stage to be a common-collector (emitter-follower) bipolar junction transistor stage or a common-drain (source-follower) or even common-source FET stage. If high input impedance is not important, the first stage might be a common-emitter circuit. FETs are normally used for the input stage and for the specific application when very high input impedance is needed. It is not unusual that the output impedance of an amplifier should be low. A common-collector circuit is then used. If very low output impedance is not needed, the last stage might be a common-emitter circuit. For the amplifying stages in between, it is common to employ common-emitter circuits because they can achieve high voltage gain. The following table offers a summary of some multi-stage amplifier constructions and their characteristics.
Stage | Characteristics | |||||
| 1 | 2 | 3 | 4 | Rin | Rout | Voltage gain |
| CE | CE | Medium | Medium | High | ||
| CE | CC | Medium | Low | Medium | ||
| CC | CE | High | Medium | Medium | ||
| CC | CC | Very high | Very Low | <1 | ||
| CE | CE | CE | Medium | Medium | Extremely high | |
| CE | CE | CC | Medium | Low | Very high | |
| CE | CC | CE | Medium | Medium | Very high | |
| CE | CC | CC | Medium | Very Low | Medium | |
| CC | CE | CE | High | Medium | Very high | |
| CC | CE | CC | High | Low | Medium | |
| CC | CC | CE | Very high | Medium | Medium | |
| CC | CC | CC | Very high | Very low | <1 | |
| CC | CE | CE | CC | High | Low | Very high |
Descriptor | R | Voltage gain |
| Low | Less than a few hundred ohms | |
| Medium | A few hundred to a few thousand ohms | Less than 50 |
| High | A few thousand to a few ten thousand ohms | 50 to 500 |
| Very high | Many tens of thousands of ohms | 500 to 5000 |
| Extremely high | Over one hundred thousand ohms | Over 5,000 |
General characteristics of typical multi-stage amplifier structures
A few considerations for multi-stage analysis:
In determining the gain of the first stage, the loading effect of the second stage on the first one must be considered. The total input resistance of the second stage presents a load to the first stage. The voltage gain of the first stage is reduced by the loading on the second stage if the effective AC collector resistance of the first stage is less than the actual value of the collector resistor of the first stage, for example.