Feedback and Amplifier Type
Amplifiers use current or voltage as input and output, so four types of amplifier are possible. See classification of amplifiers. Any of these four choices may be the open-loop amplifier used to construct the feedback amplifier. The objective for the feedback amplifier also may be any one of the four types of amplifier, not necessarily the same type as the open-loop amplifier. For example, an op amp (voltage amplifier) can be arranged to make a current amplifier instead. The conversion from one type to another is implemented using different feedback connections, usually referred to as series or shunt (parallel) connections. See the table below.
| Feedback amplifier type | Input connection | Output connection | Ideal feedback | Two-port feedback |
|---|---|---|---|---|
| Current | Shunt | Series | CCCS | g-parameter |
| Transresistance | Shunt | Shunt | CCVS | y-parameter |
| Transconductance | Series | Series | VCCS | z-parameter |
| Voltage | Series | Shunt | VCVS | h-parameter |
The feedback can be implemented using a two-port network. There are four types of two-port network, and the selection depends upon the type of feedback. For example, for a current feedback amplifier, current at the output is sampled and combined with current at the input. Therefore, the feedback ideally is performed using an (output) current-controlled current source (CCCS), and its imperfect realization using a two-port network also must incorporate a CCCS, that is, the appropriate choice for feedback network is a g-parameter two-port.
Read more about this topic: Negative Feedback Amplifier
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