S-parameters in Amplifier Design
The reverse isolation parameter determines the level of feedback from the output of an amplifier to the input and therefore influences its stability (its tendency to refrain from oscillation) together with the forward gain . An amplifier with input and output ports perfectly isolated from each other would have infinite scalar log magnitude isolation or the linear magnitude of would be zero. Such an amplifier is said to be unilateral. Most practical amplifiers though will have some finite isolation allowing the reflection coefficient 'seen' at the input to be influenced to some extent by the load connected on the output. An amplifier which is deliberately designed to have the smallest possible value of is often called a buffer amplifier.
Suppose the output port of a real (non-unilateral or bilateral) amplifier is connected to an arbitrary load with a reflection coefficient of . The actual reflection coefficient 'seen' at the input port will be given by
- .
If the amplifier is unilateral then and or, to put it another way, the output loading has no effect on the input.
A similar property exists in the opposite direction, in this case if is the reflection coefficient seen at the output port and is the reflection coefficient of the source connected to the input port.
Read more about this topic: Scattering Parameters
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