amplifiermicrowavestability
Given a general microwave amplifier circuit
For this amplifier (transistor) to be stable the the magnitude of the input and output reflection coefficient must be smaller than 1.
But why? What is an intuitive explanation for why the magnitude of the input and output reflections need to be smaller than 1 for this amplifier to be stable?
As your signal propagates through the transmission line, when it hits a discontinuity, it will reflect and start heading back. If you have multiple branches in your transmission line, each time you come to one of those intersection, you'll have part of the signal travelling in each of those branches and then back (if the branches are not terminated properly).
A signal travels at a finite speed, and if you have a fast frequency signal, it may change faster than the signal can propagate through the line. So now you have your signal interfering with its past self (reflections) which will then distort your signal.
By minimizing your reflections, you minimize the amount of interaction between the reflected signal and the incident. This then decreases your ISI (inter symbol interference), which allows for more data to pass through without issues and therefore a higher bandwidth.
A 1/4 wave stub acts as a short circuit in its own right and any capacitance to ground at the "cold" end is good practice but a bonus.
I'm not familiar with the diagram conventions - but, what are the 'oblong' and square 'blobs' (symbols) at the outboard end of the stubs (by the arrows in each case).?
If the stubs are the "tracks" from power output collector & drain respectively, and if the blobs are capacitors (as they may be) then they serve the short circuiting function and Cb x 3 in the left hand diagram are primarily bias transistor emitter decoupling which also helps with any RF that gets beyond the stub.
Added:
The text associated with the second diagram on page 22 in this document explains the biasing feed arrangement. They say that the "track + square" symbol represents a 1/2 wave stub that is tapped at the 1/4 wave point.
The 1/2 wave stub appears as an open circuit to the amplifier and the 1/4 wave tapping is a zero voltage point where bias is applied. In an ideal system this point has no signal present and no additional decoupling is needed. In real world situations there will (usually) be some sort of additional capacitive decoupling in the bias feed "to keep Murphy honest".
They say -
Best Answer
Reflection coefficient is the ratio, reflected/transmitted. If your input refection coef. is >1, you could supply the amp with 1 watt and have 2 watts reflected back out to the input port. If it continues to reflect back and forth, with the magnitude increasing with each reflection, you have an oscillation.