TABLE OF CONTENTS The stability of a transistor or amplifier circuit is a measure of the likelihood of oscillation when the circuit is energized. However, the conditions that cause oscillation are more stringent than the conditions that make a circuit unstable. Evidence of instability, or possible instability, can be seen by analyzing the reflection coefficient of a network. Consider a situation in which we have a transistor with the output-terminated with some circuit or load. We will look at the reflection coefficient at the input of the transistor either by calculation or measurement. If the magnitude of the reflection coefficient is greater than 1, more power will be reflected from the circuit than is incident upon it. This phenomenon is called reflection gain. With this combination of transistor and load, the circuit could begin to oscillate. By way of the parameter K, we can show how likely it is that the reflection coefficient will be greater than 1, using all possible ways of terminating the transistor.
First we will study the conditions that might cause the input reflection coefficient of a transistor to be greater than 1. Armed with the mapping functions from Chapter 3, we can easily find all the possible output circuits that might cause the magnitude of the input reflection coefficient to be greater than 1. The circuit in Figure 6-2 shows a simple circuit diagram with the input and output reflection coefficient of a transistor two-port. It might seem backwards to map the ?
