Theory of the Single Stage Amplifier Notes
The theory of the single stage vacuum tube amplifier is generalized and also extended to include phenomena of interest in vacuum tube and receiver design at high frequencies. General expressions are derived for the increase in the input admittance due to feedback, for the conditions for stability, and for the gain, as functions of the various parameters of the tube and the associated circuits. These expressions are then examined in some detail. Two specific results of especial interest are given: the case of feedback through a parallel path of susceptance and conductance, and the conditions which result when the transfer admittance consists not only of a real part but also of an imaginary part.
The network of a single-stage amplifier circuit
A single-stage amplifier circuit is shown in Figure 1.5. The amplifier consists of a transistor, the circuit for matching the source termination and the transistor, and that for matching the load termination and the transistor. The biasing circuit is ignored for brevity. The source and load terminations are usually 50 
. The function of the matching circuits is to provide suitable source and load conditions for the transistor such that the transistor can generate power gain for the input signal under a stable condition.
Figure 15: Small-signal high-frequency amplifier
Note that 
is the input reflection coefficient of the amplifier when output port is connected to the load with a reflection coefficient of 
Similarly, 
is the output reflection coefficient of the amplifier when input port is connected to the source with a reflection coefficient of 
The amplifier can be simplified to the network shown in Figure 1.6. Note that the network can be represented by the flow graph in Figure 1.3 if S-parameter is used to model the transistor.
