Design of Microwave Active Devices

Appendix 9

Differential Amplifier

A9.1. Differential operation of a four-port network

The operation is assumed to be differential between input ports 1 and 3 and output ports 2 and 4. We will now use the theory of mixed modes applied to the admittance parameters; similar results are obtained with impedance and scattering parameters.

The vector voltage and current mixed mode are linear arrangements of the voltages and currents at the ports. They are obtained due to the transformation matrix:

or by developing:

So these mixed-mode vectors can be written as two vectors:

– differential mode:

– common mode:

By reporting the characteristic equation for the four-port network’s admittance, we get:

which enables us to define a mixed-mode admittance matrix and a mixed-mode current noise vector:

That is:

which can be written as:

Each submatrix is characteristic of the differential operation mode ...