The black box model

The black box representation of our amplifier is shown below

(In point of fact the box could represent any system having a single input and

single output.) Note the convention assumed for the positive direction of

currents and voltages. In particular, observe the directions I 1 and I 2. It is the

accepted convention to have both currents flowing into the box. Note also that

the subscript notation '1' for input and '2' for output is used.

We have said that in the common emitter amplifier, the base is input, the

collector the output and the emitter is common to both input and output. The

transistor can therefore be pictured connected within the box as shown in the figure below

Thus, the transistor can be represented by a two port (input port and output

port), four terminal (a,b,c,d) box by extending the emitter from input to output.

Note also, in the black box approach, only signals (i.e. input and output

voltages and currents) are shown. No attempt is made to show power supplies

and the like, which are taken for granted.