The mid-frequency range

Over this range the coupling capacitors are assumed to be behaving themselves

and may be regarded as having negligible impedance (compared to the

resistances in the circuit). They can, therefore, be ignored in any calculation.

Conversely, the box's internal capacitances, which are low in value, will have a

very high impedance. However, they appear in parallel across the load and

thus can also be neglected from any calculations.

Over the mid-frequency range, then, the behavior of the circuit is assumed to

be due entirely to resistive components. A suitable equivalent circuit is shown

below.

The current gain is given by

The only thing novel is the subscript 'MF' used in the symbol for current gain. Shortly, we will be expressing G I as a function of frequency and we need some means of distinguishing between the three frequency ranges over which the amplifier can operate. We will use the subscripts 'MF' for mid-frequency, 'LF' for low frequency and 'HF' for high frequency ranges.

The voltage gain is given by

this being simply a factor times the current gain value.