Circuits’ frequency responses

We have shown how an electronic device such as an amplifier can be represented by a two port 'black box' model. In this model, though, only the resistive components of the original amplifier circuit were considered, the effects of the capacitors in the circuit being ignored. In point of fact, it is usually valid to ignore the circuit capacitances of an amplifier over its range of operating frequencies. The value of the capacitors will have been chosen to permit such an assumption. But how do we know how far the operational range of the amplifier extends? Or, to reverse the question, how do we determine the value of the capacitors, given the range of frequencies over which the amplifier must operate? It will be shown that an amplifier's frequency response has its low, mid and high ranges. The low range can be modelled by the 'black box' loaded by a series RC circuit, the mid range by a simple resistive load and the high range by a parallel RC load. The effect on voltage, current and power gains for each of these conditions will be investigated.