Why the input impedance must be smaller and the output impedance must be large?

Question

plz help me ......

thank u......

Answer 1

Normally its the other way. Because the higher input impedance the less current it draws from the source(input) and you dont want to draw a lot current from the input source and lower output impedance minimizes the loss of output volatge in case of an amp

Answer 2

It is usually the other way around.... the input impedance of an amplifier is usually very high, and the output impedance is usually very low.

If the input impedance is very high, then it will not affect the input source voltage.  Think of a voltage divider with the input source modelled as a pure voltage and a series (Thevenin) resistance and in series with the amplifier input impedance.  The higher the amplifier input impedance the higher the percentage of voltage from the source will be at the input of the amplifier.  If the input impedance of the amplifier were infinite, then *all* of the voltage produced by the source would be impressed on the input of the amplifier.  In reality an infinite input impedance is impossible, so it is made as high as possible.

The output impedance should be as low as possible so the output will "look" as close to a pure voltage source as possible.

.

Answer 3

The input or output impedences are required to be of desired value of high or low for the following easy reasons.

As per maximum power transfer therem,  maximum power is transferred to the load by a source if the internal resistance of the source circuit is equal to the load resistance of the external circuit.

This implies, if the internal resistance of the source is less the load consumes more power(i^2 R), though maximum power may not be transferred.  For this reason amplifier is designed with low output impedences.

Similarly the high internal resistance of the amplier input allows very low power consumption from the source (which is generally a very low signal generating transducer).

At times there might be a requirement where we may have to sample a signal from a low impedence source and transfer the same to a low impedence device as input then there can be a matching device like a matching transformer etc. as positioned in the question.

Further explanation can be provided only after getting further detail of the context in which the question was asked.