The amplifiers have the ability to increase the magnitude of an input signal. It is useful to be able to rate an amplifier’s ability in terms of output / input ratio. The technical terms of an amplifier’s output /input magnitude ration is gain. As a ratio of equal units power out / power in gain is naturally a unit less measurement.
Input impedance
- Some voltage source is connected at the input terminals. The circuit from the source perspective then the impedance that is offered by the circuit is known as the input impedance. The equivalent circuit from the source perspective then it will look like this. The input impedance is the impedance that is observed by the voltage source between the two terminals of the circuit. Now here we are assuming that the applied voltage source is the ideal source.
But in the actual case this voltage source will also have some finite resistance. Ideally this entire input voltage V should also appear across the two terminals of the input impedance. This finite series resistance there will be some voltage drop across this resistor. And because of that lets the voltage which appears across this input impedance is equal to V’. This V’ will be equal to V times input impedance zin divided by zin plus Rs. This finite series resistance there will be some drop across this resistor. That actual voltage which appears across this input impedance will be less than the input voltage. This phenomenon is known as the loading effect. This loading effect can be minimized by having very large input impedance. For example if the input impedance of the circuit is much larger than this series resistance in that case this voltage V’ will be approximately equal to the input voltage V. on the other end if this input impedance is comparable to this series resistance Rs, in that case this voltage V’ will be less than input voltage V. for any circuit the input impedance should be as high as possible. That the entire voltage will appear across the circuit.
Output impedance
- In this circuit we have connected some load at the output terminal. The circuit is giving some output voltage Vo. Ideally this output voltage Vout should also appear across the two terminals of the load. From the load perspective if you see it will also see some finite resistance in series with this output voltage. This finite resistance is known as the output impedance of the circuit. This finite series resistance of the actual voltage which appears across this load will be less than the output voltage. The voltage which appears across the load is equal to Vo’. this Vo’ will be equal to Vo times ZL divided by (ZL+ZO)
- This output impedance of the actual voltage which appears across the load will be less than this voltage Vo. This output impedance is comparable to the load impedance in that case the actual voltage which appears across the load will be less than the output voltage Vo. This is also one kind of loading effect. It can be minimized by having this output impedance as minimum as possible. By having this output impedance as minimum as possible we can increase the output voltage which appears across the load. In this way for any circuit the input impedance should be as high as possible while the output impedance should be as minimum as possible. So this input and output impedance are very crucial parameters when we are cascading either two circuits or systems.