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Loading effect Signal Source o/c voltage = 100mV VIN VOUT Amplifier Gain = 100 Load Consider the circuit above, where the o/p from a signal source is measured to be 100mV. If the signal source is connected to VIN of the amplifier with gain of 100, expected value at amplifier output VOUT will be (100*100mV) = 10V. Since the gain of the Amplifier should be independent of the load, we will also expect VOUT to remain at 10V, with or without the load connected. Now consider the following situation where we have included the actual thevenin equivalent models of all the blocks concerned, where ZS, ZI and ZO and ZL are the equivalent input and output impedance of each respective block.. ZS ZO VIN ZI 100VIN VOUT ZL 100mV From this diagram it is obvious that the actual VIN value will depend on the ratio of ZI & ZS, while VOUT will depend on the ratio of ZL & ZO in addition to value of VIN, To reduce this 'loading' effect at VIN and VOUT, we usually try to mismatch the values of ZI & ZS (i.e. make ZI >> ZS), as well as values of ZO & ZL (make ZO << ZL). In practice, we will try to design an amplifier such that its ZI is as high as possible (ideally ∞), and its output impedance ZO is as low as possible (ideally 0) to minimise the loading effect.