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.