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Impedance

Input Impedance is the impedance offered by the input terminals of a circuit.
It is also defined as the ratio of voltage across the input terminals to the current flowing through the input terminals.
Generally we prefer high input impedance over low input impedance .There will be no voltage drop across the source impedance when input impedance is assumed to be infinity. 

Output impedance being low is needed because a speaker is 2 ohms, or 4 ohms or 8 ohms or 16 ohms. All of these numbers are very low numbers.This is the output impedance you must match. Input impedance must be high to not “pull” on the source device. For example, a record player, or phonograph, puts out only mini-volts of voltage. So the signal is very weak before it hits the first pre-amp stage. Therefore, you can’t “pull” on that device, since the first stage in any amplifier is basically a voltage amplification circuit and then followed by power amplification circuit.

 Impedance  formula

Z=R2+(Xl-Xc)2


Z=impedance
R=resistance
Xl=inductive reactance
Xc=capacitive reactance

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