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input/output impedance considerations

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technocrat

technocrat

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why should input impedance be high in op-amps etc and output impedance be low??? or should it be vice versa in other devices ?? what's the principle involved??
 

technocrat,
The idea behind an op amp is to provide a gain block such that the transfer function of a circuit is dependend only on the parameters of the external components; not on the parameters of the op-amp. By providing a high input impedance, the op-amp will not provide a significant load on the external componets. Conversely, by providing a low output impedance, the output voltage of the op-amp will not be affected by the loading presented by the external components.
Regards,
Kral
 

in general you want to have high input impedance and low output impedance.
this type of circuit is more immune to changes in the load.

OpAmp was designed with this goal in mind.
 

just keep the voltage divider equation in mind. higher the impedence, more will b the voltage drop across the terminals. since v need most of the voltage of the source to b dropped across the op-amp i/p terminals, it needs high i/p impedence. similarly the o/p voltage of op-amp must b dropped across the load and so it v go 4 low o/p impedence. this is true for all amplifier ckts, not just op-amps.
 

    technocrat

    technocrat

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You might have already noticed that high input impedance means a very low input current which would result in quite a high current gain, mostly desired.
 

High input impedance will produce very small load effect. In other words, it will absorb very small current from the input signal soruce.
Low output impedance implies the output can drive weight load. In other words,it is equivalent to a votage source.
 

technocrat said:
why should input impedance be high in op-amps etc and output impedance be low??? or should it be vice versa in other devices ?? what's the principle involved??
Click to expand...

Actually that's not completly true. It depends on the type of OPAMP you're using. Usually, amplifiers work as VOLTAGE amplifiers, so if you need high gain you would need to make it independent of the voltaje drops casued by the input and output loads. If you have a LOW output impedance (pretty close to zero) any load you place on the OPAMP's output won't affect the OUTPUT voltaje. Now, if you have a CURRENT amplifier you would face the opposite case. In these kind of OPAMPS you have LOW input impedance, and HIGH output impedance, cause what you want is to have as low current gain lost as possible. So if you use an ouput load the ouput current won't be affected by the load value.

Hope this helps...

diemilio.
 

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