EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: joersam24 on June 03, 2012, 07:13:56 am
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Hi to all,
Can anyone explain in details why should OP AMP have very high input impedance and very low output impedance? Thanks...
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Hi to all,
Can anyone explain in details why should OP AMP have very high input impedance and very low output impedance? Thanks...
The input of an opamp is designed to be high impedance. Either it is a very small geometry transistor running at very low collector current, or a FET/MOSFET input where the only current is leakage currents. So the inputs are very high impedance by design.
Output impedance of a typical opamp is low but not very low. An opamp could easily have an output impedance of a few hundred ohms.
Everything changes when you add a feedback circuit. The feedback circuit coupled with the opamp loop gain drives the output to the exact voltage that makes the two inputs the same voltage. With feedback, the resultant output impedance becomes very low - less then one ohm.
The input impedance of the amplifier depends on the circuit. Non-inverting circuits will have an extremely high input impedance. Inverting amplifier designs have an input resistance equal to the input resistor. If the input resistor is 10K, then the final amplifier's input impedance will be 10K.
Richard.
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Thanks amspire, Does it mean that high input impedance and low output impedance of an op-amp exists only due its physical construction? I mean it exists only unintentionally? Or does it intentionally designed to serve a certain purpose or applications?
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Thanks amspire, Does it mean that high input impedance and low output impedance of an op-amp exists only due its physical construction? I mean it exists only unintentionally? Or does it intentionally designed to serve a certain purpose or applications?
I mean for solely a single op amp unit only without external components(feedback resistor or input resistor).
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Thanks amspire, Does it mean that high input impedance and low output impedance of an op-amp exists only due its physical construction? I mean it exists only unintentionally? Or does it intentionally designed to serve a certain purpose or applications?
An opamp is based on a concept that is a very high gain amplifier, with a high input impedance differential pair stage, a single pole gain fall off characteristic with increasing frequency and a wide range output. The concept has been around since before ICs existed, and it still is one of the most successful analog amplifier concepts.
There are other amplifiers, like a transconductance amplifier that is very different. An amplifier designed for video use may only need a 75 ohm input impedance. That would not be an opamp. To be an opamp, the amplifier has to match the opamp concept.
Richard.
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why should OP AMP have very high input impedance and very low output impedance?
the idea is so the input can suck little energy from source and the output can give high energy.