[SOLVED] Input Impedance of Opamp (original title restored by modertator)

[archunan_m]

Is the input impedance of an op-amp (consider it is connected in negative config) with all ideal components, being infinity is practically possible??? If the input impedance has any value???

Can someone explain how the input impedance goes infinity???

ORIGINAL TEXT RESTORED - MODERATOR

 

[mister_rf]

Re: Input Impedance of a Op-Amp

An ideal op-amp is usually can be summarized by the two "golden rules":

I. The output attempts to do whatever is necessary to make the voltage difference between the inputs zero.
II. The inputs draw no current.

Input impedance, (Zin) Infinite - Input impedance is the ratio of input voltage to input current and is assumed to be infinite to prevent any current flowing from the source supply into the amplifiers input circuitry (Iin =0). Real op-amps have input leakage currents from a few pico-amps to a few milli-amps.

Operational amplifier - Wikipedia, the free encyclopedia
Operational Amplifiers or Op-amps Tutorial
Op-amps - The Inverting Amplifier

 

[keith1200rs]

Re: Input Impedance of a Op-Amp

Is the input impedance of an op-amp (consider it is connected in negative config) with all ideal components, being infinity is practically possible??? If the input impedance has any value???

Can someone explain how the input impedance goes infinity???

Your diagram is of a transimpedance amplifier with virtually zero ohms input impedance. An inverting opamp configuration, even when drawn properly, won't have infinite input impedance.

Keith

 

[mister_rf]

Re: Input Impedance of a Op-Amp

In this configuration (transimpedance amplifier)as we have connected a feedback resistor from the output to the –input, since the main job of the op amp is to adjust the output such that the inverting input equals the non-inverting input – and input is at ground potential here, the input impedance of the circuit is nearly zero, but impedance of the operational amplifier is infinite, so no current flows into the op amp itself, and the current has nowhere to go but through the resistor. :grin:
Read here, ''Active current-to-voltage converter''
Current-to-voltage converter - Wikipedia, the free encyclopedia

 

[keith1200rs]

Re: Input Impedance of a Op-Amp

I assume the discussion is about the input impedance of the amplifier configuration. The input impedance of the opamp itself doesn't change with the configuration but if you want to consider it then you need to specify the opamp. The input impedance of the opamp or the amplifier configuration will never be infinite, nor will it ever be zero.

Keith

 

[mister_rf]

Re: Input Impedance of a Op-Amp

I was convinced we are talking about the ideal op-amp impedance, anyway good to know all details about some specific amplifier configuration.

Is the input impedance of an op-amp (consider it is connected in negative config) with all ideal components, being infinity is practically possible???

 

[keith1200rs]

Re: Input Impedance of a Op-Amp

Unfortunately questions are often ambiguous and we have to guess at the 'real' question! Maybe we will find out later what the question really meant.

Keith

 

[FvM]

Re: Input Impedance of a Op-Amp

Two general comments:

- You should not edit the text or the title of your posts in a way that changes it's meaning to the opposite, even if it turns out to be erroneous. This creates much confusion and makes the other contributions look incomprehensible. Fortunately, your initial post has been quoted in this case.

- You should not start a second thread with the same topic. Forum rules are pretty clear in this regard.

Technically, the input impedance of the transimpedance amplifier is Zin = R/(1 + A0), A0 being the open loop gain of the amplifier. The expression is valid for DC resistance as well as for (complex) AC impedance, using compley A0. You can calculate it according to OP data.

 

[keith1200rs]

Re: Input Impedance of Opamp {original title restored by modertator)

FvM's comments are valid.

Original title and text restored and thread closed.

Keith.