Operational Amplifier (Voltage Follower)

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Discussion Overview

The discussion revolves around troubleshooting issues with a voltage follower configuration using a TL071CP operational amplifier to amplify a millivolt signal from a high input resistance electrode. Participants explore potential causes for unexpected output voltage when the probe is disconnected.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant describes an issue where the output voltage is -10.2V when the probe is disconnected, suggesting a problem with the op amp configuration.
  • Another participant questions the power supply voltage, suggesting that the TL071CP may not be rated for +/-12V and could be damaged, recommending a lower voltage supply.
  • A different participant suggests verifying the pinout connections to ensure correct wiring, as incorrect connections could lead to similar output behavior.
  • One participant notes that the TL071 has a maximum supply voltage of +/-18V, arguing that +/-12V should not affect operation, and suggests checking the input sources presented to the op amp.
  • Another participant agrees with the need to check pin connections and suggests trying different TL071 chips in case of a faulty component.
  • A participant inquires about the use of offset null inputs, hinting at another potential factor affecting the output voltage.

Areas of Agreement / Disagreement

Participants express differing views on the appropriate power supply voltage for the TL071CP and whether the op amp may be damaged. There is no consensus on the cause of the output voltage issue, and multiple suggestions for troubleshooting are presented.

Contextual Notes

Participants reference the datasheet specifications for the TL071CP, indicating some confusion about the maximum supply voltage. The discussion includes assumptions about the circuit configuration and the behavior of the op amp under certain conditions.

Who May Find This Useful

Individuals working with operational amplifiers, particularly in high-impedance signal applications, may find the troubleshooting suggestions and considerations relevant.

splitendz
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Hi Guys,

I'm trying to amplify a millivolt signal from an electrode that has a high input resistance (~ 200 MOhm). To avoid losses over the series resistance I'm using a high input impedance Op Amp TL071CP as a voltage follower. The problem I'm having though is that for some reason when the op amp is powered with +-12V (or any +- votlage for that matter) I am getting -10.2V at the non-inverting input and consequently seeing that voltage at the output, even when the probe is disconnected. Ideally I would have 0V at the output when nothing is connected. Does anyone have any suggestions as to why this may be happening?! It's been bugging me for hours.

Cheers.
 
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I don't think that opamp is meant to be powered by +/-12V. The datasheet says +/-8V max. Maybe that chip is blown now. Try using a fresh chip and only +/-5V or so. Also, what is the equivalent circuit of your sensor? Does it just look like 200MegOhm to ground?
 
Also maybe check that you have the pinout correct. If you accidentally connected the output to the + input, you might get a similar behavior.
 
The datasheet says +-18V. Hmm, in the past I've never had an issue with powering the same IC with +-15V. I don't see why +-12V would affect operation?

The circuit is very simple, i'll upload an image for you to see for yourself.

Thanks :)
 

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splitendz said:
The datasheet says +-18V. Hmm, in the past I've never had an issue with powering the same IC with +-15V. I don't see why +-12V would affect operation?
The circuit is very simple, i'll upload an image for you to see for yourself.
Thanks :)
My bad, I was looking at the TLC071 datasheet and saw the +/-8V max. You're correct that the TL071 has the wider supply range, so that's not the problem. I'm not sure what to suggest, guy. A follower is a follower. Hook a power supply up to the + input and make sure that it follows that low-impedance voltage input across its input Vicm range. Then take a closer look at the other input sources that you are presenting to it. And try a few different TL071 chips -- maybe you've got a bad one.
 
I see what you're saying about +/- 18V supply voltages. ref
I would go with berkeman's 2nd suggestion and recheck your pin connections.
 
Are you using the offset null inputs?
 

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