Operational Amplifier (Voltage Follower)

In summary, the conversation discusses the use of a high input impedance Op Amp to amplify a millivolt signal from an electrode with a high input resistance. However, the person is experiencing an issue with the output voltage, which remains at -10.2V even when the probe is disconnected. Suggestions are given, such as using a fresh chip and checking the pinout and equivalent circuit of the sensor. The datasheet of the Op Amp is also referenced, and the conversation ends with a suggestion to check the pin connections and use offset null inputs.
  • #1
splitendz
32
0
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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  • #2
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?
 
  • #3
Also maybe check that you have the pinout correct. If you accidentally connected the output to the + input, you might get a similar behavior.
 
  • #4
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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  • #5
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.
 
  • #6
I see what you're saying about +/- 18V supply voltages. ref
I would go with berkeman's 2nd suggestion and recheck your pin connections.
 
  • #7
Are you using the offset null inputs?
 

FAQ: Operational Amplifier (Voltage Follower)

What is an operational amplifier (op-amp)?

An operational amplifier, or op-amp, is an electronic component that amplifies the difference between two input signals to produce an output signal. It is a highly versatile and widely used component in many electronic circuits, especially in analog and digital signal processing applications.

How does an op-amp work?

An op-amp typically consists of an inverting and a non-inverting input terminal, a power supply, and an output terminal. It uses a high-gain differential amplifier to amplify the difference between the two input signals, which is then fed through internal feedback to provide the desired output signal. The gain of an op-amp is typically very high, ranging from thousands to millions, making it an ideal component for amplifying weak signals.

What is a voltage follower and how does it work?

A voltage follower, also known as a unity-gain amplifier, is a basic configuration of an op-amp where the output voltage follows the input voltage without any amplification. It has a very high input impedance and a very low output impedance, making it ideal for impedance matching and buffering applications. The output voltage of a voltage follower is the same as the input voltage, which makes it useful for isolating and stabilizing signals.

What are the applications of a voltage follower?

The primary application of a voltage follower is to provide impedance matching and buffering in electronic circuits. It is commonly used to isolate a high-impedance signal source from a low-impedance load, preventing any changes or distortions in the signal. Additionally, it can also be used as a voltage regulator, to provide a stable output voltage regardless of input voltage fluctuations.

What are the advantages of using a voltage follower?

There are several advantages of using a voltage follower in electronic circuits. It has a very high input impedance, which minimizes the loading effect on the input signal source. It also has a very low output impedance, which makes it an excellent buffer between two different circuits. Additionally, it provides unity gain, which means it does not amplify or distort the input signal, making it ideal for signal isolation and stabilization.

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