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Problems with using LM324 as an inverting amplifier

ZanShini

Mar 2, 2018
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As part of my project I need to invert a pulse train without changing it's amplitude. Hence I want to use an Op-Amp as an inverting amplifier with unity gain. I'm using the circuit given in the following tutorial.

https://www.electronics-tutorials.ws/opamp/opamp_2.html

And also the schematic I'm using.

https://imgur.com/a/pu8L3

I chose LM324 to implement the circuit. I've used a single Op-Amp i.e. pins 1, 2 & 3, two 1K resistors for input and feedback resistances and connected pin 4 to battery's +ve terminal and pin 11 to the
-ve terminal. I tested it with a 9v battery and as far as I understand, the output should be -9V. But my multimeter shows the output fluctuating around 0.98V.

I want to know if I'm doing something wrong or if LM324 can't be used as an inverting amplifier?
 

duke37

Jan 9, 2011
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You cannot get a negative output if you do not supply a negative source.

What do you mean by inverting?
If the signal is 0 to 5V, do you want 5V to 0 or 0 to -5V?
 

ZanShini

Mar 2, 2018
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You can't invert the output without having a split-rail power supply.
You mean I need the supply to be greater than the output I desire right? That is if I desire to get -5v output with 5v input, I need to supply the opamp with +Xv and -Xv where X>5 right? Please excuse my lack of basic knowledge in Electronics.
 

ZanShini

Mar 2, 2018
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That is a non-inverting amplifier, the opposite of what you want.

ak
Actually in some video I heard that in Psipce schematic the + terminal is the inverting one. As you can see the output is -9v. It confused me too how the voltage input at + can give output in reversed polarity.
 

ZanShini

Mar 2, 2018
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You cannot get a negative output if you do not supply a negative source.

What do you mean by inverting?
If the signal is 0 to 5V, do you want 5V to 0 or 0 to -5V?
Yes I want the output to be -5V if I give 5V input. And in my project I want to invert the polarity of a pulse train.
 

BobK

Jan 5, 2010
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Assuming you mean a pulse that goes from 0 to V.

You can invert a positive going pulse by using a transistor and two resistors. Same circuit can also change the level of the high portion of the signal.

You can also do it with a single supply opamp. You will need to bias the + input to half the voltage you want out.

This circuit will invert a 0 to 5V pulse train. By changing the biasing point and the gain you can also make the output have any level you want.

upload_2018-3-2_10-19-3.png


Bob
 

AnalogKid

Jun 10, 2015
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Yes I want the output to be -5V if I give 5V input.
That seems pretty clear. To do this, you will need a rail-to-rail opamp powered by at least +5 V and -5 V, or a conventional opamp powered by +8 V and -8 V or more.
And in my project I want to invert the polarity of a pulse train.
That is much less clear. The problem is that in electronics the word "invert" can mean two different things. Inverting a voltage is what you describe above. But inverting the polarity of a pulse can mean inverting its *logic* polarity. In this case the signal still goes from 0 V to +5 V, but the high and low parts of the pulse wave are reversed, or "inverted". The times when the input pulse is at a logic 1, the output is at a logic 0, and vice versa.

To help clear up things, tell us what is generating the input signal, and what the output signal goes to.

ak
 

BobK

Jan 5, 2010
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Okay, so my post is not what you wanted. Use an opamp with dual supplies, as AK said. Change the + input to go to ground, instead of the voltage divider in my circuit, and you are done.

Bob
 

ZanShini

Mar 2, 2018
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Thanks a lot to everyone who responded. It helped clear my basics about an Op-Amp and I learnt a lot from you guys. The circuit is working now. I hope for your help in the future too.
 
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