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Unity-gain inverter problem

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tdkehoe

New Member
I need to invert a small DC signal (0 to +0.8 volts). I built a unity-gain inverter according to the schematic in "Engineer's Mini-Notebook: Op Amp IC Circuits," but it doesn't work. I have a 4-volt power supply to the op-amp (LM358), a 1K resistor between the + (non-inverting) input and 2 volts, a 1K resistor between the signal input to the - (inverting) input, and a 1K resistor between - (inverting) input and the output. The result is that the - (inverting) input is always at 2V and the output is always at 2V. I believe that the problem is that with a single-supply op-amp you have to put capacitors on the input and output, but this blocks the DC input voltage. I.e., a single-supply inverting op-amp works fine for AC signals, but with a DC signal you have to use split power supplies.

I tried using a 74xx04 inverter but the output is either 4V or 0V, it can't output an analog DC signal.

Any suggestions for solving this problem?
 

Papabravo

Well-Known Member
What is your meaning of invert? If your op amp had bipolar power supplies of let us say +12V and -12V then a unity-gain inverter would map the inputs in the following way:
Code:
0.8V --> -0.8V
0.6V --> -0.6V
0.4V --> -0.4V
0.2V --> -0.2V
0.0V -->  0.0V
With a unipolar supply the unity gain inverter will not be able to invert at all. It's output is limited to at least GND and Vcc assuming it can even get that close to the power supply rails.

Why on Earth would you expect a digital device like a 74xx04 to invert an analog signal. That expectation is just off the charts.
 

tdkehoe

New Member
Here is the schematic. I was expecting results like this:

0.8V --> 2.0V
0.6V --> 2.2V
0.4V --> 2.4V
0.2V --> 2.6V
0.0V --> 2.8V

This is with the non-inverting input at 2V (with a 4V power supply). I was expecting the signal to flip from negative to positive in relation to the non-inverting input. Would this circuit work if the input voltage were above 2V:

2.8V --> 1.2V
2.6V --> 1.4V
2.4V --> 1.6V
2.2V --> 1.8V
2.0V --> 2.0V

If so, maybe I could use a summing amplifier to raise the input voltage.
 

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  • inverter.pdf
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BrownOut

Banned
If the voltage at the resistive divider is really 4v instead of 5v shown in your schematic, then you should get your second scheme:

2.8V --> 1.2V
2.6V --> 1.4V
2.4V --> 1.6V
2.2V --> 1.8V
2.0V --> 2.0V

Also:

1.8v --> 2.2v
1.6v --> 2.4v
etc.


That is, assuming your opamp is speced to operate at 4V.
 
Last edited:

AllVol

New Member
Here's just a suggestion:

Since the LM358 can be used either single or split supply, follow Forrest's schematic exactly, but ground the v- pin.

And since the LM358 is a dual device, simply construct another unity inverter and have the results you a looking for.
 
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