# Confusing results from voltage amplifier circuit

Discussion in 'General Electronics Discussion' started by Shakswithlaserbeams, Mar 18, 2016.

1. ### Shakswithlaserbeams

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Mar 18, 2016
So I am trying to make a voltage amplifier that raises a intermittently pulsed 3.3V signal to 5V. I've built a circuit based off a diagram I've found:

Let's just say I don't know very well what I'm doing, and I don't know If I've interpreted how to implement the digital ground here correctly. The circuit I have now looks like this:

and generates the correct voltage when R1=1.5M and R2=1K !

The OP amp is an NTE823, and it's powered by 5V DC. I am testing the voltage output with two AA batteries, grounding to the battery and measuring at Vout. What kind of monster have I made?

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Dec 18, 2013

3. ### Shakswithlaserbeams

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Mar 18, 2016
Well the gain does not match the expected values as given by the equation in the first diagram, so something is definitely made wrong. However, upon posting this, I realized that my circuit does not have resistance between the inverting input and ground. Moving that resistor to the correct position seems to fix the issue. So I may have solved my own problem. Thank you though!

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Dec 18, 2013
No probs

5. ### dorke

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Jun 20, 2015
IS your signal a 3.3V "digital square wave" ?
You wont to shift it from 0-3.3v to 0 to 5.5v?

Why are you using an audio power amp here?
Obviously not the way to go...

6. ### duke37

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Jan 9, 2011
You seem to have both R1 and R2 connected to ground, this is wrong.
I do not know where they should go without looking up the op-amp connections.

7. ### AnalogKid

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Jun 10, 2015
Not even close.

1. Your chip is identical to the National Semiconductor LM386. This is a small audio power amplifier. It is *not* a traditional opamp in any sense, so traditional opamp circuits will not work with it.

2. Your schematic has several errors.

3. By your own equation, you are operating at a gain of 1501, so with a 3.3V input the output is trying to produce 4,953 V. That is a comma, not a decimal point; the circuit is trying for 5 kV. This will not work for at least three different reasons.

The fact that the output goes up and down when the input goes up and down does not mean that you actually solved your problem. Read the LM386 datasheet to get a much more detailed explanation of how this device works.

ak

davenn likes this.
8. ### davennModerator

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Sep 5, 2009
indeed, and then use a proper op-amp

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Jun 21, 2012