# Inverting OpAmp question

Discussion in 'General Electronics Chat' started by RFeng, Jun 26, 2012.

1. ### RFeng Thread Starter New Member

May 30, 2012
27
0
Hi,

I simulated the below inverting OpAmp:

and got the below results:

I don't manage to understand why is it that for input vapc0 = 1.035V we get at the output vapc = 1.967V.

It looks like that: vapc = 3V - vapc0; I don't understand why.

Thank you.

Last edited: Jun 26, 2012
2. ### nomurphy AAC Fanatic!

Aug 8, 2005
567
12
R7/R6 is a gain of 1, not 100dB, because they are both 1K ohms. To get "only" 80dB of gain, R7 would need to be a 10 Meg.

What is the point of inducing +1.5V offset into the positive pin?

Last edited: Jun 26, 2012
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3. ### crutschow Expert

Mar 14, 2008
12,540
3,074
The gain from the non-inverting input to output is (1 + R7/R6) = +2 (when R6 is grounded or connected to a low impedance source). The gain from the inverting input to output is -R7/R6 = -1. The two signals sum at the output accordingly.

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4. ### RFeng Thread Starter New Member

May 30, 2012
27
0
Hi crutschow, Thanks!

I think I understand you.
Is it correct to say that in general:

Hi,
Thank you too!
I do not know the purpose of the 1.5V offset, as I'm currently learning the entire system (which the circuit is just part of it).
The system is an Automatic Gain Control unit.
How did you get the 80dB? (if R7 = 1MΩ).

5. ### nomurphy AAC Fanatic!

Aug 8, 2005
567
12
Sorry typo, 10 Meg -- the point being that generally it is stretching the usefulness of most op amps.

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6. ### crutschow Expert

Mar 14, 2008
12,540
3,074
You got it.

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7. ### RFeng Thread Starter New Member

May 30, 2012
27
0
Thank you dear fellows

8. ### upand_at_them Active Member

May 15, 2010
246
29

1. R6 and R7 create a voltage divider.
2. Because you have feedback the op-amp will try to make vapc such that the inverting input is the same as the non-inverting input.
3. The non-inverting input is 1.5V.

So you have a voltage divider with 1.035V, a 1k resistor, 1.5V, a 1k resistor, and then some unknown voltage vapc. Solving for vapc gives you the result you got in simulation.

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9. ### WBahn Moderator

Mar 31, 2012
17,454
4,701
Huh?

If

$
Vout = V_+ \left( 1\;+\; \frac{R_7}{R_6} \right) +\;V_-\left(-\frac{R_7}{R_6} \right)
$

Then that means that

$
Vout = V_+ \;+\; (V_+\;-\;V_-) \frac{R_7}{R_6}
$

Since (V+) - (V-) is going to be a tiny voltage, this would say that Vout is always equal to V+.

The simplest way (at least to me) to get the transfer characteristic is to analyze the circuit. The current flowing from the input is:

I = (Vin-V+)/R6

The output voltage is:

Vout = (V-) - I*R7

Assuming V+ ~= V-, we have

Vout = (V+) - ((Vin-V+)/R6)*R7

Vout = (V+)(1+ R7/R6) - (Vin)(R7/R6)

NOTE: I now see how the error in the OP's equation was easily missed.

For V+ = 1.5V and R7=R6, this reduces to

Vout = 3V - Vin

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10. ### crutschow Expert

Mar 14, 2008
12,540
3,074
I don't follow the logic of your post. You start and end with the same equation while implying that there is some error.

You say "Since (V+) - (V-) is going to be a tiny voltage, this would say that Vout is always equal to V+." I don't understand why you say that. (V+) - (V-) may or may not be a tiny voltage. It depends upon their relative value.

What error in the OP's equation are you talking about?

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11. ### WBahn Moderator

Mar 31, 2012
17,454
4,701
I am assuming that V+ is the voltage at the noninverting input of the opamp and V- is the voltage at the inverting input. This is almost always what I see these terms to mean in an opamp circuit. Using one term to mean the voltage at one of the input pins and the other to mean the voltage someplace else seems a bit ackward to me, but I guess if the intent was to mean both to be the voltage someplace else and that it just turns out that V+ also happens to be the voltage at the noninverting input, then I see what you are saying.

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12. ### JMac3108 Active Member

Aug 16, 2010
349
66
RFeng,

The easiest way to analyze this kind of circuit is using superposition. The basic procedure is to consider one input at a time while grounding the other, then add the results.

In your circuit, first ground the 1.5V source and conside the input. The circuit becomes a simple inverting amp with equation, Vout1 = -Vin(Rf/Rin).

Then ground the input and consider the 1.5V source. In this case the circuit becomes a simple non-inverting amp with equation, Vout2 = Voffset (1+Rf/Rg).

Combine the results and you'll get your equation. Vout = Vout1 + Vout2.

If you do a few of these, you'll get where you can analyze them in your head. Its a really simple way to approach this kind of circuit.

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13. ### RFeng Thread Starter New Member

May 30, 2012
27
0
Hi,

Thank you all for your inputs!
You've given me a great help.