# How do the math of this op-amp........?

Discussion in 'Analog & Mixed-Signal Design' started by f8voa, Jul 13, 2018.

1. ### f8voa Thread Starter New Member

Jun 5, 2013
6
0
Ho can I do the math of this configuration ?
V+ = 5VDC
V++ = 1V
V-OUT = +/- 1.44V

I like to calculate V-OUT for multiple V++

Thanks , Marco

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2. ### WBahn Moderator

Mar 31, 2012
23,554
7,207
This looks like homework. Is it? If so we can move it to the Homework Help forum.

What do you mean by Vout = +/- 1.44 V? Where does that come from?

Before worrying about other values of V++, show what you get for the value given. Why/how did you arrive at that value?

3. ### f8voa Thread Starter New Member

Jun 5, 2013
6
0
Not at all , I'm 64 Years old and try to understand this problem . I know opamps but not this one ( never saw it before )
It is a part of an electronic test equipment .
Marco

4. ### f8voa Thread Starter New Member

Jun 5, 2013
6
0
This 1.44V . I made the circuit in LTSpice and became this result after running the circuit .

5. ### crutschow Expert

Mar 14, 2008
20,813
5,916
Doesn't make any sense.
Post your simulation .asc file directly to this forum, no need to ZIP it.

6. ### f8voa Thread Starter New Member

Jun 5, 2013
6
0
Herewith the simple asc.file .
What do you mean with :" Doesn't make any sense "?

Marco

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8. ### f8voa Thread Starter New Member

Jun 5, 2013
6
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OK , thanks a lot , I can see at the end a little bit light now .
I'l try to do it that way in Spice .
Marco

9. ### crutschow Expert

Mar 14, 2008
20,813
5,916
V-OUT = +/- 1.44V

10. ### f8voa Thread Starter New Member

Jun 5, 2013
6
0
As I said before : approx 1.44V ( +/- means approx ) - LTSpice results

Thanks , Marco

11. ### WBahn Moderator

Mar 31, 2012
23,554
7,207
Ah, there's the source of confusion (probably an English as second language issue). The notation "+/-" does not mean "approximately". It means, literally, positive or negative. So the supply voltages of a bipolar opamp might be given as +/- 12 V meaning +12 V and -12 V. Or the input signal range might be +/- 10 V meaning from -10 V to +10 V. To indicate "approximately", you should use the tilde character, so ~1.44 V. The special symbol ≈1.44 V is even better.

12. ### WBahn Moderator

Mar 31, 2012
23,554
7,207
So if the opamp is doing what it should, when V++ = 1.0 V, then the voltage at the other input will also be very close to 1.0 V. That means that a current of 1 V / 47 kΩ or about 21 μA will flow. A look at the data sheet

http://www.onsemi.com/pub/Collateral/1N914-D.PDF

Shows that at this current the typical forward voltage drop is about 425 mV, so we would expect Vout to be ~1.42 V, which is very close to the LTspice sim results.