# What Vcc value should I typically use on an LM324 in PSpice?

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1. Feb 16, 2016

### kostoglotov

I've checked documentation and it says anywhere between 3 and 30V to supply the LM324. But I couldn't find further guidelines.

My text explained that op amps need external power supplies, but provided no guidelines as to what value to make that supply value.

When analyzing this following circuit in PSpice, changing the power supply to the LM324's (even between 3 and 30) drastically changes the output voltages.

The top op amp has no feedback loop, so I figure its power supply should be higher than 5V to get to a saturation response.

This is my circuit in PSpice as drawn up in PSpice with some values shown next to Voltage sources.

According to the text, v0 should be -4.992V.

Now, if I take those op amp power supplies both to 30V, v0 becomes -5.005V...much closer to the text answer. Increasing op amp power supply beyond 30V causes no further changes, it seems to max out at -5.005V.

How are you supposed to know what power supply to use on an op amp? How does it depend on the circuit you're analyzing? What are some general guidelines?

2. Feb 16, 2016

### analogdesign

First off, both op amps have feedback. They share components in their feedback loops, that's all.

As to what power supply you should use with an op amp the first place to start is with the datasheet for the op amp. That will tell you what values of supply it will accommodate. Then you can choose the supplies based on the dynamic range you need in the circuit. You want to be sure that all the op amps stay within their linear regions (usually some fraction of the supply voltage) under all conditions.

The LM324 can handle 32V between its rails. Since the "text answer" is negative, the textbook is assuming split rails. Try VCC = 15V and VSS = -15V (instead of 0V) and see what happens.

3. Feb 17, 2016

### kostoglotov

It didn't change the result by much :P

4. Feb 18, 2016

### donpacino

what that does tell you about Op amps?

hint. this is why Op-amps are very very useful.

5. Feb 18, 2016

### kostoglotov

Well, actually, the output voltage changed an order of magnitude when I halved the voltage at the power supply. What didn't change the output much was switching the power terminals on the op amp from (+30 to 0) to (15 to -15).

6. Feb 18, 2016

### Staff: Mentor

Just using calculations and ignoring the power supply values at first, what do you get for the input and output voltages of the opamps in your first schematic?

Then you just need to ensure that your power supplies for the opamps are wide enough to stay away from those input and output voltages. As pointed out, you can refer to the LM324 datasheet to see what the input and output voltage ranges are with respect to the power supply voltages.

7. Feb 18, 2016

### jim hardy

EDIT oops Berkeman got there first ! Right On ....

Find by analyzing your circuit the most positive and ,most negative input voltages your opamp will 'feel' at its inputs.
Also the most positive and most negative output voltages your circuit will need from the opamp .

LM324 can drive its output to within ~1.5 volts of positive supply, so V+ needs to be at least that much above most positive output you'll need.
LM324 can drive its output to negative supply but only at low current, so give it a volt or so "headroom" there, too . Make V- a volt or so more negative than most negative output. In a pinch you could get away with less negative side headroom provided the opamp only has to accept a few microamps into its output pin. See chart beow.

LM324 has similar headroom requirements on its inputs. That's why you have to consider both input and output.
From LM324 datasheet

Spend some time with datasheets they're a wealth of practical information