Simple Op Amp test

Thread Starter

cparke

Joined Aug 28, 2017
77
I want to do a very basic, high-level Op-amp test without building a circuit with resistors, etc. Just a meter, to the extent possible.

So I tried powering it up on 5V and just measuring voltage at the inverting, non-inverting, and output pins. I got 0.2V on the two input pins, and 3.8V on the output pin. Same thing every channel. I'm not very knowledgeable about this component, but I assume this might be expected without a ground attached?

What other simple tests could I do? For example, I've learned that a transformer or battery pack center tap would be a suitable ground, and using that at both the inputs should give me 0V out? But what about on a separate power supply?

Are these valid test of anything related to the op amp?
 

MrChips

Joined Oct 2, 2009
26,480
No, you cannot test an op-amp like that.
For starters, tell us the part number of the op-amp.

This is the simplest test you can perform.
Connect the output pin to the inverting input.
Assuming that the supply voltages are connected correctly and Vin is in between the range of the supply voltages,

Vout = Vin.

1653879465649.png
 

crutschow

Joined Mar 14, 2008
30,100
In MrChips' circuit, you could use a potentiometer (e.g. 10kΩ) connected between the V+ supply and ground, with the wiper supplying Vin to generate a varying voltage to test the amp.
Note that, unless the op amp is a rail-rail type, the output will not go to the supply rails.
 

dl324

Joined Mar 30, 2015
14,686
So I tried powering it up on 5V and just measuring voltage at the inverting, non-inverting, and output pins. I got 0.2V on the two input pins, and 3.8V on the output pin. Same thing every channel. I'm not very knowledgeable about this component, but I assume this might be expected without a ground attached?
That's not a valid test. Operating the opamp open-loop will just amplify the offset voltage. The output will probably be at one extreme or the other. The output voltage swing depends on the opamp.

To test opamps, you need an oscilloscope, a signal source, and test circuits. Among the parameters you'll want to test are gain, bandwidth, and slew rate. You need to vary the gain and frequency of the input signal so you can check gain at different frequencies; particularly the frequency where gain rolls off to 1. You also want to measure unity gain slew rate with a square wave input.
 

Thread Starter

cparke

Joined Aug 28, 2017
77
No, you cannot test an op-amp like that.
For starters, tell us the part number of the op-amp.

This is the simplest test you can perform.
Connect the output pin to the inverting input.
Assuming that the supply voltages are connected correctly and Vin is in between the range of the supply voltages,

Vout = Vin.

View attachment 268317
The Op-Amp is an LM324.

So, if I understand your drawing (thank you so much!), I should use just a single power supply, connect it to VCC+ and VCC- like I was suggesting. Then jumper the inverting input pin to the output pin and check the voltage between output and the power supply negative? Reading should be full voltage of the power supply?
 

Thread Starter

cparke

Joined Aug 28, 2017
77
In MrChips' circuit, you could use a potentiometer (e.g. 10kΩ) connected between the V+ supply and ground, with the wiper supplying Vin to generate a varying voltage to test the amp.
Note that, unless the op amp is a rail-rail type, the output will not go to the supply rails.
But I don't see any ground in his circuit. Unless we're talking about the power supply negative (which should be connected to ground)
 

eetech00

Joined Jun 8, 2013
3,206
Per post #2 and #3. Connect like this:

1653887794698.png

When you vary the pot resistance, the input voltage will vary. The output voltage should follow the input voltage.
However, the ouput voltage can only go as high as ~3.5v (1.5v less than the 5v supply voltage).
 

Thread Starter

cparke

Joined Aug 28, 2017
77
When you vary the pot resistance, the input voltage will vary. The output voltage should follow the input voltage.
However, the output voltage can only go as high as ~3.5v (1.5v less than the 5v supply voltage).
Much appreciation for your very concise and helpful diagram and explanation!

This would seem to explain, therefore, why I am getting 3.8V (it's the ~3.5V maximum) at Vout when nothing is connected to Vin+ or Vin-.

The potentiometer definitely would help further prove that the Op Amp is functioning as expected, I'll see if I can do that. I'm assuming I can simply substitute the negative on Vsupply for both ground connections? And Vout as well should be measured on the meter relative to ground (Vsupply-)
 

tepalia02

Joined May 13, 2022
37
Without adding any further components, I can only suggest unity gain buffer as someone has already suggested above. For other OP-AMP based circuits, you'll need to add some resistors at least.
 

BobTPH

Joined Jun 5, 2013
5,227
The Op-Amp is an LM324.

So, if I understand your drawing (thank you so much!), I should use just a single power supply, connect it to VCC+ and VCC- like I was suggesting. Then jumper the inverting input pin to the output pin and check the voltage between output and the power supply negative? Reading should be full voltage of the power supply?
The reading would depend on voltage on the + input. If the input is floating, the reading could be anything. An opamp input is never floating in a valid circuit.

Bob
 

atferrari

Joined Jan 6, 2004
4,630
That "no resistors" limitation (which I suspect comes just from your poorly informed imagination) is keeping you away of many possible tests of basic configurations maybe with 3 to 7 resistors.

Testing a car with no wheels could be done but with them...
 

Audioguru again

Joined Oct 21, 2019
4,793
The LM324 quad and LM358 dual have PNP input transistors. The 0.25mA bias current pulls floating inputs to a positive voltage and the input offset voltage difference between the input transistors causes the opamp to amplify it for the output to be high or low.
 
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