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testing the opamp

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tonyke

New Member
I try to test my LM741CM. By connecting both inputs to a certain voltage level(open loop, no feedback), the output turns out to be 10.925V, which I assume will be zero. I am using +12V and -12V as power. I know this might be a silly question, but what's the problem?
 

confounded

New Member
if all youve done is connect Vcc and Vee and measured output you are seeing the effects of non zero input caused by voltage offsets. You op amp has a open loop gain of 200,000 so non zero input voltage is magnified at output to give about 11v which is max it can output with 12v vcc.
What exactly are you trying to test?
 
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tonyke

New Member
i am just trying to test whether the opamp is dead. i connect the two input together, so there should be no input difference. How come I get 11V?
 

smanches

New Member
The difference is probably very near zero volts, but not quite. But since the amp has a 200k gain, the small voltage is going to look very large.

Even if 10.925 was actual output voltage (ie not clamped by the input voltage), 10.925 / 200,000 = .0546mV difference in inputs. That's quite small.

Feedback will reduce the gain on the amp, giving you more reasonable values. Am I right on that? I barely know op amps.
 
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MikeMl

Well-Known Member
Most Helpful Member
You are right on. The OP needs to review the "input offset" and "open loop gain" sections of the 741 data sheet. The OP is quoting the behavior of an "ideal" op=amp; the 741 is far from ideal...:D
 

crutschow

Well-Known Member
Most Helpful Member
You need to provide feedback to test the op amp. Connect the (-) input to the output. This will provide a gain of plus 1 from the (+) input. Then the output will be equal to the (+) input until you reach the output voltage limits of the op amp.
 

tonyke

New Member
yes, carl, I am just curious why it will perform that. Even I simulate this in multism, it gives the same result. By the way, it can't be the input offset. I have looked it up in the datasheet, the offset is only several mv.
 

Torben

Well-Known Member
yes, carl, I am just curious why it will perform that. Even I simulate this in multism, it gives the same result. By the way, it can't be the input offset. I have looked it up in the datasheet, the offset is only several mv.
Question: What is several mV multiplied by 200,000?
Answer: Lots.

The problem can very well be input offset. Without feedback the op amp is going to try to amplify that offset as much as it can and you'll see a huge error at the output.

Try the given suggestions. You may be surprised at the result. :)


Torben
 

audioguru

Well-Known Member
Most Helpful Member
The input offset voltage for a uA741C opamp is typically plus or minus 1mV but can be as high as plus or minus 5mV at room temperature.
Its DC voltage gain is typically 200,000 so of course its output will be slammed against a supply voltage (1.2V away without a load) without any negative feedback to reduce the gain.

Did you try to null the input offset voltage with a trimpot as shown in the datasheet?

I don't think Multisim is smart enough to know about input offset.
 

Speakerguy

Active Member
Any opamp with only power and ground (or V+ and V-) connected is going to rail out, or worse yet oscillate between rails.
 
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audioguru

Well-Known Member
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I don't think the inputs of the opamp were connected to 0V. They were just connected together and were floating. The input transistors are NPN so the inputs would float low until they are below the allowed input common-mode voltage range.
 
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