Some issue with the UA741CP op amp

Thread Starter

circuitnaiver

Joined Oct 1, 2021
22
Hi, All,

I use UA741CP to amplify a 0Volts, 2.5volts square wave. The design circuit is shown below. I use RG=RF=1K ohm. VCC=15Volts, VDD is connected to 0V. Pin1 and Pin5 are not connected to anything. The output is supposed to be 5Volts in this case. However, it gives me 1.2 volts instead. Can anymore help me out with this? Thank you in advance.

1633989462576.png


Here is the datasheet:
https://www.digikey.com/en/products/detail/texas-instruments/UA741CP/382197?utm_adgroup=Integrated Circuits&utm_source=google&utm_medium=cpc&utm_campaign=Dynamic Search_EN_RLSA_Cart&utm_term=&utm_content=Integrated Circuits&gclid=Cj0KCQjwwY-LBhD6ARIsACvT72OyoSaOfkpwHz2pMEJBI6GXvZj_dBEYQs2F5jEmYdMb5n6U5tWMa-gaAqStEALw_wcB
 
Last edited:

crutschow

Joined Mar 14, 2008
28,170
You are not the first to to have a problem from ignoring the common-mode input voltage range of the old 741 (below).
Many of those who contact this forum about problems using the 741 involve exceeding those limits.

1633990518993.png
Note that the input voltage cannot go higher than about 3V below the plus supply voltage or or lower than about 3V above the minus supply voltage.
Since your minus supply voltage is ground, the minimum input voltage is about +3V, (so 0-2.5V won't work properly).
You either need to add a minus supply of a minimum -3V, or use an opamp whose input can go to ground with a grounded negative supply, such as an LM324.

Also 1kΩ resistors are rather low value for standard op amps.
Values around 10kΩ are more typical.
 
Last edited:

Thread Starter

circuitnaiver

Joined Oct 1, 2021
22
You are not the first to to have a problem from ignoring the common-mode input voltage range of the old 741 (below).
Many of those who contact this forum about problems using the 741 involve exceeding those limits.

View attachment 250031
Note that the input voltage cannot go higher than about 3V below the plus supply voltage or or lower than about 3V above the minus supply voltage.
Since your minus supply voltage is ground, the minimum input voltage is about +3V, (so 0-2.5V won't work properly).
You either need to add a minus supply of a minimum -3V, or use an opamp whose input can go to ground with a grounded negative supply, such as an LM324.

Also 1kΩ resistors are rather low value for standard op amps.
Values around 10kΩ are more typical.
Thank you very much for the information!

I think LM324 would work for my case, because I only have a single power supply.
 

Audioguru again

Joined Oct 21, 2019
3,833
The LM324 is also old and has 4 opamps in it. It produces a kink in its output waveform at half its supply voltage called "crossover distortion". It might not affect your 0V to 5V output with your high 15V supply.

An LM358 has only 2 of the same opamps as in an LM324 and is in a smaller 8 pins package.
I agree that higher value resistors should be used.
Disable the unused opamps.
 

Thread Starter

circuitnaiver

Joined Oct 1, 2021
22
The LM324 is also old and has 4 opamps in it. It produces a kink in its output waveform at half its supply voltage called "crossover distortion". It might not affect your 0V to 5V output with your high 15V supply.

An LM358 has only 2 of the same opamps as in an LM324 and is in a smaller 8 pins package.
I agree that higher value resistors should be used.
Disable the unused opamps.
Thank you for the reply.
I need the output value to be around 6volts with around 2.7 volts input. It seems like LM358 also works. But which one would work better in this case? Do you have any suggestions?
 

Audioguru again

Joined Oct 21, 2019
3,833
The LM324 and LM358 have a very slow slew rate of 4us/volt which will cause the narrow 1kHz output pulse (50us) to almost resemble a narrow triangle.
EDIT: The old 741 opamp slew rate is a little better at 0.5us/volt so the pulse will have slanted sides.
 

Audioguru again

Joined Oct 21, 2019
3,833
So what you mean is that, it might not make the 1khz 10% duty cycle signal work?
Simply do the math. The LM358 has a slew rate of 4us per volt and you want a 6V output which will take 4us x 6= 24uS.
Your 10% duty cycle rectangular pulse of a 1kHz signal has an output duration of 50us so the output pulse is a triangle waveform that barely reaches the 6V. 24us up, 2us at 6V then 24us back down.
 
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