lm741 help -- Sine wave amplifier

Thread Starter

Zeeus

Joined Apr 17, 2019
406
Tried to do this..no knowledge on op amp..will read AOE but just decided to briefly visit an old circuit from a beginner book (Electronic circuits for the evil genius)

Wrong? :

Since feedback is applied, input to inverting is 4.5V which is equal to output voltage

It checks out with multimeter but now when I connect function generator (without turning it on) to capacitor, the input to inverting goes down thereby taking output close to 9V (How to fix?)

To fix, figured needed to add pot on offset pin, it worked at first : the output was close to 4.5v..When I applied sine wave input though, the output was an attenuated sine wave

I was expecting to get (100meg/1k) inverting sine wave gain : guess this should be the answer if input to non inverting is 0V but from the book and spice, seems the right answer should be a square wave

someone please enlighten me
 

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Thread Starter

Zeeus

Joined Apr 17, 2019
406
What are the supply voltages on your LM741?
Datasheet states minimum supply voltages are ±10V.
oh really??

Used 9v and 0v...The book uses that..schematic is from the book


What should be output waveform with input sinewave and what is gain too
 

AlbertHall

Joined Jun 4, 2014
8,313
What type of capacitor did you use for C2?
If you used an electrolytic then that is probably the problem. The leakage current of the capacitor must flow through the feedback resistor and as it is a very high value will mess up the DC voltages.

In the simulation, add a resistor across C2 to represent the leakage resistance. With 10MΩ the DC output rises to 8.09V
 

Thread Starter

Zeeus

Joined Apr 17, 2019
406
What type of capacitor did you use for C2?
If you used an electrolytic then that is probably the problem. The leakage current of the capacitor must flow through the feedback resistor and as it is a very high value will mess up the DC voltages.

In the simulation, add a resistor across C2 to represent the leakage resistance. With 10MΩ the DC output rises to 8.09V
Thanks...C2 is input cap..changing it to 0.1UF gives square wave output and dc output is 4.5v

but using this small cap puts f3db high? (guess to adjust R4)

"add a resistor across C2" : in parallel? Tried it, output about 8v like before

"The leakage current of the capacitor must flow through the feedback resistor and as it is a very high value will mess up the DC voltages." Every capacitor does this?
 

AlbertHall

Joined Jun 4, 2014
8,313
Different types of capacitors have different leakage currents and electrolytics are worst. You should be OK with ceramic or film capacitors.

"add a resistor across C2" : in parallel? Tried it, output about 8v like before
Doing this in the simulation (definitely not in the real circuit) will show you the effect of the capacitor leakage current on your circuit.
 

bertus

Joined Apr 5, 2008
20,042
Hello,

Reading the datasheet, the LM741 needs at least ± 5 Volts.
Even then the output swing is minimal:

LM741_intersil_output_swing.png

Also it will not be possible to create a gain of 10000 on 1 kHz as in your schematic.
The GBW is only 1 MHz.

Bertus
 

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Thread Starter

Zeeus

Joined Apr 17, 2019
406
Hello,

Reading the datasheet, the LM741 needs at least ± 5 Volts.
Even then the output swing is minimal:

View attachment 178274

Also it will not be possible to create a gain of 10000 on 1 kHz as in your schematic.
The GBW is only 1 MHz.

Bertus
The output is not a sine wave as expected by me..was just testing op amp: no study yet...Please delete this thread

edit : oh maybe it is sine wave but since the gain is huge and above supply then it becomes square wave?

Noticed this before : to get square wave from sine wave : transistor amplifier with large gain to go above supply : output becomes square wave
 

AnalogKid

Joined Aug 1, 2013
8,100
I was expecting to get (100meg/1k) inverting sine wave gain
Not on this planet. First, look up the gain-bandwidth product of a 741, divide that by 1 kHz, and see how much gain is available from the chip. Spoiler - it is not 100,000.

Second, at a gain of 100,000, an input offset voltage error of 45 uV will saturate the output. Compare this to the input offset voltage spec on the datasheet.

So will an input signal level of 45 uV if the input offset voltage error happens to be 0.000000 V. How do you get the signal generator output to be that level?

So will an input bias current error of 45 nA.

I suggest reducing the series feedback resistor to 3.0 K to see if the opamp behaves as expected.

ak
 

Audioguru

Joined Dec 20, 2007
11,251
The design of a 741 opamp is 53 years old! Why use it today? Kiss it goodbye and bury the old thing.
The 741 opamp design is so old that it will not work with a feedback resistor as high as 10M.
Why are you attempting to get a voltage gain of 10,000 times anyway?
 

Thread Starter

Zeeus

Joined Apr 17, 2019
406
The design of a 741 opamp is 53 years old! Why use it today? Kiss it goodbye and bury the old thing.
The 741 opamp design is so old that it will not work with a feedback resistor as high as 10M.
Why are you attempting to get a voltage gain of 10,000 times anyway?
LOL.. This is like the 6th time I'm reading you saying this

Thanks
 

atferrari

Joined Jan 6, 2004
3,441
LOL.. This is like the 6th time I'm reading you saying this

Thanks
Well, meantime, I hope yours is the last time you try to use that opamp.

With Audioguru I learnt a lot about opamps. Instead of counting the above, follow his advice.
 

Thread Starter

Zeeus

Joined Apr 17, 2019
406
Well, meantime, I hope yours is the last time you try to use that opamp.

With Audioguru I learnt a lot about opamps. Instead of counting the above, follow his advice.
Yes.. Understood current source to bias npn then 2 diode drops then PnP from him (or her?)

Going to get chips tomorrow

Just view the lm741 posts similar to this
 
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