Help! Maximum gain of LM741 op amp

Thread Starter

momotee

Joined Jan 18, 2012
14
Hi, I have to implement an inverting gain of 10000.
I configured a LM741CN op amp as an inverting amplifier.
Using +/- 15Vcc.
Assuming no saturation of the signal, can the gain of 10000 be realized?

Or do I have to split the gain into different parts,
Eg. 100 and 100 (using to op amps) to get 10000.
 

ssnyde

Joined Apr 4, 2012
19
Hi,

I think more important than the gain is the amplitude of your input signal. To not saturate an inverting amp with gain 10k and +/-15V, your input signal has an absolute value of < 1.5mV. Input offset voltage is typical 2mV, so your output will saturate with your input grounded. So you will need to use the offset null terminals to trim that out. Also keep the resistors under 1meg or at least under 10 meg or input bias current will cause an offset.

I think you're better off using two op-amps.
 

justtrying

Joined Mar 9, 2011
435
the other way to deal with offset is to split it into two parts and use a high pass filter in between two gain stages to remove the DC offset. This could help as sometimes it is not possible to fully null the op-amp (I've had that problem before).
 

Audioguru

Joined Dec 20, 2007
11,249
The lousy old 741 opamp is 44 years old and is too noisy (hiss) for high gain.
Its gain at 50Hz is not much more than 10000 so there will not be much negative feedback so the output will be distorted.
Use a low noise audio opamp instead.
 

#12

Joined Nov 30, 2010
18,223
You are definitely going to have to use a nulling circuit to get rid of the DC offset at the input. Theoretically, the 741 can do 50,000 gain unless I read that wrong and its 50,000 gain-bandwidth product, at which point you are asking it to go ten times as fast as it can go. That wiil require two stages. Try it with one stage and find out!

If it's incredibly picky and distorted, use two stages.
 

Ron H

Joined Apr 14, 2005
7,014
I agree with Audioguru on the choice of op amps.
Regarding offset voltage, the OP said that the input is a 50Hz sine wave. Justtrying posted the solution to the offset problem, except I would AC-couple both stages.
 

Ron H

Joined Apr 14, 2005
7,014
AC coupling as suggested in post #4?
Here is what I wrote:
Justtrying posted the solution to the offset problem, except I would AC-couple both stages.
Justtrying did post that in post #4.
I suppose a gain of 100 in each stage would allow a single cap to be used, as suggested in post #4. My bad.:(
 

Thread Starter

momotee

Joined Jan 18, 2012
14
You are definitely going to have to use a nulling circuit to get rid of the DC offset at the input. Theoretically, the 741 can do 50,000 gain unless I read that wrong and its 50,000 gain-bandwidth product, at which point you are asking it to go ten times as fast as it can go. That wiil require two stages. Try it with one stage and find out!

If it's incredibly picky and distorted, use two stages.
Assuming a two stage gain of 100 and 100,
if the output of the first stage already saturates, it means that the second stage also saturates right?

That means for a gain of 10000, and a Vcc of +/-15V,
my input signal cannot be more than 1.5mV,
regardless of how many stages of gain I use?
 

Audioguru

Joined Dec 20, 2007
11,249
A lousy old 741 opamp with a supply that is plus and minus 15V has its output saturate at about plus and minus 13V peak. With a gain of 10,000 its input can be no more than 13V/10,000= 1.3mV peak which is almost nothing. The noise level might be more.
 

#12

Joined Nov 30, 2010
18,223
Yes, 15 divided by 10,000 is .0015, no matter how many stages you use.

or, as Guru said, 13 divided by 10,000 is .0013

no matter how many stages you use.
 
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