# LM341 circuit with a gain of 1000

#### kris_maher

Joined Apr 24, 2009
90
Hi, Me and my group are working on an ECG circuit. The one where the doctor in hospitals stick the electrodes and measure your electrical pulses. Anyways our lab tutor wants us to use only 1 opamp, he wants an LM341 only and it has to provide a gain of Av = 1000. The input frequencies will only be between 1-40Hz sine wave. The datasheet says the typical gain as 200, not even 1000. I designed a 2-stage amplifier with the LM341, the first stage amplies to 100 and the 2nd stage amplifies by 10 giving an overall gain of 1000. However the tutor said he wont accept it since he only wants 1 LM341 to be used. So my group member proposes we use this circuit a few days later when we met up (another group used a similar one - they 'googled' it and the tutor accepted):

With R3 at 100k, and R1 at 100Ohm so Av = 1000. Our power sources are 2x 9V batteries, V+ has +9V. and V- has -9V. The problem looks somewhat like this (I know lame drawing), it simply does not seem to amplify.

I've included as an attachment my original design in multisim of the 2-stage amplifier using 2 LM341s that I did.

Any ideas? Thanks guys!

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#### kris_maher

Joined Apr 24, 2009
90
Thanks ill check it out but just a question. Getting a LM741 with a gain of 1000 is possible right? It should be I think since its very low frequencies.

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#### SgtWookie

Joined Jul 17, 2007
22,221
An LM741C has a minimum specification of 15V/mV when the load is >2k Ohms.
It has a very slow slew rate, and low bandwidth. However, 15V/mV equates to a minimum open-loop Av of 15,000.

Be certain that the values of resistance that you use for your feedback are high enough so that the output of the opamp is not heavily loaded.

#### Wendy

Joined Mar 24, 2008
22,469
On post 1 look at adjusting R1 and R2 (or R3 and R4) to get the gain you need, per the paragraph Wookie suggested.

#### jans123

Joined Jan 30, 2010
20
You probably must have a real HI-z input for the signal you want to measure, unless you don’t want to bias the nerves and muscles.
On the budget side try CA3130 or CA3140 or perhaps some more modern… and perhaps just a two amp instrumental circuit design with the (+) inputs only in serial with some 100k…1M to keep your “victim” safe, preferably two or more resistors in series of the signal to increase the length of any creeping currents, just for safety. Use connectors with Teflon housing and shielded cables with Teflon insulation. Eventually you could use BNC-cables and connectors.
But, whatever you do, keep anything that can carry any substantial current, like the shield of the cable away from any human contact. - SAFETY FIRST!!!
Your 100kOhm input –I guess it is far to low - my guess only ;-)
With the old OP-s I mentioned you will most likely get a drift, but you will probably get a signal to play with, then, if you still are interested you can do tests with more expensive circuits, but I would have started here.
But again SAFETY FIRST!!!

By the way according to Google the LM341 is a 3-terminal voltage regulator, not an op-amp.

Just found a note on how to use the LM741 for a similar purpose.
http://www.univie.ac.at/CGA/courses/BE513/EKG/lab.html
I won't try it, not if someone else makes the wiring....
Where on earth is this thing grounded (safely)?

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#### Wendy

Joined Mar 24, 2008
22,469
Remember, it is a homework assignment, where the OP must use what is dictated.

I wonder if a differential circuit is actually the best given the constraints.

The comment about the 741 does make me wonder if he is limited to to the LM341. Perhaps a LM341 was meant to be a LM741?

#### jans123

Joined Jan 30, 2010
20
Remember, it is a homework assignment, where the OP must use what is dictated.

I wonder if a differential circuit is actually the best given the constraints.

The comment about the 741 does make me wonder if he is limited to to the LM341. Perhaps a LM341 was meant to be a LM741?
I think so too...
Found a ECG-note (se my link above). How ever that is in my opinion a unsafe circuit that never will be accepted in a medical device.

#### kris_maher

Joined Apr 24, 2009
90
Oh yeah just wanna know, the design I originally did with the multisim simulation on attachment, does that look correct?
thanks

We are basically having V1 voltage being supplied by the left arm, and V2 supplied by the right arm, by electrodes of course.

#### SgtWookie

Joined Jul 17, 2007
22,221
Oh yeah just wanna know, the design I originally did with the multisim simulation on attachment, does that look correct?
No.

Look up the datasheet for an LM741.

See how to connect the offset null inputs properly.

#### Audioguru

Joined Dec 20, 2007
11,249
The circuit is far too simple to be an ECG machine. The oscilloscope is probably showing the mains hum it picks up.
An ECG circuit uses an instrumentation amplifier that has at least 3 opamps plus well-matched resistors, not a single opamp. The circuit for an ECG circuit is shown on the datasheet of most instrumentation amplifier ICs such as the AD620A which I show here:

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#### Wendy

Joined Mar 24, 2008
22,469
I'm still wondering if the OP really has to use a LM741. This is not the most capable chip in the world.

#### SgtWookie

Joined Jul 17, 2007
22,221
Yeah, it's not the greatest circuit.

It's just a basic introduction to differential amplifiers. The instrumentation amplifier was introduced on the 1st page I linked to.

The CMRR will be lousy unless the resistors are well-matched. However, it's more of an introduction to the concept rather than a practical device. As long as they can see something resembling a heartbeat on the O-scope, it's a success.

#### kris_maher

Joined Apr 24, 2009
90
The circuit is far too simple to be an ECG machine. The oscilloscope is probably showing the mains hum it picks up.
An ECG circuit uses an instrumentation amplifier that has at least 3 opamps plus well-matched resistors, not a single opamp. The circuit for an ECG circuit is shown on the datasheet of most instrumentation amplifier ICs such as the AD620A which I show here:
That I know, this is only meant to be one of the parts of the entire system.

And yes, our lab tutor only wants an LM741. I've told my group mates the lm741 is not good but we got to do what the instructor wants us to use.

Yeah with this we are only measuring heart beats.

#### Audioguru

Joined Dec 20, 2007
11,249
Yeah with this we are only measuring heart beats.
No.
You are only measuring mains hum pickup.

The input impedance of your circuit is only 100k ohms for the inverting input and is 200k ohms for the non-inverting input so its inputs are not balanced.
The input impedance of each input of an instrumentation amplifier IC is many megohms and both inputs are the same.

The antique LM741 is very noisy (snap, crackle, pop and hiss) but an intrumentation amnplifier IC is very low noise.

The LM741 opamp has an input offset voltage and input offset current that are typically 67 times higher than an instrumentation amplifier IC.

EDIT:
I notice that you shorted pin 1 to pin 5 on both 741 opamps. Then they probably will not work properly.
Didn't you look at its datasheet where a 10k trimpot connects to pin 1 and pin 5 and to the negative supply voltage to null the input offset voltage?

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