Not getting proper gain in opamp LM324N

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In summary, a user is experiencing a problem with their instrumentation amplifier circuit built using the LM324N chip from ON semiconductor. They chose resistor values to give a gain of 640, but when checked, the gain was only around 175. The user is unsure of the problem and is using a split supply of +12V and -12V. They also have not connected the unused pins on the fourth LM324 opamp. Another user suggests using a follower configuration and matching resistor values closely. They also mention that the LM324 has a unity gain bandwidth of 1MHz and a limitation on frequency. The concept of rail-to-rail ICs is also discussed, which refers to opamps that can swing their output voltage close to the
  • #1
arunbhaskar
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I used LM324N(4 opamps on 1 chip) from ON semiconductor to build an Instrumentation Amplifier circuit. I chose the values of the resistors so as to give a gain of 640. But when i checked the gain on the CRO the gain was around 175. What problem could this be?
Datasheet link:http://www.onsemi.com/pub_link/Collateral/LM324-D.PDF"
 
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  • #2
What impedance is the output driving? What is your Vcc? Please post your circuit.

Bob S
 
  • #3
I used this circuit:
http://img195.imageshack.us/img195/9979/instamp.jpg
I observed the output directly across the CRO.
So don't have any idea about output driving impedance...
 
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  • #4
arunbhaskar said:
I used this circuit:
http://img195.imageshack.us/img195/9979/instamp.jpg
I observed the output directly across the CRO.
So don't have any idea about output driving impedance...

What voltages are you using for your power supply rails? What voltages are your inputs? (not super-important for experimenting, but what are you doing to tie off the 4th LM324?)
 
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  • #5
that fig 13 equation is missing something
 
  • #6
LM324 has compatibility for split supplies and single supply.
I used split supplies +12V and -12 V. Yeah I forgot to mention that I wasn't getting the output with +5V and ground. I'm sorry but I'm very new to op-amps. I left all pins of op-amp 4 open.
What do you mean by tie off? Am I supposed to connect the pins of opamp which is not being used?
 
  • #7
I did the experiment on a trainer kit and rigged up the circuit using patch cords. Is there a chance that I'm not getting proper gain because of noise picked up by the cords?
 
  • #8
arunbhaskar said:
What do you mean by tie off? Am I supposed to connect the pins of opamp which is not being used?

It's good practice to tie off unused gates and opamps/comparators/etc. In this case, just ground the + input and configure it as a follower.

On your gain question, what is your input signal (both differentially and common-mode)? With your target gain, what would that give you at the output?
 
  • #9
Could you also indicate the frequency and level of the input signal. The LM324 has only 1MHz unity gain bandwidth, along with fairly modest slew rate capability - see datasheet enclosed.

Your circuit uses three amplifier cells, and I can't say exactly what the bandwidth would be by just looking at it, but I would guess that it won't be more than a few kHz for a gain of 640. Perhaps therefore what you are seeing is a frequency limitation.
 

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  • #10
with lm324 you must ensure your circuit operating voltages stay well clear of the supply voltages. This is not a rail-to-rail op-amp.

With an instrumentation amp circuit show, it is very important to match the like-value resistors as closely as possible.
 
  • #11
You should start out with C = a = b = 1, and R = R1 = 10k, and then increase C to 5, and a and b to ~10. This will give a gain of ~5 x 21 = 105. You should avoid any feedback resistors less than ~ 2K or or over ~100K. The gain-bandwidth product of the LM324 is ~1 MHz, so for a gain of 175, you should expect a small-signal bandwidth of ~5 KHz.

Bob S
 
  • #12
What are rail to rail ICs. I tried to find out on the internet but didn't find a satisfactory explanation. Can anyone explain the exact meaning of rail to rail...
 
  • #13
arunbhaskar said:
What are rail to rail ICs. I tried to find out on the internet but didn't find a satisfactory explanation. Can anyone explain the exact meaning of rail to rail...

That refers to an opamp that can swing its output voltage very close to the power supply rails, and may also accept input voltages that are close to the power supply rails.

So for example, if you are running the opamp with +/-12V supply rails, the inputs and outputs can swing very close to those supply rails.

The original LM741-type opamps were not able to do this, because they were made up of bipolar transistors and needed some headroom from the rails at their inputs, and could not drive their output to more than a volt or two from the rails. The newer CMOS opamps, at least the more expensive ones, can have much closer to rail-to-rail performance.
 

1. What is an opamp LM324N and how does it work?

The LM324N is a type of operational amplifier (opamp) commonly used in electronic circuits. It is designed to amplify small signals and perform mathematical operations on them. The LM324N contains four opamps in a single package and can operate with a wide range of supply voltages.

2. What are some possible reasons for not getting proper gain in opamp LM324N?

There are several potential causes for not getting proper gain in an opamp LM324N. These can include incorrect wiring, improper power supply, incorrect component values, and external interference or noise.

3. How can I troubleshoot and fix a low gain issue with an opamp LM324N?

To troubleshoot and fix a low gain issue with an opamp LM324N, you can start by checking the circuit for any errors in wiring or component values. You can also try adjusting the power supply voltage and adding decoupling capacitors to reduce noise. If the problem persists, you may need to replace the opamp or check for external interference.

4. Is it possible to get too much gain from an opamp LM324N?

Yes, it is possible to get too much gain from an opamp LM324N. This can happen if the feedback resistors are too large or if the opamp is configured in an inverting amplifier circuit with a gain greater than the opamp's open-loop gain. In these cases, the output may saturate at the power supply rails, resulting in distortion or clipping of the signal.

5. Can the opamp LM324N be used in high-speed applications?

No, the opamp LM324N is not suitable for high-speed applications. It has a limited bandwidth and slew rate, which can lead to distortion or signal loss at higher frequencies. For high-speed applications, a different type of opamp with a higher bandwidth and slew rate would be more appropriate.

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