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  #1  
Old 08-04-2009, 04:25 AM
jbjb jbjb is offline
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Default LM741 voltage follower circut

Hello all

I've been working on a simple voltage follower circuit that will be used to protect an analog output on a data acquisition/output board that is connected to a computer. The voltage follower will go between the analog output and will drive a single LED. I planned to vary the outputted voltage to vary the current that goes through the LED.

Even though this circuit is really simple, it hasn't been behaving as it should. Two problems I've had with it are:

The Op amp is still drawing current from the source, even though it should have a nearly infinite input impedance. The current it draws from the source is about the same amount of current that the LED is receiving.

What gives?

This problem has been super frustrating because it should be so simple. I've tried building the circuit in different ways and am not sure if I've fried something or not.

I've attached my schematic. The op amp is powered by one 9V battery. The input can go from +10 to -10v, though I'm basically planning to use in the 3-4V range.

Thanks!
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File Type: png vfollower1.png (32.1 KB, 87 views)
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  #2  
Old 08-04-2009, 05:15 AM
t_n_k t_n_k is offline
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Looks like you have a couple of issues (at least)

1. There are no ground references on your simulation schematic.
2. There is no series resistance for the LED.
3. You will probably need to use a split supply with center ground.
4. I don't think the LM741 is spec'd to run off a 9V supply.

Be advised - Several folk on this forum have a serious disliking for this op-amp.
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Old 08-04-2009, 05:28 AM
jbjb jbjb is offline
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Thanks for the reply.

I meant the drawing to be a representation of the circuit that I actually prototyped on a breadboard and so the problem of current being drawn from the source is an actual one, not just one from simulation. Though yes, you're right -- I should put the ground in there else the simulation doesn't work.

I have tried connecting it up using a "virtual ground" by using 2 9V batteries and two 4.7KΩ resistors connected as such:

+ ----------------- +9V
9V \ 4.7
- /________virtual ground
+ \
9V / 4.7
- ----------------- -9V

with still the same problem that current is being drawn from the analog source.

I thought the LM741 was the "go to" workhorse amp? Id be willing to swap for something equally cheap and available.
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  #4  
Old 08-04-2009, 06:41 AM
t_n_k t_n_k is offline
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It would be useful if you draw the complete circuit as it is intended to function. Show all connections including the input and LED with ground points.

I'm not sure what your design concept entails with respect to driving the LED.

Are you hoping to see the LED brightness change as the analog output changes?

If the input to be buffered goes negative then you would need some steering diodes to still have the LED illuminated with negative inputs.

Keep in mind also that the LM741 will draw some quiescent current.

If you are using two 9V batteries then you don't need to create a virtual ground with the 4.7k resistors - the common connection point for the two 9V batteries can be the reference or ground. Did you follow what I said about needing a series (limiting) resistor for the LED?
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  #5  
Old 08-04-2009, 10:24 PM
Audioguru Audioguru is offline
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The input voltage of a 741 opamp does not work when it is near its negative supply voltage. The input of an LM358 works fine.

I could not remove all the Chicken Pox dots all over your schematic.

EDIT: Two ways to do it.
Do it like this:
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File Type: png opamp follower.PNG (11.1 KB, 84 views)

Last edited by Audioguru; 08-04-2009 at 10:37 PM.
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