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Basic Electronics » Op-amp not working

November 22, 2010
by lcruz007
lcruz007's Avatar

Hello,

I'm currently working on an operational amplifier circuit. According to my calculations, the circuit I built should give me a gain of around 1000 (30dB). However, it seems that the input voltage is dropping a lot when I check the op amp's output.

I've tried the 741 and the LM358N and none worked for me. I have also tried changing the resistors values but I don't see a considerable change. What am I doing wrong?

Here are the schematics:

http://img708.imageshack.us/img708/3939/opamp.jpg

Thanks in advance!

November 22, 2010
by rajabalu21
rajabalu21's Avatar

Have a look at this link Page 11 first diagram. Please make sure that Ground and 0 volts are connected. I am guessing you are trying to use the single supply configuration as described in the above mentioned link and so 0 volts is your ground. I also think that your inputs are inverted.

-Raja

November 22, 2010
by mrobbins
(NerdKits Staff)

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Hey Luis,

What's the signal coming in from your function generator? For example a full description might be: 100mV peak-to-peak sine wave at 1kHz, with a DC offset of 2.0 volts (such that the voltage is between 1.95 and 2.05 volts at any given time).

I suspect that you don't actually want ground to be connected to the non-inverting (+) input of the op-amp, because ground is also the negative supply voltage. Therefore, your inverting (-) input is always going to basically be at or above the voltage of the non-inverting (+) input, which means your output is always going to be driven to the low output limit of the op-amp.

When you are working with single supply op-amps, you've got to center your signals around something other than ground. For example, you could make a voltage divider with two 100K resistors and create a 2.5V mid-level reference.

Mike

November 23, 2010
by lcruz007
lcruz007's Avatar

Hello,

I have tried with several signals. The function generator is sending a sine wave output of 500mV of amplitude and 30-100Hz. However, I even tried using my microcontroller to output an square wave (5V of amplitude) in a variety range of frequencies (100Hz-1Khz for instance).

Do you think having a virtual ground would improve it? I'll try using the voltage divider using some resistors and let you about anything I notice and see if it works. Otherwise, I'll post my new schematics.

Thank you!

November 23, 2010
by mongo
mongo's Avatar

From the looks of it, there is a gain of 1000. If your input is 5vP-P, you would expect an output of 5000 v. For a single stage, the negative input would be the correct input to use with the negative feedback to control the gain. I would suggest coupling the input from the signal generator capacitively instead of directly. I would also reduce the 1Meg resistor to something more realistic, like 10K for about a gain of 10 to start with.

Keep in mind, the output will be about 180 degrees out of phase so when the input signal goes positive, the output goes negative.

November 24, 2010
by BobaMosfet
BobaMosfet's Avatar

lcruz007-

You've got it wired as a virtual ground on the non-inverting input. It won't do what you want to do. Go reread the spec-sheet and pay particular attention to summing node, and max input and output voltage.

BM

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