Operational Amplifiers question

[iamtrout]

Say that you've got an operational amplifier, with the +/- VCC set at +/- 5V. Why does the gain decrease as the voltage input into the amplifier increases? Is it because the saturation point at +/- 5V is being approached?

 

[Gibson486]

Doesn't it depend how you use it? I means, if you have unity gain, then it wouldn't really matter.

 

[iamtrout]

We haven't learned anything about unity gain 🙁

This is just for an ECE lab, and all we're doing is hooking up an op amp, increasing the voltage in, reading the output, and dividing them to get the gain, but I have no idea why the gain starts decreasing as voltage in goes up.

The textbook is no good because it treats all op amps as ideal op amps.

 

[TuxDave]

Yes, as you approach +/- 5V, the transistors that make up the amplifier leave its saturation region (high output resistance/high gain) enter it linear range (low gain/low output resistance).

 

[jmcoreymv]

In an opamp, your output voltage cannot exceed your power supply rails. So if you are supplying +/- 5V, once your output hits those levels it will clip and stay at that level. It only acts with a specific gain in its linear range. Also the true gain will depend on the frequency of the input as well (and the chip being used).

 

[Gibson486]

Unity gain is just when the out put is hooked to the input, so gain is 1 or 0dB. If anything, i would think it would be constant, but, like you said, that is only with an ideal op amp.

 

[iamtrout]

Thanks for the input so far. I'm just a bit confused on the data, because it feels to me like it's losing a lot of gain very early...

Rails are set at +/- 5V.

Vin.......Vout.......Gain
1.3.......-2.6........2.0
1.5.......-2.9........2.0
1.7.......-3.3........2.0
2.0.......-3.4........1.7
2.2.......-3.4........1.5

Now, I have absolutely no "feel" for when gain should drop, but Vin is only at 2.2V, less than half of the saturation point, but gain has already dropped 25%??

Does this look normal?

 

[Gibson486]

Why is Vout negative? I mean, you could have a negative gain, but if this is intro stuff, i don not inderstand whythey would have you get a negative gain.

Also, if your school uses cheap op amps (most do) then youwould porbably start clipping when you do not want too....which explains why gain begins to lower quickly.

 

[TuxDave]

Depending on the op-amp you're using, it also may have a limited output range. Do you have the white papers? Also are you calculating small signal gain? Because if you are and you're using DC voltage, you need to take the dVout/dVin.

 

[iamtrout]

Eh, I dunno. The gains were calculated to be negative, I just didn't write them in. All we're doing is reading off the oscilloscope. I guess cheap op amps is a good reason to use in my lab report 🙂

 

[hypn0tik]

Originally posted by: Gibson486
Why is Vout negative? I mean, you could have a negative gain, but if this is intro stuff, i don not inderstand whythey would have you get a negative gain.

Also, if your school uses cheap op amps (most do) then youwould porbably start clipping when you do not want too....which explains why gain begins to lower quickly.

Inverting configuration = negative gain. The inverting configuration is actually a little easier to analyze than the non-inverting configuration.

What resistors are you using?