[SOLVED] Opamp works fine with DC - but getodd values with AC

[kochevnik]

Hi all -

I'm a beginner

I have an inverting opamp circuit (similar to attached) that is working just fine with DC on the I- pin but when I set up my function generator to output an AC signal of roughly the same voltage P-P the opamp is outputting a similar sinewave, but it's not amplified very much.

The opamp is a MCP6241-E/P with 550khz gain bandwidth product (datasheet attached). I'm powering it with a voltage 5.35 V on Vdd and Vdd/ 2 = 2.66v on I+ and this opamp is supposed to go rail to rail (which it does with DC just fine).

The basic circuit I have is with a 1 M ohm resistor from Vout to I- and then I have a bunch of different resistors I try on Vin 800k 467k 325k 200k etc this lets me easily watch the change on Vout as I change the gain. This all works great with DC - I get almost the exact right voltages on Vout that would be predicted from the opamp formulas.

So now I hook up a 150khz - not quite a clean sine wave - from my signal generator but fairly close, then I use the EXACT same setup, only adding a 4u7 cap on Vin (I was under the impression this is a DC blocking cap) and then check Vin and Vout on my scope while I change the resistor values on Vin (same values I have listed above).

The output is a nice clean sinewave, but the peak to peak voltage does not vary much (5% maybe) as I try the different resistor combinations.

For example - my not so clean sine wave going into Vin is 0.285v P2P and on Vout I get a nice clean sinewave of 0.530 volts P2P - for ALL of the different resistor combos above. I switch them and I get some small changes in Vout, but nothing like the changes I got with the DC testing.

And a few odd things - If I run Vin directly into pin2 without a resistor or cap, I still get a nice sinewave on Vout, only the voltage is now 0.23v P2P not the 0.53 volts I get when I try all the resistors.

So what gives ? What am I doing wrong ?

Thx -k

 

[keith1200rs]

150kHz? Your opamp only has 550kHz GBW so you will get very little gain at 150kHz. Your resistors are set to give you a gain of 100 in the diagram. Try a lot lower frequency - say 1kHz.

Keith

 

[kochevnik]

Thanks - appreciate the replies - but the circuit is just an example - remember that the test resistors I used were 1M ohm for the feedback resistor and 810k ohms for the resistor on Vin - this gives a gain of -0.810 for an inverting amp.

Or for one of the other resistors I tried (325k ohms) this gives a gain of
-0.325

according to this calculator :

**broken link removed**

Example :

r1 = 1000
r2 = 810
r3 = 0
r4 = 1
v1 = 0.1425 (1/2 of .285v ?)
v2 = 2.66
Vp = 5.35
Vn = 0


gives a Vout = 4.70v

changing v to 0.285v (?) still gives 4.58v

Neither of these is close to what I actually saw on the scope.

- - - Updated - - -

- - - Updated - - -

I think I got the answer to this - figure 2-15 in the spec sheet for the opamp shows the max output voltage swing and it is limited @ 150khz.

Thx keith for your help.

Solved.

 

[keith1200rs]

I was just going to say that you will be slew rate limited so while you could get a small gain at 150kHz, that would only be with a small signal.

Keith

 

[LvW]

I was just going to say that you will be slew rate limited so while you could get a small gain at 150kHz, that would only be with a small signal.
Keith

Yes - of course, it is the limited slew rate (0.3V/usec) that is responsible for the problems.