Hi all,

I've got a LM 272, setup as an inverting amp with an amplification of 1(1k, 1k).
The source voltage is 30 V, i've used two resistors with capacitors to create a 15V (center).

The data sheet mentions the 272 has a GBWP of 350 kHz.
If i look at the response from a sinewave:
at 200 kHz, input 1Vpp I get a decent 1Vpp inverted signal from the opamp.
at 200 kHz, input 10Vpp I get a distorted triangular shaped response, Vpp = 2.25.
at 50 kHz, input 10Vpp I get a decent 10 Vpp inverted signal from the opamp.

What is happening here? why is the opamp unable to follow the signal at the combination of high frequency and high voltage?
My presumption was that the GBWP is not related to absolute voltages, is there a different spec that limits?

Thanks,

 

Hi,

In a word, slew rate. Oh that's two words

 

I couldn't find an LM272, but the L272 datasheet says it has a slew rate of 1V/us so to get an output change of 10V is going to take 10uS so a full cycle will take 20uS which makes the maximum frequency at which 10Vpp is possible 1/20uS or 50kHz.

 

Ah..

Specified is 1V/us, which is indeed what I am measuring as maximum rate.
Learned a bit,

Thanks.

 

If i look at the response from a sinewave:
at 200 kHz, input 1Vpp I get a decent 1Vpp inverted signal from the opamp.
at 200 kHz, input 10Vpp I get a distorted triangular shaped response, Vpp = 2.25.
at 50 kHz, input 10Vpp I get a decent 10 Vpp inverted signal from the opamp.

What is happening here?

When you're discussing distorted waveforms, it's best to post pictures...

 

Specified is 1V/us, which is indeed what I am measuring as maximum rate.
Learned a bit,

If you want to learn a bit more, study some opamp circuits and you'll see what causes slew rate limiting.

 

I couldn't find an LM272, but the L272 datasheet says it has a slew rate of 1V/us so to get an output change of 10V is going to take 10uS so a full cycle will take 20uS which makes the maximum frequency at which 10Vpp is possible 1/20uS or 50kHz.

Faster op-amps tend not to have the required voltage rating - that can be corrected by casc-oding the output with a common base stag.