I'm simulating an LM833 in inverting mode, very simple. just the two resistors. And according to all datasheets the GBP of this amplifier is typicaly 15 Mhz. This means that in the circuit of my amplifier, that has two resistors of 10K, the gain is 1, so the bandwidth of my circuit is half the GBP (correct me if i'm wrong, i'm new in this stuff). Why the simulation in pspice show me that pole in 9Mhz?? Did anyone had the same problem?
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Hello toto,
This is a good question.
I have only the model from ONSEMI. Their model has a GBW of about 13MHz calculated from a measured/simulated open loop frequency rsponse at 100kHz. The open loop unity gain frequency is 10Mhz in the LM833 model from ONSEMI's webpage.
Don't rely on the datasheet here. You should measure GBW of your model before you try with the theory.
My model gives a -3B-bandwidth of 9Mhz for a gain of 2 or -1. From the theory we would expect a BW between 5MHz and 6.5MHz for this gain setting.
What theory do we have in mind?
Fc = GBW/gain Fc=-3dB corner frequency
This theoretical model assumes a perfect 90 degree phase shift of the open loop gain. Do we have that in our LM833 SPICE model? The answer is no! We have phase a shift of about 128degree at 10MHz (0dB gain). The SPICE model and the real opamp are at least a second order system whereas the theoretical model is based on a first order model. Now choose a gain of 100. Here we have 90dgree phase shift in our SPICE model too. The simulaton gives a banwidth of about 130kHz. This is a good match with the theory. (13Mhz/100) Summary: We can use the the therotical formula for bandwith calculation only up to a frequency of about 1/10 of GBW, because the phase shift is increasing for higher frequencies.
I have to admit that I never thought about that before, because opamps are normally never used so close to their GBW. GBW is a very weak specified parameter. It may vary from 10MHz to 20MHz from part to part for the LM833. Be aware that the opamps from all manufacturers have a large tolerance of their GBW.
Best regards, Helmut
I have simulated with LTspice.
Thak you Hetmul, that was very useful. I'm taking a course in college of design with operational amplifiers, and the idea of the first lab is to push the model to it's limits, and books normally avoid this kind of problems. Helmut Sennewald ha escrito:
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Hello Toto,
thanks for your feedback. Could you please forward my answer to your teacher and other course attendees too. Maybe they also have never thought about that as I did in the past. Does anybody in this course know LTspice?
Best regards, Helmut
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