LTSpice: Problem with open loop gain analysis of an OP-AMP

[Mad I.D.]

Hi.

In my project I'm using AD8051 opamp from Analog Devices. I have downloaded the
SPICE model and included it in a spice schematic.

I would like to simulate the open loop gain of the opamp. The problem is that I
get peculiar plots from it. Gain at DC equals 0dB and the GBW is too low for
that kind of amplifier. Obviously something is wrong. If I try to simulate
LTspice intrinsic opamp model, I get expected results.

The major difference is that AD8051 model is a single supply model with 5V on
V+. Maybe I have to set up some kind of Q point first, for example 2.5V (half
the supply) but I don't know how. I have tried several options with no success.

My .asc file is included in this post.

If someone can help with this I would be very thankful.

Ivan, student.

 

[keith1200rs]

Have you a gif or jpg of the schematic?

Keith

 

[Mad I.D.]

Re: LTSpice: Problem with open loop gain analysis of an OP-A

keith1200rs, that you for your interest in this.
The problem was solved couple of minutes ago.

If you are still interested in the schematic and the solution let me know and I will put all information here.
Basically, single supply opamps have to be biased before running the AC analysis. I didn't know how to bias it and still calculate the open loop gain.

There is a SPICE trick by which one can change value of the resistor depending on which simulation is running, i.e my feedback resistor has a Value=10k AC=100G. That is, 10k sets the Q-point, while 100G basically removes feedback.

Ivan

 

[keith1200rs]

Ivan,

If you have solved it, that is fine - I was planning to help but don't have Ltspice on my mobile phone so can't read the asc file.

You are right - you need to make sure you bias the amp in its input/output range when doing any simulations. Displaying the DC bias can help track down suspicious simulations.

Keith

 

[electrocomm]

Re: LTSpice: Problem with open loop gain analysis of an OP-A

If you are still interested in the schematic and the solution let me know and I will put all information here.


Ivan[/QUOTE]

I tried to simulate an opamp in LTSpice using one AC 1 source connected to the noneinvertin input, a feedback resistor for decoupling the loop in AC (AC=1G), but i dont get what i expected. Don't you think that an aditional capacitor (1F cap) is needed between AC source and noninverting input for decoupling in DC? Can you post some information that you think might help me? Please.
Thank you so much!