fully differential opamp implementation

google "three opamp differential amplifier". Or cheat and use a Spice vcvs, which is a perfect diffamp.

John

John Larkin a écrit :

But the output is not diff.

Or cheat and use a Spice

But the output isn't referenced WRT to common mode. You'll have to use 2 VCVS for this, or 3 if you don't want to reflect the output current to the CM ref input.

I'll knock one out for you for $25K, payable upfront ;-)

...Jim Thompson

Thanks very much John for the response.

I was designing my fully differential opamp using "three opamp differential amplifier" concept initially because it only requires two basic opamps and 4 resistors which my simulator has all the components.

Unfortunately, after I connect the input of the three opamp differential amplifier to the output of my modulator, I get "timestep too small" error message when simulate. Do you know how to debug this problem?

Currently, my circuit can simulate fine using vcvs as you suggested.

Do you know if there are any disadvantage for using vcvs instead of normal fully differential opamp? Would it affect my output signal spectral analysis significantly?

Thanks again for your assistant

Hi Fred Are you talking about "common mode feedback circuit"?

I was reading about that when I was researching about fully differential opamp. Unfortunately, I can't find a detailed explanation about the CMFC(i.e. the articles only show the schematics but not the values I should use for MOSFET, capacitor, resistor and reference voltage). So I really don't know how to integrate CMFC with opamp.

Do you have experience implementing the fully differential opamp? Can you send me a netlist of that, so I know what values to put for each components

Thanks

Thanks for the offer Jim, but I am a poor engineering student. So I can't afford that kind of money

But I think I can offer you $1 if you can help me design a fully differental opamp. $2 if you can help me design a chopper stabilized amplifier

"Perfect" VCVS has no delay or phase shift, infinite bandwidth... all of the constituents of instability.

Back up and study how to model an OpAmp, with at least one pole.

...Jim Thompson

Is that what he meant by "fully differential"?

Whatever.

John

If you are trying to evaluate the spectral properties of a diff-output mixer, the vcvs is literally ideal. An opamp thing will just add its own complications and abberations.

If you eventually want to build the circuit, you will need a diffamp, I suppose. Or a transformer. There are some nice, integrated, very fast diffamps around these days.

John

Hi,

If he search the spice models he should look by the factories...

may be a good start.

Marte

Thanks, I guess I need to learn how to use Pspice now.

Can Pspice generate the Power Spectral Density of a signal? I need to do some spectral analysis in my circuit

Thanks

With the 3 opamp IA circuit you can just add an identical output differential amp stage with the inputs inverted to obtain the other output signal.

Yes.

Yes. You can do a spectral analysis in Probe, or can define distortion analysis in the setups to get a table.

...Jim Thompson

Hi tiger,

You do not need to switch the program. Multisim can use (p)Spice models as well as LTSpice for example. The changes that may have to be made are very little and almost known in the support forums.

Marte

... who prefers to use LTSpice

Even it can, and even if you do it right on simulation, on the real world will be different (could be very different at high frequencies) Focus on fully differential OA datasheet available at Analog Devices, Texas Instruments and Linear Technology. There are a lot of articles about those. Some clues: AD8132, THS4121, LT1994

The cold end of the resistor going into the + input of the 3rd amp is the output reference; it need not be ground.

John

Hi All Does anyone have experience using multisim?

I keeps getting a "timestep too small" error message when I simulate. I tried all the debugging method I found online, but non of them solves my problem

Any input/assistant on this would be really appreciated

thanks

Hi John I tried to implement the circuit in Fig4 of the article "A COMS Chopper Amplifier" by Christian C. ENZ.

Can you take a look at the circuit diagram. If you can't find it online, I can email you a copy

Is there something wrong with the amplifier connection because the circuit shows that all the - inputs are connect to the output and the

Thanks

Why don't you post the schematic to alt.binaries.schematics.electronic ?

We can't see the schematic because it's locked up by IEEE... seeable by fee only :-(

...Jim Thompson