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Design Languages >> Verilog-AMS >> OTA verilog-AMS model
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Message started by Visjnoe on Dec 24^th, 2005, 2:29am

Title: OTA verilog-AMS model
Post by Visjnoe on Dec 24^th, 2005, 2:29am

Hi,

I'm a bit stuck on generating a realistic OTA model, especially with regard to slewrate.

My main idea was to implement a simple small-signal model for the OTA, something like this:

I(out) <+ gm*V(inp,inn);
I(out) <+ V(out)/rout;

This approximation is valid in the region abs(V(inp,inn)) < SQRT(2)*(Vgs-Vt) with Vgs-Vt the (equilibrium) overdrive voltage of the differential input pair. This is in fact the classic 'gm*Rout' model of a simple OTA.

Now, I could include slewrate limiting by using the slew() function on the OTA output.

However, I would like to be able to use the OTA in different surroundings, meaning that I would like to provide some sort of maximum current for the OTA and then let the slewrate arise from the capacitive load the OTA sees at its output.

In contrast, the slew() function imposes a minimum/maximum slewrate, independent of the capacitive load/OTA surroundings. I find this to be a disadvantage for system-level design.

Now, my problem is to unify the small-signal (gm*Rout) model and the slew-rate behavior of the OTA, which is clearly large signal.

At the moment, I only see 1 solution: if the input voltage is smaller than SQRT(2)*(Vgs-Vt) I implement the behavior is shown in the code above and otherwise I would implement it as follows:

I(out) <+ Imax*dir; (dir being -1 or 1)

So, using different branch contributions for small-signal and for large-signal behavior.

Does anyone have some experience with this? All comments are welcome.

Although being regarded as a basic building block, how come I find a OTA harder to model than most other blocks?:)

Kind Regards

Peter

Title: Re: OTA verilog-AMS model
Post by sheldon on Dec 24^th, 2005, 4:28am

Peter,

Try limiting the gm * Vin function. You can use the
arctangent function or something like

I( out) <+ 1/ ( 1/ gm *V(inp, inn) ) + ( 1 / Imax) ;

This functions limits the output current

gm * V(inp,inn) >> Imax --> Imax
gm * V(inp,inn) << Imax --> gm * V(inp, inn)
gm * V(inp,inn) = Imax --> gm * V(inp,inn)/2

The sign can be handle inside of arctangent function.
You will still need to figure out the sign separately
using this approach.

It can be computationally expensive to exactly mimic
the MOS differential pairs limiting characteristics.

Best Regards,

Sheldon

Title: Re: OTA verilog-AMS model
Post by Visjnoe on Dec 26^th, 2005, 4:17am

Sheldon,

thanks a lot for your answer. I was aware of the tanh() approach, but your alternative was new to me.

Kind Regards

Peter