OTA design simulation in LTspice

[sudhanwaaa]

Hi,I have designed an OTA circuit and I want measure transconductance ,dc-gain,GBW in LTSpice. And One more thing,Should I connect load capacitor of 1pF for this measurement?
Basically,I am new to OTA simulation,so any help will be appreciated.

 

[Nig]

Hello,

You need to run DC simulations to find the operating point of various devices ( like to check whether they are in saturation or not).
AC simulation (Freq sweep from 10 - UGB*10) can be used to find the transconductance, DC-gain and Gain Bandwidth product.
Connecting a load cap ( 1pF in your case) is mandatory to measure the Bandwidth and stability(if applicable) of the OTA. Load cap will not effect DC simulations.

 

[Audioguru]

A capacitor with a value of only 1pF is nothing. One picoFarad is one-millionth of one microFarad.

 

[erikl]

A capacitor with a value of only 1pF is nothing. One picoFarad is one-millionth of one microFarad.

Right. And the input cap of the connected gate on the same chip including routing could well be in the order of 100 femtoFarads. So 1pF is surely more than nothing.

Hi AudioGuru! We live in the era of sub-micron, no: sub-100nanoMeter structures!

 

[Audioguru]

I have never designed or built an IC, instead I buy ones that are already made.
If I make an OTA with a 2N3904 and 2N3906 at the output then its output capacitance is 9.5pF plus at least 6pF for the pcb, before a load is added.

 

[erikl]

I have never designed or built an IC, instead I buy ones that are already made.

Good. I have done both.

If I make an OTA with a 2N3904 and 2N3906 at the output then its output capacitance is 9.5pF plus at least 6pF for the pcb, before a load is added.

You are totally right. And I've designed and layouted many OTAs (still in sub-micron processes) with output caps in the order of 300fF and input caps around 100fF, and perhaps 100fF more for the connection. So there are people for whom 1pF is nothing at all, and there are other people for whom a pF is a good value for an expected output cap for an OTA. Please!!!

 

[sudhanwaaa]

Can someone be specific about simulation process,I am using LTspice,I have applied 10mV signal at inverting and non-inverting terminals out of phase by 180.So once I am in AC analysis,is it Icap/(V1-V2) plot that i need to analyze?

 

[FvM]

By definition, transconductance is measured with shorted output. You'll also measure output impedance and can calculate voltage gain with different output loads, e.g. load capacitor.

In so far, output capacitance is irrelevant for the parameter measurement, but of course matters for the application circuit.

 

[erikl]

Can someone be specific about simulation process ...?

Sure, quite specific, see here: View attachment Cadence_Characterizing_Differential_Amplifiers.pdf