AC analysis issues

[addda]

I have created a CD4007UBE NMOS transistor using a virtual enhancement mode 4 terminal mosfet from Multisim. To test if the imported model was correct I've designed the circuit and compared the DC, transient, and AC analysis to simulations conducted in Mircrocap and PSpice (for the same circuit).

Everything is in the agreement except the AC analysis. The phase response is completely different from Mircorcap and PSpice...The midband gain is in the agreement with the other two softwares, however the low frequency and the high frequency response is way different in Multisim. 

I cannot figure out why this would be the case..... I am using bypass caps in my circuit and I shouldn't be seeing any DC gain...Multisim is showing a DC gain where there should be none. 

Any suggestions will do...I really would like to figure this issues out...

[yyao]

Hi there,

This sounds funny. Can you post your test bench, SPICE model and expected results for us to take a look?

Also, are you using the correct MOSFET level?

Thanks.

[addda]

Thanks for the reply...

Yes, it is definitelly funny... I really hope that I've just made a silly mistake. 

Anyway, I have attached the circuit (vin has no DC offset so the gate is biased at 0 V), spice model used in Microcap and Multisim, and the expected AC analysis results from Microcap. 

Thanks.

[addda]

MOSFET Level 1 was used in Microcap so I am using Level 1 in Multisim too.

[addda · Accepted solution]

I have fixed the problem.

[yyao]

Just out of curiosity, what was the root cause?

I simulated this is LTSpice which generated the same result as Multisim. PSpice 16.0 calculated the circuit to be in cut off. Was it a biasing issue?

[addda]

The circuit is biased properly...The reason I used this exact circuit was because I know it works, meaning the hardware performs as well as the design.

The issue was that I missed the vertical scale option in AC analysis simulation window. All I had to do is set that to Decibels and just plot v(vo)...this solved the problem.