For those of you that uses Advanced Design System (ADS), from this circuit: enter image description here

Fundamental Frequency for transient simulation tool is set to 1GHz.

Yet, I'm getting this output: enter image description here

Main question:

Since this is Common Drain, VOUT should at most have the same amplitude as VIN. Probably even negative shift, since I didn't change the voltage threshold (I haven't got to tweaking parameters yet). So what's up with the positive DC bias?? Why is the amplification significantly less than unity?

Other questions:

1.) Also, what's that Transient Response at the very first cycle of VOUT? Why would a semiconductor have a Transient Response? Is it because of the "inductance" on the drain and source of a MOSFET?

2.) Lastly, why is it that this simulation does not work on very low frequencies? I first started with 1Hz scale for everything (just change all units from 1GHz below to 1Hz). The M1 just acted like a 1kOhm constant resistor.


Because of recent response, I bothered to simulate this in the 1kHz scale (replaced all units from GHz in the pics to kHz, did the same with Fundamental Frequency) -- really should've done that in the first place (stupid me). This is the output:

enter image description here

Even here, the DC bias is nearly the same and the amplification is much, much less. It also lags by 90°. Why?

I'm starting to think it's either just using the wrong component or there was a set of parameters to fill-in for this component to get this to work.


the continuation of this discussion:

Keysight's Advanced Design System users, what's wrong with this simulation - II

  • 1
    \$\begingroup\$ You are simulating at 1GHz!!! The MOSFET is doing nothing at that frequency, its just conducting a bit, The output is due to capacitive coupling gate to source. 1GHz! \$\endgroup\$ – user1582568 Feb 16 '16 at 12:45
  • \$\begingroup\$ Did you have a look at that mosfets datasheet? Things like turn on relay time and so on? And I am pretty sure it works fine at 1kHz, your expectation is just wrong \$\endgroup\$ – PlasmaHH Feb 16 '16 at 12:49
  • \$\begingroup\$ @user1582568: I see. So that's why there's a Transient Response. I thought this (GHz) was the fundamental frequency to make the simulation run, seeing that this package is for HF and that I was getting a flat line on VOUT at the Hz range (as stated in 2) \$\endgroup\$ – kozner Feb 16 '16 at 12:50
  • \$\begingroup\$ @PlasmaHH: I think you should see my second point above. \$\endgroup\$ – kozner Feb 16 '16 at 12:51
  • \$\begingroup\$ @kozner: Your expectation is just wrong there, your "gain" will probably like 0.3 or something there \$\endgroup\$ – PlasmaHH Feb 16 '16 at 12:53

If you look at your MOSFET's datasheet page 2 (like @PlasmaHH proposed above).

You'll see that it is not able to work properly at 1 GHz since it's turn-on and turn-off delay times are about tens of ns which obviously waaaay too slow to operate at this frequency. You should try with a component better suited for RF applications (or just decrease your operating frequency).


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