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trouble with getting correct ro through sp analysis in spectre

snaildr

Hi there,

I'm pretty new to Spectre, and was simulating some basic stuff. Here's a question about getting ro using sp analysis that puzzls me.

I'm under the impression that for the drain-source impdance, the large signal value is 'ron' which is Vds/Id, and the small-signal value is ro==1/gds, which is dVds/dId. So, for small-signal gain, I expect to use ro to match my hand calculation to simulation, and it did work.

I also want to do a sp analysis at the output port of a single-transistor common-source amplifier. I set the load to be an RF choke so that the output impedance should be the drain-source impedance. In the MOSEFT and bias condition I used, ron=40 Ohm, ro=1/gds=220 Ohm, so there's a big difference between the two. However, when I look at the Z11 (input impedance looking into the output port, i.e. drain-source port), the real(Z11) is close to ron (40Ohm) ! I varied bias voltage, MOSFET dimensions etc, this issue remains. It's supprising to me that as a small-signal simulation, sp analysis picked the large-signal impedance!

As a control experiment, I replaced the 'port' element with a 50 Ohm resistor and a 'vsin' element in series and watch the voltage before and after the 50 Ohm resistor, it indeed appears that the voltage ratio is about 0.8, so the impedance should indeed be ~250Ohm.

 

ro is a key parameter in circuit design and being able to get the correct value through sp analysis is very important in RF designs. I'd really appreciate your take on this!

 

Ran 

  •  Hi Ran,

    Without seeing your netlist/model files, it's difficult to say what is going on. 

    I suspect it's a setup issue - or a misunderstanding of how the port operates.

    sp is a small signal analysis, like ac.   

     

    I recommend contacting Cadence Customer support ( http://support.cadence.com ) with your netlist/model files (You cannot post model files here...they belong to whomever developed them) so an AE can look at this and assist you.

     

    best regards,

     

    Tawna

  • Issue resolved, I was looking at Z22 to get the port2 input impedance, but Z-parameter is really Z-parameter not (s11+1)/(1-s11). So, to get the port input impedance one should look at ZM parameter or equivalently the Z-smith of S-parameter.

     

    Thanks Tawna for your kind reply.

     

    thanks,

    Ran 

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