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["Sathish" <d.sathish@xxxxxxxxxxxxx>]

Happy & Prosperous New year guys......

Regards,
Sathish.

' The Huge Tree is hidden in a Seed '
Swami vivekananda....

----- Original Message -----
From: "Bi Han" <mike_bihan@xxxxxxxxxxxx>
To: <si-list@xxxxxxxxxxxxx>
Sent: Tuesday, December 31, 2002 7:23 PM
Subject: [SI-LIST] hspice simulation issue in VCO


>
> I have got a problem with hspice simulation of VCO.
>
> When I am simulating netlist as below, the simulation results will be
incorrect.
> As we can see, the resistor R1 & R2 is the loss resistor of spiral
inductor.
> If we tune their value to 0, the simulation results will be very weird.
That is:
> the voltage at output node will be greater than the supply voltage.
>
> Also, the .lis file sometimes contains lines as follow:
>
> "warning" negative-mos conductance =......
>
>
> Some very kind people did helped me to explain it. I will quote their
reply at the end of this mail.
>
> however, i will still hold the question that why not MOSFET limiting
effect work to limit their output?
>
>
> * # FILE NAME: /DISK2/WORK/HBI/SIMULATION/PAN/HSPICES/SCHEMATIC/NETLIST/
> * Generated on Dec 31 01:38:15 2002.
> MN2 NET21 NET23 0 0 N18 L=500E-9 W=235E-6 M=1.0
> MN1 NET23 NET21 0 0 N18 L=500E-9 W=235E-6 M=1.0
> C0 NET23 NET9 1E-12 M=1.0
> C1 NET21 NET9 1E-12 M=1.0
> L1 NET3 NET6 3E-9 M=1.0
> L2 NET3 NET22 3E-9 M=1.0
> R3 0 NET9 3E6 M=1.0 **DC path
> R1 NET6 NET23 20.0 M=1.0 **loss resistor of spiral inductor
> R2 NET22 NET21 20.0 M=1.0 **loss resistor of spiral inductor
> R5 VCC NET3 300.0 M=1.0 **resistor cause voltage drop
> VDD VCC 0 2.5
> * Include files
> * End of Netlist
> .TRAN 1.0000 5.0000 START= 0.
> .TEMP 25.0000
> .OP
> .save
> .OPTION INGOLD=2 ARTIST=2 PSF=2
> + PROBE=0
> .END
>
> _______________________________________________
>
> Hi,
>
> It is not werid to have voltages greater than the supply rails when you
have inductors in your circuit. More common even, if the series parasitic
resistors are zero, which means that the quality factor is infinite.
Remember that the inductor keeps the current through it continuous, which
can make a huge voltage among the terminals. Have you never tried a big
inductor in parallel with a 1.5V battery and suddenly disconnected the
batery, holding the inductor terminals in contac with you??
>
> The negative conductance can arise from a self heating of the transistor.
It depends on the model you're using. If it is due to self heating, when
your transitor is driven by a big GVO (Vg-Vth), some models take into
account the increase on the substrate temperature givin rise to a reduction
of Id as Vd increases. That means that the output conductance becomes
negative.
>
> What model do you use?
>
> ______________________________________________________________________
>
> I used to think this was weird also, but now I look at it like this:
>
> the inductor/capacitor form a tank circuit... and it is very literally
that, a tank that stores. This is what allows it to get above the supply
voltage, storage. So lets say that you store some amount during one cycle,
then next cycle, because your gain is greater than one, you have all the
energy you had previously, plus a little more. And so on, and so on. It is
only the resistive and nonlinear part of the circuit that ends up limiting
this amplitude.
>
> There are other circuits called "pump" circuits that use a somewhat
similar method of getting "dc" voltages above the supply. They pump a
current onto a capacitor then switch it to add to the supply. As long as it
is driving a large impedance (i.e. a capacitive load) then it isn't really
supplying much current and the power lost is minimal.
>
> Hope this helps. The above explanation was also good
>
>
>
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