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# about C-V curve......

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#### Ding Yan

##### Newbie level 4
cv curve

Hi guys. Can anybody give me some hints on how to plot the C-V curve of a PMOS varactor. I am using cadence ADE and I am trying to find out the linear tuning region. Thanks

Yan

c-v curve

The I V curve will look like an ordinary diode. I suspect what you want is a graph of the small signal capacitance verses reverse bias.

One of the problems of using an ordinary capacitance meter is the large signal used for the measurements.

I suggest that you make an LC oscillator and measure the frequency vs. bias and back out the capactiance from a calculation knowing the L.

Even better would be using the data sheet data.

cv curve of diode

Ding Yan said:
Hi guys. Can anybody give me some hints on how to plot the C-V curve of a PMOS varactor. I am using cadence ADE and I am trying to find out the linear tuning region. Thanks

Yan

You can simply run Spice (Spectre) AC simulation using PMOS varactor model and by sweeping its voltage, then plot capacitance (equal to imaginary part of the current divided by angular frequency [omega]) versus applied voltage.

pmos c v curve

Ding Yan said:
Hi guys. Can anybody give me some hints on how to plot the C-V curve of a PMOS varactor. I am using cadence ADE and I am trying to find out the linear tuning region. Thanks

Yan

place an ac voltage source with dc voltage as vin, ac_mag=1/(2*pi*1) to the positive node of your varactor, while ground the negative node with 0V. Run an ac analysis using design variable as a sweep and set the frequency as 1. Also, pick up vin and choose the sweep range in the form. Measure the ac current goes into the positive node of your varactor.

i = (2*pi*1 )*C*ac_mag

the measure current will be equal to C

varactor spectre

flatulent said:
The I V curve will look like an ordinary diode. I suspect what you want is a graph of the small signal capacitance verses reverse bias.

One of the problems of using an ordinary capacitance meter is the large signal used for the measurements.

I suggest that you make an LC oscillator and measure the frequency vs. bias and back out the capactiance from a calculation knowing the L.

Even better would be using the data sheet data.

Actually, I am building a LC tank. I am just trying to find out the region with good linearity. Thank you for your sughuestion.

Yan

Added after 1 minutes:

timof said:
Ding Yan said:
Hi guys. Can anybody give me some hints on how to plot the C-V curve of a PMOS varactor. I am using cadence ADE and I am trying to find out the linear tuning region. Thanks

Yan

You can simply run Spice (Spectre) AC simulation using PMOS varactor model and by sweeping its voltage, then plot capacitance (equal to imaginary part of the current divided by angular frequency [omega]) versus applied voltage.

Thank you.

Added after 1 minutes:

wpchan05 said:
Ding Yan said:
Hi guys. Can anybody give me some hints on how to plot the C-V curve of a PMOS varactor. I am using cadence ADE and I am trying to find out the linear tuning region. Thanks

Yan

place an ac voltage source with dc voltage as vin, ac_mag=1/(2*pi*1) to the positive node of your varactor, while ground the negative node with 0V. Run an ac analysis using design variable as a sweep and set the frequency as 1. Also, pick up vin and choose the sweep range in the form. Measure the ac current goes into the positive node of your varactor.

i = (2*pi*1 )*C*ac_mag

the measure current will be equal to C

Thank you for your sharing.

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