A Question about Vgs?

[kakajeej]

Hello Everyone.

This might seem to be a stupid question for some people but I have just started with Analog Designing so please bare with me. The problem is that I simply cant figure out the exact value of VGS for a circuit( lets say) an amplifier. Supposedly all the other things are known like device sizing and overdrive voltage etc. what value of VGS should I use while doing the simulation? Is it possible that because I know the overdrive voltage, I can simply use this formula Vov=VGS-Vth? or can i use a chart base approach by plotting drain current against VGS of any transistor?

A quick reply will be very much appreciated.

Thanks in advance.

 

[palmeiras]

Hello Kakajeej,

I didnt understand your question. If you have the values of overdrive voltage (Vov=VGS-Vth), you have the values of VGS. Moreover, as you say, if you have all other information, current and size of devices, you can use the traditional equation to estimate the size of VGS: I=1/2*u*cox*W/L*(Vgs-Vth)^2.
If I still didnt answer your doubt, please, reformulate your question.

 

[kakajeej]

Thanks a lot for your help.

I think I should have mentioned this thing earlier that is the Threshold voltage a constant ( means technology dependent) or does it change based on the region of operation of the device? The equation that you wrote holds true for the strong inversion region but not for the weak inversion. See, I just want to figure out the exact value of Vth based on the region of operation. How can I do that?

 

[palmeiras]

Ok, I understood your question
The threshold voltage can change with the bias. For instance, considering the first-order model, the threshold voltage can be described by equation:

where, you can see that it depends on the bias, for instance the source-bulk voltage. However, in practice, the threshold voltage also depends on other things, for instance, the size of the device. Devices with the same bias, but with different sizes, has different values of threshold voltage.

But in a first-order approach, the threshold voltage does not change if the device is working in strong or week inversion. So, if you have the technology parameters (doping - Nsub, etc) you can calculate the threshold voltage of your device.

Did I answer your question?

 

[prcken]

you can use hspice simulator to plot Vth versus Vgs.
you can get a bunch of information with parameters like L of your device, temperature, process corners, operation regions.....