How to measure the capacitance of the NMOS used as a varactor

Thread Starter

gkini19

Joined Jun 15, 2021
17
Hi All,

I am using the circuit shown below. I want to have a tuning range of 2-4 GHz for LC VCO. I want to use the varactor in this configuration.

My question:

1) How can I find the capacitance of the transistors used as a varactor? which analysis should I use and which plot will be useful?

2) To have a tuning range of 2 GHz how many stages of such Varactors should I have? is there a method or a way to choose this?



Thanks all in advance
 

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DickCappels

Joined Aug 21, 2008
7,905
Bipolar transistors are good for this. The first place I would check is the data sheet. Many transistors, and in particular those intended for high frequencies have charts showing collector-to-base capacitance as a function of collector-to-base voltage.

Is there any particular transistor in which you are interested?
 

Thread Starter

gkini19

Joined Jun 15, 2021
17
Bipolar transistors are good for this. The first place I would check is the data sheet. Many transistors, and in particular those intended for high frequencies have charts showing collector-to-base capacitance as a function of collector-to-base voltage.

Is there any particular transistor in which you are interested?
I am not sure about the type as of now. I would like to know how the capacitance is calculated and I can probably do the same if in case I have a different transistor in mind
 

sparky 1

Joined Nov 3, 2018
593
This video gave me confidence doing varactor setup also implimentation of jigs is shown with the correct procedure I would think.
My capacitance meter survived the project however I ended up ordering a very specific hyper abrupt varactor but learned the basics with this.
The video and Wes Hayward's book that is mentioned has that board. It inspired me to build a few socketed boards like shown and it was a lot of fun. You could aquire enough data to charecterize the voltage and capacitance. Finding diode junctions that yield useful capacitance curves helped me to appreciate the science behind varactors. You can try different varactors in circuit, tuning by feel intuitively can lead to the math to gain better control.
By studying data sheets I have collected inexpensive diodes that would save money however those that buy tuning circuitry are intuitively prone to precision but sometimes cost savings vs precision control is preferred such as low quantity production run.

 
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