Hi all,

I want to create a Colpitt's VCO which will supply a large voltage to a large load at high frequency. This is a challenge so I have been looking at buffering the output stage. Are ICs or just simple BJT buffers better for this application? Are there any principles that you have to keep in mind when designing this? (I can't find literature on how you can just adjust your current design)

 

Welcome to AAC!

We cannot even begin to give you any answers because your question lacks all the essential details.
What do you mean by "a large voltage to a large load at high frequency" ?

Engineering 101 mandates that you must supply numbers. "Large" and "high" are just qualitative words with no quantitative design specifications.

 

Hi Jane,
Is this a College project?

Mod

 

Welcome to AAC!

We cannot even begin to give you any answers because your question lacks all the essential details.
What do you mean by "a large voltage to a large load at high frequency" ?

Engineering 101 mandates that you must supply numbers. "Large" and "high" are just qualitative words with no quantitative design specifications.

Hello, thanks for getting back to me
- I am hoping to operate my VCO between 150MHz and 500MHz
- The load may change from an initial 5MOhms to 1MOhm or 50MOhms
- 0.1V across load

I'm looking for resources and principles rather than a specific design but I'm very happy to take any suggestions you have re these values too!

 

Hi Jane,
Is this a College project?

Mod

A research project. I've not done an RF design module in my EE degree so starting from the ground up but I would like to find resources that could help me understand the issues involved at a high level. Thank you.

 

Anyone know how to couple a high input impedance buffer (e.g. https://www.analog.com/media/en/technical-documentation/data-sheets/MAX2470-MAX2471.pdf) to a VCO output where the loading is very important?

 

An interesting project. First transistor I thought of was BFR93A. I know there are a few UHF Colpitts using that.
Just for fun I searched for a datasheet for high gain npn wide bandwidth transistor, this was one of the first results.
The datasheet had some helpful information.
55GN01FA - RF Transistor (onsemi.com)

The BFR93A datasheet
BFR93A NPN 6 GHz wideband transistor (nxp.com)

Good prototyping sometimes comes from experimenting, improving and having confidence in working with sometimes very small copper engraving.
if you use stripboard there are some tools that everyone should be aware of that just cutting a trace with a dremol our and exacto knife is ok,
A very talented hand engraver knows that the quality is in the design of the high speed steel engraver gouge tool. This opens up many possibilities for using vero stripboard to prototype using surface mount parts. An example of a basic drill type cutter.

Making a V gouge that will cut the size track you may need. Always were face shield and safety glasses.