Varactor question...

[Dave]

What is the proper method for using a varactor to tune a circuit? Deliver
positive voltage to the cathode and anode to ground, such that the voltage
is "felt" but no current flows? It's been 30 years, and I don't remember.
Will check the books, but would like input form the gurus if it's not too
much of a pain.

Thanks...

Ignorantly yours,

Dave

 

[Tim Wescott]

On Thu, 04 Dec 2008 22:08:01 -0600, Dave wrote:

What is the proper method for using a varactor to tune a circuit?
Deliver positive voltage to the cathode and anode to ground, such that
the voltage is "felt" but no current flows? It's been 30 years, and I
don't remember. Will check the books, but would like input form the
gurus if it's not too much of a pain.

Yes, you want the diode to be reverse biased. Varactor action happens

because the conducting regions of the diode form the plates of a
capacitor, while the depletion region forms the dielectric. As the
reverse bias gets larger the depletion region grows; with the increasing
width of the depletion region comes reduced incremental capacitance.

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html

 

[Dave]

"Tim Wescott" <tim@seemywebsite.com> wrote in message
news:c4edncWq6IHyMqXUnZ2dnUVZ_vjinZ2d@web-ster.com...

On Thu, 04 Dec 2008 22:08:01 -0600, Dave wrote:

What is the proper method for using a varactor to tune a circuit?
Deliver positive voltage to the cathode and anode to ground, such that
the voltage is "felt" but no current flows? It's been 30 years, and I
don't remember. Will check the books, but would like input form the
gurus if it's not too much of a pain.

Yes, you want the diode to be reverse biased. Varactor action happens
because the conducting regions of the diode form the plates of a
capacitor, while the depletion region forms the dielectric. As the
reverse bias gets larger the depletion region grows; with the increasing
width of the depletion region comes reduced incremental capacitance.

--
Tim Wescott

Yeah, that's what I figured. I didn't have it reverse-biased., but there
was no actual current flow due to the configuration I had it in. Easily
fixed.

Thanks much!

Dave

 

[Eeyore]

Tim Wescott wrote:

Dave wrote:

What is the proper method for using a varactor to tune a circuit?
Deliver positive voltage to the cathode and anode to ground, such that
the voltage is "felt" but no current flows? It's been 30 years, and I
don't remember. Will check the books, but would like input form the
gurus if it's not too much of a pain.

Yes, you want the diode to be reverse biased. Varactor action happens
because the conducting regions of the diode form the plates of a
capacitor, while the depletion region forms the dielectric. As the
reverse bias gets larger the depletion region grows; with the increasing
width of the depletion region comes reduced incremental capacitance.

And the same thing happens with BJT c-b capacitance which is why apparently
stable circuits may oscillate slightly when the output driven towards the
supply voltage. The c-b capacitance increases, reducing the phase margin and
away you go !

Graham