Diode question

Any PN junction has a voltage dependent capacitance. Some have much more dramatically varying capacitance than others.

The 1N914 is pretty tame. See figure 7 of:

Large area diodes have larger swings, partly because the junction capacitance is lots larger than the stray capacitance. Take a look at figure 4 of: for 3 amp rectifier. Schottky diodes also have large capacitances. See figure 3 of: for a 3 amp unit.

I understand that you can use the base-collector junction of a small-signal transistor for this, but that's about all I know.

Do a web search?

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

Yes. The depletion region in any diode has capacitance; for best Q, however, you want a large capacitance and low series R, and 1N914 is not as good a candidate as a power Zener diode in either respect. Power rectifiers have deliberately added series resistance, so have good C but mediocre R...

Of course, you want to bias the diode well under its breakdown voltage.

Also, you want the bias voltage to be a lot higher than the signal voltage or you'll get nonlinear effects from the capacitance changing with the signal. I've used a 1N4xxx rectifier that way, but just for fooling around. For example, a garden variety SiC 4A Schottky will vary from 150pF at 0V to 10pF at -300V and 7pF at -600V

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

A garden variety red LED works for this.

--
Ben Jackson AD7GD

http://www.ben.com/

I have used both 1N4005 and 1N4007, they both work, but have quite different C vs V curves, I have them plotted somewhere, but can't find them.

Barry

It's possible there is info in the old Motorola data books. The

1N4001~1N4005 were the same, and 1N4006 and 1N4007 were the same as each other, but different.

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

In the past, some small metal can power rectifiers were useable as a varactor up to the 100MHz region. Play around with the lowly 1N4007 and such...

depend

FSK or PSK PWM, interesting way to put it! :-)

What's that do to spectral purity I wonder? I suppose it would be harmonic and easy to filter though.

Tim

-- Deep Fryer: A very philosophical monk. Website @

As well as all common diodes (as the other posters explained), you can also use small MOSFETs as varactors.

With MOSFETs, the drain and source are connected to the tuning voltage "Vtune" and the gate is connected to the RF signal (LC tank etc.). The capacitance vs. voltage of the MOSFET is not very linear, but then that may not matter. In voltage controlled oscillator circuits (VCOs), if the swing is very large, then this actually makes the tuning curve smoother. This is a little counter-intuitive at first, but it works. If you consider a "worst possible" varactor characteristic where the capacitance is either "high" or "low" and switches at a certain threshold, then even this allows a VCO to be tuned quite smoothly, because during the oscillation cycle of the VCO, the varactor will switch between the "high" and "low" capacitance states, and the proportion of the time that it is in each state will depend on the tuning voltage, a sort of PWM.

Page 29 shows the use of MOS varactors

Chris

Exactly, there is some harmonic distortion, but it's easy to filter out. If the Q of the oscillator tank is high (which is desirable for many reasons) then the harmonic content of the tank voltage will be pretty low anyway.

Chris