Transistor or LED as Varicap?

[zlatkoMM]

Hi to all...
I am interested is there a way to build transistor or LED
circuit which act as varicap diodes?
We all know that transistor with shot connection between
emiter and colector (NPN) respond on reverse voltage
and act as varicap.
Is there anyone with experience in this area?
Or someone who already build such a LC tank in first place
for FM range 88-108MHz.
Same is for LED diodes?
any help is welcome !

 

[Ivan_Ryger]

Dear ZlatkoMM, the varactors based on NPN transistor have usually low Q when the C-B junction is used (due to high series resistance). On the other hand, this junction has usually high Miller capacitance, what is desirable for varactor operation. Therefore, the C-B junction capacitance change is suitable for short or medium-wave oscillators (several MHz). The B-E junction has lower series resistance, and therefore you can experience higher Q than the above mentioned counterpart. On the other hand, the width of base area is small and the emmitter is highly-doped, so the breakdown voltage of this junction is very low (usually about 5V) and such varactor is quite noisy near Vbr.

 

[zlatkoMM]

Ivan
Thanks for reply...
so you think that if i want use this circuit on FM range i must use
option B_E for higer freqs?...right?
By the way you say noisy...well reall varicap diodes are also noisy
if you compare them with real variable capacitor..right?

 

[Ivan_Ryger]

Varicaps are, somehow, less noisy as the avalanche region is quite far away from their standard operation region.
You may probably use the transistor BE junction. It can be an interesting experiment to prove it, but I would advise to use some common varicaps in the final solution.
Mainly for the case of undefined base doping profile, when compared to abrupt or hyperabrupt varicaps.

Yes, certainly, the real variable capacitor with moving part is theoretically noiseless.

 

[zlatkoMM]

Yes..ok..
Reson why i would like to try transistors or LEDS is that with all that
varicaps which i have i cannot tune whole FM range on easy way....
i need something like BB909...but i don't have this diode
which work ok in area from 0-3V cca (45-32pF)
also i have found on one site where is presented Varicap circuit with
2 ordinary RED LED (5mm) as good candidate for FM range.

 

[jiripolivka]

Using transistor diodes or LEDs to vary their capacitance by DC bias is possible but as their junctions differ from those of varicaps or varactor diodes, their performance will not be so good.
Varicap diodes were specifically designed for tuning resonant circuits. With a reverse voltage > 3...5V, their leakage resistance is high, so they behave like good tuning capacitors.
You can try any other diode but the result will be poor. If you want to be a good designer, follow good advice and specifications.

 

[zlatkoMM]

Everybody knows that varicap is the best option....
BUT this is a alternative way in specific situation