Varicap

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A varicap is made to have a wide range of capacitance change when its reverse voltage is changed. But an ordinary diode or transistor junction has a narrow range of capacitance change when its reverse voltage is changed.
 
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audioguru said:
A varicap is made to have a wide range of capacitance change when its reverse voltage is changed. But an ordinary diode or transistor junction has a narrow range of capacitance change when its reverse voltage is changed.
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i mean, can i use a signal diode + capacitor to replace a vericap
 
It is VARICAP, not vericap.
Any signal diode is a varicap but its range of capacitance change is very low. An extra capacitor is not needed because when a diode is reverse-biased then it does not conduct and has a small capacitance that changes a little when the reverse bias voltage is changed.
You might need a coupling capacitor to block the bias voltage.

Look at the datasheet for a 1n4148 small signal diode. When its reverse bias voltage is 2V then its capacitance is typically 0.84pF as shown on a graph in its datasheet. With a reverse bias voltage of 15V then its capacitance is a little less at 0.81pF.

The capacitance of a 2N4401 transistor changes from 9pF at 0.1V to 2.3pF at 50V.
 
To be more positive - you can quite easily use a diode as a varicap - but you want a decent size diode (such as a 1A rectifier), this will give a MUCH better varicao action than a small signal diode, but still nothing like a proper varicap.
 
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Thanks for the correction. The problem i am having is with bb139 and it likes for my 4km Fm transmitter, i could not get one thats the reason i'm looking for alternative. I knew the capacitance of a signal diode is too small to serve as varicap, reason i'm trying use an external capacitor with. Pls recommendation a solution. Am waiting pleeease...
 
I know of no substitute for a varicap. Adding an external capacitor to a standard diode will not increase the range of the capacitance variation.

Have you tried to find a replacement equivalent?
 
Your problem is that you are looking at very old circuits on a website in a 3rd-world country so the parts are not made anymore.
You should look for a modern circuit on a Western website.
 
audioguru said:
Your problem is that you are looking at very old circuits on a website in a 3rd-world country so the parts are not made anymore.
You should look for a modern circuit on a Western website.
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nop! I included varicap diode in my design because i want to be able to adjust my transmitting frequency to any range within the Fm band, and i did not want to use a trimmer/ pot capacitor. It seems i dont have anymore alternative than resorting to it. Dont u av any solution?
 
crutschow said:
I know of no substitute for a varicap. Adding an external capacitor to a standard diode will not increase the range of the capacitance variation.

Have you tried to find a replacement equivalent?
Click to expand...

Nop. I contacted a dealer and he promised to supply me but am not sure meant it. Do u any replacement recommendation?
 
Varicap diodes (varactors) were made 30 to 40 years ago. Most are not made anymore.
Some pirate FM transmitter kits still use varicap diodes. Look at Ramsey and a few other Pirate FM transmitter kits.
 
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