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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.
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.
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.
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.
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...
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.
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.
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?
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.
Although Audio Guru is correct about RF technology shifting away from varicaps - they are still made, even by large companies such as NXP (https://www.nxp.com/products/diodes/varicap_diodes/#products). Farnell still has 44 parts in its catalogue, hopefully decent distributors in your country will also stock a selection of varicaps.
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