FET transmitter

[Electronman]

Hello guys,

I wanna know your idea about the bellow circuit which I want to make?
As you see it uses 2 N-Channel JFET transistors. I never have built a FM transmitter with FETs so don't know how good the bellow circuit will work finally?

By the way, the BF245 transistors that I got have an unclear mark, so how can I test them to see if they are BJT transistors or FETs?

**broken link removed**

Thanks a bunch

 

[mvs sarma]

Hello guys,

I wanna know your idea about the bellow circuit which I want to make?
As you see it uses 2 N-Channel JFET transistors. I never have built a FM transmitter with FETs so don't know how good the bellow circuit will work finally?

By the way, the BF245 transistors that I got have an unclear mark, so how can I test them to see if they are BJT transistors or FETs?

**broken link removed**

Thanks a bunch

Physical examination under an led or monochromatic light source, you can read it.
next if the fet gate is terminated,the device works like a constant current source when connected to power supply. you can have an source resistor of say 1K
and the voltage dro across it would indicate that it is FET
for BJT if you terminate the base, there would be no current flow in the C_E path
a link possible is given here
**broken link removed**

 

[audioguru]

It uses an old crystal microphone. Where will you find one today?
If you use a modern electret mic then it will need a preamp to drive the vari-cap diode.

 

[Electronman]

Physical examination under an led or monochromatic light source, you can read it.
next if the fet gate is terminated,the device works like a constant current source when connected to power supply. you can have an source resistor of say 1K
and the voltage dro across it would indicate that it is FET
for BJT if you terminate the base, there would be no current flow in the C_E path
a link possible is given here
**broken link removed**

No I am not able to read it. Part of the number has removed.

Is it the only way to test it? Any better way?

 

[Electronman]

It uses an old crystal microphone. Where will you find one today?
If you use a modern electret mic then it will need a preamp to drive the vari-cap diode.

Fortunately I have an old one.

What is your idea about the main circuit? What about its range?

 

[audioguru]

What is your idea about the main circuit? What about its range?

5mA x 9V= only 45mW. Its range is 10m to a cheap radio, 20m to a clock radio and across the street to a very sensitive home stereo or car radio.

It doesn't have pre-emphasis (treble audio frequencies boost) like FM radio stations have so it will sound muffled on an FM radio (they all have de-emphasis (treble audio frequencies cut).

 

[kchriste]

No I am not able to read it. Part of the number has removed.
Is it the only way to test it? Any better way?

The following will not test whether the JFET is functional or not, but it will confirm whether it is a JFET or a bipolar transistor.

Measure with an ohm meter between what you think is the Drain and Source terminals (See the datasheet). You should have a few hundred ohms in both directions (apx 300-700Ω). A bipolar transistor will read open between its' collector and emitter on the ohms scale.