BF245A in simple oscillator. Suitable replacements?

[neazoi]

Hi I want to experiment with this simple oscillator that uses a BF245A transistor. It uses the internal capacitances of the fet to minimize component count.

1. Can I use J310, because I do not have a BF245A?

2. Are there any other types of more powerful transistors I can use, so as to convert it to a more powerful oscillator? (irf511?-although not jfet depletion)

3. Also the motorola datasheet states two pinouts for the BF245A, how should I know which pinout is the true if I buy one?

 

[johnjoe]

2. If you take some power transistors, the oscillator will oscillate (if it oscillates) at much lower frequency because of the increased parasitic capacitances. Just compare the values of BF245 and IRF511. If you need more power, just build or oscillator with an additional amplifier.

3. The pinout is shown clearly. Your BF245A is "style 23" so 1-gate, 2-source, 3-drain.

1. Just from the feeling, the J310 should work, but I think you have to rework the coil for the same oscillation frequency.

By the way, for this oscillator you need some JFET.

 

[FvM]

Also the motorola datasheet states two pinouts for the BF245A

No. Different pinouts for BF244 and BF245. If you don't know which you have, use an ohmmeter to identify the gate terminal.

The circuit can be expected to work with almost any NJFET (or PJFET if you change the supply polarity). Cgs will affect the oscillation frequency.

 

[neazoi]

Thank you all,
I have tried several n-jfets but the mpf102 and j310 I had on hand worked ok, with the j310 to give more power output, about 4vpp at 1Mohm without significant distortion. I did not have the bf245a on hand to try. I used a t50-2 toroidal core with 13t in total and about 3-5T the tap, which gave about 21.5MHz. At 4T the output voltage was lower but the oscillator could work from 4-6V, with varied output power (max 1vpp at 1M). At the 5th turn tap, the oscillator worked only on 3v but with about 4vpp at

I am curious if the circuit could work with a mosfet, even with higher power mosfets.

 

[biff44]

good luck. finding a new transistor to fit into an existing oscillator circuit, especially one with very special transistor parasitic capacitance requirements, sounds like a nice project for the winter. warm up your soldering iron, you will need it.

 

[johnjoe]

I am curious if the circuit could work with a mosfet, even with higher power mosfets.

Somehow it will work but not on this frequency. The circuit utilizes the parasitic capacitance of the JFET. Power MOSFET have significantly larger caps, in this case you have to rework the feedback. Why you don't use another topology with a power mosfet?

 

[neazoi]

Somehow it will work but not on this frequency. The circuit utilizes the parasitic capacitance of the JFET. Power MOSFET have significantly larger caps, in this case you have to rework the feedback. Why you don't use another topology with a power mosfet?

I am just curious about the 2-components circuit, no other reason.
I suspect hand effects will play a significant role, since the frequency depends as well on the parasitic (usually small) capacitances, in the prototype I built they did.

 

[FvM]

BF245 total gate capacitance in this circuit is 6-7 pF, resulting in about 6 kohm characteristic LC impedance at 3.7 MHz, that's really large. Surely sensitive to hand effects.

Consider that the 2 component circuit can't work with enhancement FETs, e.g. any standard MOSFET. You need additional gate bias and also current feedback for a stable bias point. About 4 -5 additional components.

 

[neazoi]

BF245 total gate capacitance in this circuit is 6-7 pF, resulting in about 6 kohm characteristic LC impedance at 3.7 MHz, that's really large. Surely sensitive to hand effects.

Consider that the 2 component circuit can't work with enhancement FETs, e.g. any standard MOSFET. You need additional gate bias and also current feedback for a stable bias point. About 4 -5 additional components.

Thanks FvM! interesing point. Are you aware of any depletion power mosfets, just to try as an experiment.