40 mhz oscillator

[izach]

need help with  a astable oscillator running at 40 mhz

i am using the standard transistor astable oscillator design and feeding it with a variable voltage therefore getting a variable amplitude  of 100vpp 40mhz signal problem is that after a while it heats up and dies, thinking that maybe the transistors are no good for the job using bd241c. was wondering if i could use mosfets instead ?

 

[Alun]

Why don't you post the schematic?

Don't you mean 100mVpp as 100Vpp would fry most small signal transistors for sure. ;D

 

[audioguru2]

A BD241C is a PNP 100V/3A power transistor. It doesn't have any gain above 3MHz.
Post your circuit for us to see how it could provide 100Vp-p at 40MHz without melting.
In an ordinary astable multivibrator circuit, its base-emitter junctions would try to operate well above their max rating of only 5V.

 

[izach]

two things,just to get everything straight,

1)a bd241 c is a npn device @ 3 amps
2) i am trying to build this circuit but i have realised that the max i can get the oscillator to do is 4mhz
3) working the midnight oil caused me to miscalculate my figures as i was drawing 50 watts off a 10 watt resistor.
4) wowuld love to post my drawing but i use ares lite and dont know how to convert it any suggestions. i will post it as it is quite a good sine wave generator
up to 4mhz and the heating problem has beeen fixed.
5) problem is i am now trying to design a 40mhz oscillator that will drive a air coil with secondary output with 100Vpp yes 100Vpp

 

[audioguru2]

Hi Izach,
You are absolutely correct, a BD241C is NPN. Motorola's datasheet is very confusing. :-[

The transistor is much too slow for 40MHz since it doesn't have any gain above 3MHz because its rise-time, storage time and fall time are much too long.

I think the BD241C is an audio power transistor, good to about 50kHz.
40MHz is VHF so try VHF transistors or Mosfets used in radio transmitters. ;D

View attachment 37446

 

[izach]

hi audioguru

thanks for the advice you are correct in saying this is a audio device and there is no gain at 3mhz.

please inform me how to download the schematic and then i will show you how my latest design looks see if you have any suggestions

many thanks
IAN

 

[izach]

managed to find a way to send the diagram

goodie

View attachment 37448