izach Posted August 4, 2005 Report Share Posted August 4, 2005 need help with a astable oscillator running at 40 mhzi 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 ? Quote Link to comment Share on other sites More sharing options...
Guest Alun Posted August 4, 2005 Report Share Posted August 4, 2005 Why don't you post the schematic?Don't you mean 100mVpp as 100Vpp would fry most small signal transistors for sure. ;D Quote Link to comment Share on other sites More sharing options...
audioguru Posted August 5, 2005 Report Share Posted August 5, 2005 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. Quote Link to comment Share on other sites More sharing options...
izach Posted August 6, 2005 Author Report Share Posted August 6, 2005 two things,just to get everything straight,1)a bd241 c is a npn device @ 3 amps2) i am trying to build this circuit but i have realised that the max i can get the oscillator to do is 4mhz3) 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 Quote Link to comment Share on other sites More sharing options...
audioguru Posted August 6, 2005 Report Share Posted August 6, 2005 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 Quote Link to comment Share on other sites More sharing options...
izach Posted August 7, 2005 Author Report Share Posted August 7, 2005 hi audioguruthanks 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 suggestionsmany thanks IAN Quote Link to comment Share on other sites More sharing options...
izach Posted August 7, 2005 Author Report Share Posted August 7, 2005 managed to find a way to send the diagram goodie Quote Link to comment Share on other sites More sharing options...
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