quincy Posted June 1, 2007 Report Share Posted June 1, 2007 i'd like to know a lot of stuff about some of the audio oscillators that are projects here.i thought id just start a general discussion :Dto start off, i was wondering about the kind of transistors that can be used for the reverse bias oscillator.-can you use NPN darlingtons?and were can you put a potentiometer or varistor in the circuit to control the frequency?-thanks, quincy Quote Link to comment Share on other sites More sharing options...
AN920 Posted June 1, 2007 Report Share Posted June 1, 2007 Changing the value of the resistor will vary the frequency. This type of oscillator works because of a negative resistance region created at the reverse breakdown potential level. You should not make the resistor too low and damage the transistor.I have done some experiments on this type of circuit and found that the upper limit of oscillation to be around 200kHz. I used it as a VCO by varying the applied voltage between 8-30V. It gave quite a good saw tooth wave with constant amplitude of 3Vp-pI even managed to get it oscillating with a low frequency crystal (32kHz) but it is a bit tricky to get going. The output is then a nice sine wave. Quote Link to comment Share on other sites More sharing options...
quincy Posted June 1, 2007 Author Report Share Posted June 1, 2007 thanks!but what about the transistors as far as darlingtons?what does anyone think a darlington npn might change in the output? Quote Link to comment Share on other sites More sharing options...
AN920 Posted June 1, 2007 Report Share Posted June 1, 2007 I have not tried darlington's but due to their structure they will be very much slower in switching if they switch in this mode at all. I have a feeling they won't work. Quote Link to comment Share on other sites More sharing options...
quincy Posted June 1, 2007 Author Report Share Posted June 1, 2007 ah. i see. well, i might try some anyways, considering i get them all for 75cents at axmanill try any other transistors i can get for cheap there, and update this page with which work.or maybe i could update the project? does anyone know how i might do that?anyways, thanks Quote Link to comment Share on other sites More sharing options...
AN920 Posted June 1, 2007 Report Share Posted June 1, 2007 Ok, I was wrong. They do work, and quite well. Just hooked up one here in the lab. Quote Link to comment Share on other sites More sharing options...
quincy Posted June 1, 2007 Author Report Share Posted June 1, 2007 yaay!i wish i had a labwhat transistor did you use?i was thinking of using mj9001or 9000or 2n9000 i cant remember, obviouslybut something from motorola thats one of the round ones, and i think its 40v npn and darlingtonsupposedly used for switching applicationsdo you have any idea if that would work alright?editi guess ive been rather vagueim trying to find the datasheet Quote Link to comment Share on other sites More sharing options...
AN920 Posted June 1, 2007 Report Share Posted June 1, 2007 I don't know about power darlingtons. I implemented the small signal darlington in a design in this thread.http://www.electronics-lab.com/forum/index.php?topic=11442.msg62103;topicseen#new Quote Link to comment Share on other sites More sharing options...
quincy Posted June 2, 2007 Author Report Share Posted June 2, 2007 okay, im not asking you anymore, but ive tried a 2n4922 (http://www.onsemi.com/pub/Collateral/2N4921-D.PDF)and a 2n6037http://pdf1.alldatasheet.com/datasheet-pdf/view/21704/STMICROELECTRONICS/2N6037.htmland i cant get it to work.i also used:1k pot9v battery1uf 50v electrolytic capacitor8ohm, probably .5w speakerwill all npn transistors work that arent darlingtons? Quote Link to comment Share on other sites More sharing options...
AN920 Posted June 3, 2007 Report Share Posted June 3, 2007 Use the darlington I used or just a common bipolar like a 2N3904 or 2N4401. Power darlington's internal structure is different with low value resistors to help speed up switching time. Quote Link to comment Share on other sites More sharing options...
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