The Oscilator projects

[quincy]

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 :D
to 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

[AN920]

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-p

I 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.

[quincy]

thanks!
but what about the transistors as far as darlingtons?
what does anyone think a darlington npn might change in the output?

[AN920]

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.

[quincy]

ah. i see. well, i might try some anyways, considering i get them all for 75cents at axman
ill 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

[AN920]

Ok, I was wrong. They do work, and quite well. Just hooked up one here in the lab.

[quincy]

yaay!
i wish i had a lab
what transistor did you use?
i was thinking of using mj9001
or 9000
or 2n9000 i cant remember, obviously
but something from motorola thats one of the round ones, and i think its 40v npn and darlington
supposedly used for switching applications
do you have any idea if that would work alright?
edit
i guess ive been rather vague
im trying to find the datasheet

[AN920]

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

[quincy]

okay, im not asking you anymore, but ive tried a 2n4922 (http://www.onsemi.com/pub/Collateral/2N4921-D.PDF)
and a 2n6037
http://pdf1.alldatasheet.com/datasheet-pdf/view/21704/STMICROELECTRONICS/2N6037.html
and i cant get it to work.
i also used:
1k pot
9v battery
1uf 50v electrolytic capacitor
8ohm, probably .5w speaker

will all npn transistors work that arent darlingtons?

[AN920]

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.