hello everybody, i've been having quite a bit of trouble with a radio circuits (which im sure just about everyone knows cuz that mostly all i tak about). Maybe the reason that im having sooooo much trouble is the kind of transistors im using . . . radio shack's. I've heard quite a bit of bad things about radio shack components.
Ive been using the 2n3904, and all of them register a hfe of 20 or so (its possible that my multimeter is broken or something). I purchased most of em from different radio shacks on different occasions. Anyone had any similar experiences?
wow chemelec, thats reaaly generous of you. i wish i knew u, u sound like a nice guy . nothanx though, im gonna go buy some @ an electronics supply store or get some off ebay, like a pack of 100.
Mouser has 2N3904's from Fairchild for $5.00 per hundred. As to the hFE of the ones that you got from Radio Shack, I suspect that the tester that you are using is the cause of your lower-than-expected readings. If you look at a **broken link removed**, it becomes obvious that the typical minimum DC gain starts low (around 40) with very small (0.1mA) collector currents, peaks with collector currents around 10mA (to a typical minimum of 100 and a maximum of 300), and then falls off again as the collector current goes up beyond that point, falling off again to around 30 with 100mA collector current. All of those specs are at 1V VCE. It's a safe bet that your tester is either applying a different test potential, or else is supplying a current in excess of 100mA... IOW, something doesn't match the test conditions given in the datasheet.
wow chemelec, thats reaaly generous of you. i wish i knew u, u sound like a nice guy . nothanx though, im gonna go buy some @ an electronics supply store or get some off ebay, like a pack of 100.
Zach - keep in mind that the circuit design or your construction techniques may be contributing to the poor performance. I've noticed that many people (not sure if you are one) ask for simple circuits - a great place to start for sure. Unfortunately the circuits are designed with simplicity in mind - they work and demonstrate the principle but they may not be robust in terms of stability, reliability, predictability, etc.
Often I'll see postings about FM broadcast transmitter problems where the builder is having problems. Some of the time they describe problems that are inherent in the design - they didn't do anything wrong. RF circuits, as you go up from broadcast band, to MW, SW, VHF are increasingly sensitive to construction techniques, vibration, temperature, enclosure (or lack thereof). In some cases the design, even with the very best of construction techniques may still not perform well. The simple FM transmitter is a great example where I think some people expect too much.
What you might try doing is building something that is well documented in a magazine article or handbook. Adhere to the design/construction as described by the author to the best of your ability then see where you come out. Often the authors describe the reasons for what they've done - a great way to unravel the mysteries behind a design.
Do what you can to find a local mentor who might assist in debugging or troubleshooting. Doing it yourself is great but a great compliment to this is someone who has been there and done that. I am fortunate that I have people in my area who are willing to do that for me - and where I can, I provide guidance to those who are following me up the learning curve.