Colpitts oscillator that dont oscillate! - Page 1
Forum section: electronics / beginners

author: Boschi
author_rank: Regular Contributor
timestamp: December 13, 2016, 09:21:15 pm
content: Im trying to do an arduino inductance meter, based around a colpitts oscillator.About 6 monts ago i did mi first trial of this circuit and it worked flawlessly, but it was a ugly layout and i stripped it apart.Now im doing this circuit again, but i cant get it to oscillate.The circuit is here:https://soldernerd.com/2014/12/14/arduino-based-inductance-meter/The schematic download is on the bottom of the pageI soldered it on perfboard, its dimensions (just the oscillator, i havent included nothing beyond c9) are about 3*3 centimeter.The supply voltage is pretty clean, so the problem shuldnt be there.What could be the problem?

author: Audioguru
author_rank: Super Contributor
timestamp: December 14, 2016, 12:16:32 am
content: I did not see a schematic in that very long blog. Please post the schematic on this thread.

author: Farley
author_rank: Regular Contributor
timestamp: December 14, 2016, 01:45:43 pm
content: Did you try different inductor values across X3-1 and X3-2? I didn't read the entire blog so I'm not sure what range of inductance it is designed to work with.

author: Boschi
author_rank: Regular Contributor
timestamp: December 14, 2016, 02:38:07 pm
content: @Audioguru: here we are@Farley: i have also suspected about a faulty inductor, so i placed another one across the terminals, but nothing.about the range, it doesent specify nor the max value nor the minimium, but from in the comment it says it can go down to about few hundred nH, then the leads inductance become a problem, and by doing some math i think it could go up to 1mH, probably even more.thanks for the answers

author: orolo
author_rank: Frequent Contributor
timestamp: December 14, 2016, 05:35:19 pm
content: In the circuit, the bjt seems to be operated at Ic = 0.5 mA. For the BFR92A, that seems to be insufficient to reach full hfe, which is not that high to begin with (hfe = 90 typ, 65 min @ 15mA). Besides, if the oscillator fails to start, the main suspect is not enough transconductance. Both facts suggest modifying the DC bias so as to increase Ic to at least 1mA.

author: Boschi
author_rank: Regular Contributor
timestamp: December 14, 2016, 06:09:00 pm
content: To do so i have to change R3 and R4 to 5K each?Inviato dal mio A0001 utilizzando Tapatalk

author: orolo
author_rank: Frequent Contributor
timestamp: December 14, 2016, 06:41:04 pm
content: No, that achieves nothing. Try reducing the emitter resistor to 1.8k for 1mA. Other idea is to raise the bias point.

author: orolo
author_rank: Frequent Contributor
timestamp: December 15, 2016, 05:46:09 pm
content: Hi. I've got some free time, so I decided to build something like your circuit, to see if it'd work. The Colpitts oscillator is the same, but I decided to make some changes:1. Instead of a fancy UHF transistor, I used a jellybean BC550C in the oscillator. Everything else in the Colpitts is the same.2. Instead of the comparator, I used a emitter follower buffer (BC550C) and then a switching transistor (2N2369A). At first I used another BC550 as a switcher, but it was too slow from 3MHz on.The circuit is simple, it took me little more than an hour to build, and it works all right. I attach the schematic, a picture of the circuit (the small mushroom is the 2369A, the other two are the 550's), and the oscilloscope capture of the output for a short circuit, a 30uH axial inductor, and a 1000uH axial inductor.Perhaps your circuit is failing by some other reason than biasing. And you only need three more or less jellybean bjts to build an equivalent working circuit. The switching transistor is biased quite hard (10mA), because at first I was using a BC550 as the switcher. I think that with a 2369 the biasing can be quite relaxed.

author: Boschi
author_rank: Regular Contributor
timestamp: December 15, 2016, 07:21:20 pm
content: Thanks for the dedicationI think the problem in my circuit is the transistor, the emitter is at ground level, so probably i screwed it up somehow.One question, how do you decide if a transistor is suitable or less for this application?For example i have a bunch of bc337, and i see that their current-gain-bandwidht product is 210Mhz, so for a 10mA collector current and a gain of 100 their max frequency is about 200Mhz, well above 7Mhz.Culd they do the job or not? And even more important, why?

author: orolo
author_rank: Frequent Contributor
timestamp: December 15, 2016, 09:38:19 pm
content: I'm sure that one of the experts will correct me if I make a mistake. My understanding is:In this circuit the bjt is in common base configuration, and the gain will roll off because base resistance together with diffusion capacitance. So the ft figure for the transistor describes well the current gain, and you should expect that transconductance falls to 25% at 7MHz. When the Colpitts oscillator starts, it works as a negativa resistance amplifier, where the neg resistance is the product of transconductance by the reactances of the capacitors (1nF gives about 23 Ohm at 7MHz). At 1mA gm=40mS, so at our frequency it falls to 10. So the negative resistance is 0.01*23*23 = 5.29 ohms. That should be enough to start and maintain oscillation unless the inductor has quite a bit of series resistance.So the BC337 should be more than enough. I've seen schematics of Colpitts oscillators used with transistors working at half their ft, so yours should have no problem. If you're planning to reduce the emitter resistor quite a lot, you should use a choke in series.