Colpitts Oscillator - did it work ? - Page 1
Forum section: electronics / beginners

author: lordvader88
author_rank: Frequent Contributor
timestamp: December 29, 2016, 09:22:48 pm
content: Yesterday I learned of and looked up Colpitts Oscillator and so threw it together from herehttp://www.electronics-tutorials.ws/oscillator/colpitts.htmlI made this 1 with pretty much the same Rs and CsAnd my DMM said it was oscillating at about 1.8kHz or maybe 2.4kHz. So I wanted to hear it and spent 1-2 hours trying to get a little mini speaker hooked up to it, and I tried a 1 BJT driver, and none of it worked, and then I didn't know if I had a peizo buzzer or a speaker or what was wrong or if I just was doing it all wrong for setting up the trannieSo I was like thisLOL, gotta love those catsSo I built a little 555 circuit to make sure I could work the speaker, and it worked fine. So I checked the Colpitts Osc with my DSO and it wasn't showing any noticeable oscillation, just a fixed 2V output from the 741 opamp, and hence why the speaker did nothing.I never bothered with any equations to even see what freq. it was supposed to work at, or the amplitude, but it was fixed on my scope.So what am I missing here ? I'll play with it again now soon, but my DMM does work fine for Hz in my limited experience, and the scope confirms it usually.

author: Andy Watson
author_rank: Super Contributor
timestamp: December 29, 2016, 10:17:14 pm
content: Quote from: lordvader88 on December 29, 2016, 09:22:48 pmNegative power supply?

author: lordvader88
author_rank: Frequent Contributor
timestamp: December 29, 2016, 11:10:21 pm
content: IDK, I used 0-12V. The article says "Note that being an inverting amplifier configuration, the ratio of R2/R1 sets the amplifiers gain. A minimum gain of 2.9 is required to start oscillations.", I used 3.9k/1k so I got that part.I haven't touched yet todayOn DC and AC coupling the only variation looked like background noise it was so small, I'm not sure what range the sine wave is supposed to be over.A few months back I knew the basics of calculating op-amp circuits, time to learn again.

author: Andy Watson
author_rank: Super Contributor
timestamp: December 29, 2016, 11:59:06 pm
content: Your op-amp circuit will (try to) output a signal that goes positive and then negative. The negative portion requires a negative power supply. In the case of the 741 you will need a negative supply that exceeds the output voltage by approximately 3V (IIRC). What/where have you connected pin 4 of the op-amp?Alternatively, you could make a potential divider to generate 6V from your 12V supply and use this to drive the positive input of the op-amp (pin 3).

author: Audioguru
author_rank: Super Contributor
timestamp: December 30, 2016, 01:51:28 am
content: The lousy old 741 opamp is 48 years old. Get a better newer opamp and bias its (+) input at half the supply voltage with two resistors making a voltage divider..

author: lordvader88
author_rank: Frequent Contributor
timestamp: December 31, 2016, 12:33:02 am
content: I have an old printer SMPS that puts out/in -12/12V, and some other SMPS with -5V rails, must try later

author: LvW
author_rank: Frequent Contributor
timestamp: December 31, 2016, 11:11:55 am
content: Quote from: Audioguru on December 30, 2016, 01:51:28 amAudioguru - don`t you think that biasing with "half the supply voltage" applies to an amplifier with unity DC gain only? Here, we have a DC gain of (1+3/1.2)

author: Zero999
author_rank: Super Contributor
timestamp: December 31, 2016, 04:55:36 pm
content: Quote from: LvW on December 31, 2016, 11:11:55 amThe DC gain needs to be changed to 1, whist keeping the AC gain the same.

author: Andy Watson
author_rank: Super Contributor
timestamp: December 31, 2016, 05:05:40 pm
content: Quote from: LvW on December 31, 2016, 11:11:55 amThe DC gain is unity.

author: Audioguru
author_rank: Super Contributor
timestamp: December 31, 2016, 05:38:50 pm
content: Of course there must be a coupling capacitor feeding the 1k input resistor of the opamp so that the opamp has a DC gain of only 1. It also might need an output coupling capacitor.

author: Andy Watson
author_rank: Super Contributor
timestamp: December 31, 2016, 05:50:24 pm
content: Quote from: Audioguru on December 31, 2016, 05:38:50 pmIt already hasC1 and C2. Remove these from the circuit (as would be the case for DC) and it becomes obvious that the op-amp has unity gain, DC feedback - via two paths.QuoteYep!

author: Zero999
author_rank: Super Contributor
timestamp: December 31, 2016, 06:57:32 pm
content: Quote from: Andy Watson on December 31, 2016, 05:05:40 pmOf course it is. The DC voltage at the inverting and non-inverting inputs closely follow the output.