Colpitts Oscillator - Page 1
Forum section: electronics / beginners

author: combsbt
author_rank: Newbie
timestamp: April 22, 2015, 04:40:47 am
content: So in this video on sparkfun:https://www.sparkfun.com/news/1796the guy explains a Colpitts Oscillator and shows it working. Figure 3 on the wikipedia pagehttp://en.wikipedia.org/wiki/Colpitts_oscillatoris the same as the one in the sparkfun video. I commented on the sparkfun page with my question but I doubt it will ever get seen so I'm going to reproduce it here."So I made one of these following the same schematic shown on the white board. It has the signal out in a weird spot. I was not able to see a signal there, just the input dc voltage, which makes sense as there is a direct path through the inductor to that “output” and the inductor acts as a dc short circuit. However, when I probed the emitter of the transistor I saw the expected signal.This diagram is consistent with other ones I’ve seen and even the one on the wikipedia page for Colpitts Oscillators. How could you not get a DC short circuit with that setup? I must be missing something."I used an toroidal inductor that I pulled off an old board, I measured it's inductance to be around 215 millihenries by finding its resonance with a capacitor of known value, as well as by using this methodhttp://www.dos4ever.com/inductor/inductor.htmlwhich agreed on the value.I used 3 220pF caps (the only difference in the diagram in the video and the wikipedia one is that in the video C2 is replaced by two capacitors in series) and calculated the frequency of the oscillator to be 1.27 MHz. When I probe the emitter of the transistor I get 1.39 MHz which is close enough for me. However, I can remove C1 resulting in a slightly larger amplitude signal at 1.85 MHz. I still don't understand the part that is labeled output, what the point of C1 is, or how the circuit would work loaded (it seems to just draw dc current if I add a resistor from where I'm seeing the signal to ground).My background is in Physics and I'm still relatively new to the electronics design scene, but I can handle any of the math and theory you want to throw at me. I'm just trying to figure out what exactly is going on here. We covered very simple circuit concepts in physics classes but not so much of amplifiers and oscillators.

author: combsbt
author_rank: Newbie
timestamp: April 22, 2015, 05:05:42 am
content: I just realized I could probably use an op amp as a buffer on the output signal and load that if I wanted to drive something with this signal. I watchedhttp://www.eevblog.com/2014/04/06/eevblog-600-opamps-explained/but that is the extent of my knowledge on amplifiers. I'd still appreciate any insight on the nature of the colpitts oscillator though.

author: Dago
author_rank: Frequent Contributor
timestamp: April 22, 2015, 06:18:23 am
content: Could you maybe try to explain a little better where do you think there would be a short?The inductor works as a biasing inductor, it is indeed a "short" for DC, it transfers DC power to the oscillator tank. But at the frequencies the oscillator is operating it has very high impedance and basically none of the RF-energy gets through it.

author: combsbt
author_rank: Newbie
timestamp: April 22, 2015, 07:23:47 am
content: Ok, that makes sense if its actually oscillating. Why am I just seeing the DC input voltage there when I probe it? I don't think I understand the tank circuit in this setup. Is it just the series L and C1 or is C2 part of the tank as well as providing the feedback?

author: Dago
author_rank: Frequent Contributor
timestamp: April 22, 2015, 10:49:27 am
content: Quote from: combsbt on April 22, 2015, 07:23:47 amMy guess would be that the oscilloscope probe is loading the circuit (making the tank Q-value smaller) enough that it stops oscillating or the oscillation is so small you cannot see it with an oscilloscope. Remember that the probe is not some magical invisible thing that does not affect the circuit in any way, but it is an impedance to ground. At DC the impedance is usually 1 Mohm, but at higher frequencies it drops rapidly and be in the range of 100 ohms at these frequencies!

author: Neganur
author_rank: Unknown
timestamp: April 22, 2015, 12:38:06 pm
content: I remember that wiki circuit, a group built it in the lab some years ago and it just didn't want to start oscillating.I'll have a look at my old notes when I'm home tonight.

author: Neganur
author_rank: Unknown
timestamp: April 22, 2015, 10:04:17 pm
content: I didn't find what I was looking for but I think it was the feedback that is not behaving well.Attached circuit is a little nicer, you can lower R3 a little if the output is distorted/clipping.