Colpitts Adventure

When I was young…

I used to dread this little oscillator. I think part of the issue was any time I tried to build this thing, I didn’t have access to an oscilloscope, so I’d usually be trying to check if it was oscillating by placing a tiny ear-phone on the output — inevitably I’d be rewarded with a DC current flowing through my ear-piece indicating that the circuit was oscillating as well as a 2 week old rat corpse.

I’ve decided to tackle this circuit again; just real quickly on bread-board; because I need to escape my childhood fear of this circuit. I’m also sort-of interested in seeing how well this circuit could work as an inductance meter.

The Colpitts oscillator is incredibly well documented everywhere on the internet and I don’t really want to talk about it much. Briefly; it uses a single transistor to keep exciting a tank circuit. This circuit is analogous to repeatedly banging a tuning fork to keep it ringing.

The Build

I’ve drafted this schematic as a rough map to the electronic wilderness:

Potentiometer R1 is used for bias adjust on Q1’s base.; I was too lazy to work out what this should be. I figure I’ll just start at zero and wind my way up from there. Don’t judge me.

The rest of the circuit I just copied from someone’s instructable.  I said don’t judge!

Now that I have a rough guide, it’s time to give the breadboard the business and  actually build a Colpitts oscillator! .

I dig around in the bag of unsorted junk to grab a small transistor. The very first one I pull out is a BC547 which is an NPN, hooray! Looks like I’ve won the silicon lottery today, so I’m off to a good start..

Alrighty let’s pop this little gal into the bread board:

Look at this little gal, standing so majestically with three feet knee-deep in the bread-board.  I can’t really go back now. She wouldn’t have any friends to play with.

We should add the components to bias the base first:

This little BC-547 transistor has never felt bias of the like I’m giving it here — and it’s adjustable.

I really like these orange capacitors, they’re so bright and cheerful. I only have them in 1uF though, shame.  This particular one

I now need to get the breadboard holes nice and moist to slot in the 68 ohm resistor between the emitter and ground.

Everything has turned sideways, but I’m not going to let that slow me down. I’m basically at the half way mark now. I’ve got to add in the coup de grace, which is obviously the tank circuit.

Here’s the two capacitors that make up 1/2 of the tank circuit, they’re not as nice as the orange one.

I found this sexy wire-wrapped donut in a switched-mode PSU. I salvaged it for it’s aesthetic beauty. But perhaps it will also work as an inductor in this circuit.

I’ve almost finished. The Last step is to add in the feed-back capacitor C4. Frustratingly I can’t find anything in the right size though. I’ll have to resort to my secret bag of AC protection capacitors.

The 400V blue capacitor makes the green and orange capacitors look puny in comparison despite it being only a tiny fraction of their capacitance. Hopefully they don’t become insecure.

With the last capacitor in place the moment of truth has arrived.  I hook up the power, turn it on, twiddle the bias adjust photometer …. And there’s no oscillations.  El-zilcho. The dead-rat is back and my childhood fears are being re-lived in this moment.

I didn’t take any images of the oscilliscope but I can simulate the signal in ascii art for you:





I immediately start poking the circuit with my finger. Perhaps a few good pokes will start it oscillating? seems like I can’t poke charge into the tank circuit, at least, no today.Better get serious and start multi-metering things.

I measure some voltage on the emitter of the transistor, so we’re running some current through the sexy donut, but it’s not vibrating. Because not much else can go wrong; I hazard a guess that the feed-back capacitor C4 is too small and may be acting as a high-pass filter and filtering out the signal required to oscillate, so I made it lots bigger, 100nF


Success!  A sine-wave so clean and stable that my underwear is getting moist just thinking about what i could use this sine-wave for.

Oh.. as a little side-note here; I also tried increasing feed-back capacitor C4 all the way to  a 47uF capacitor. With this larger capacitor in place, the circuit oscillates with a much wider range frequencies. It seems like if you’re in doubt, use a large cap for the feed-back capacitor in this circuit.

Now that the oscillator actually, well, oscillates. I decided to get sciency and measure the resonant frequency with a known inductor, and compare it to the resonant frequency as predicted by math.

In my junk parts bag I found this even sexier black-donut of an inductor. The data-sheet indicates that its value is 68uH, which happens to be my favorite inductance!

After the sexy black-donut is wired in I measured a resonant frequency of about 227khz. Next step is to calculate what the resonant frequency should be, using maths, or at least, an online-calculator that does maths for us.


I’ve entered the frequency we measured (227khz), and the capacitance here represents the total capacitance in the tank-circuit. In my case this is a 0.1uF and a 0.01uf. The total capacitance of these two in-series is 9.09nF.

So, we expected to get 68uH, but we got 54uH. I guess this is in the ballpark…or at least in the same suburb. Maybe the capacitors or inductor values aren’t accurate? They’re only rated to be within 10%. A more likely explanation may be that internal capacitance in the inductor is causing a difference between the expected and actual values. Because this circuit will be impacted by the inductor’s indutance and capacitance both, perhaps it may not make the best inductance meter, shame.

Ok. Well. I’m over this circuit. Until next time! Bye!


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