Electronic – Colpitts oscillator not oscillating

analogoscillator

I have tried breadboarding a simple Colpitts oscillator, just to see how it works (and to get to use my 'scope for something more interesting than measuring static voltages).

I have been following this example, specifically the second circuit design:

A Colpitts oscillator http://www.play-hookey.com/oscillators/lc/images/colpitts_oscillator_cb.gif

I'm feeding it 5 V, the resistors are all at 1k, L is a .22 µH fixed inductor, Q is a 2N4401, C1 and C2 are .001 µF ceramics and the unlabeled cap at the base is a 220 pF ceramic (maybe way too low?), and I'm probing between emitter and ground.

Now, admittedly, these values are more or less randomly chosen from my component drawers. In this case I am not interested in obtaining a specific frequency as long as it's low enough for me to measure it (50 MHz), so I figured I could just throw in any values for the caps and the inductor, as long as they were high enough – I've read that this can actually be a pretty accurate method of measuring capacitance and inductance respectively, based on the frequency you get.

Questions:

  1. Why is the circuit not oscillating? I'm measuring a DC voltage of 1.87 V at my probe point.
  2. How do you calculate the proper resistor values (or ratios)?
  3. What's the base cap used for? Just power decoupling?
  4. Am I probing in the right place?

Best Answer

My answer:

1.) I do not know because I didn`t recalculate the circuit. However, it is YOUR task to find a suitable design (not using random parts values), see point 2).

2.) At first, you must understand the circuit (why it can oscillate). There is a frequency-selective feedback network with a bandpass characteristic (L in parallel with C1 and C2). Do not overlook that the supply voltage is identical with signal ground.

Hence, at the midband frequency the phase shift will be zero. A part of this signal (depending on the C1-C2 ratio) is fed back to the emitter establishing the required positive feedback (loop gain).

3.) It is the task of the base capacitor to keep the base at signal ground (transistor in common base configuration and positive gain, see 2).

4.) The classical (normal) output for common base stages is at the collector .

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