operational-amplifieroscillator
I use the following Colpitts oscillator schematic and I always need to put the scope probe on the inverting input of the opamp to make it start. Otherwise I get a DC voltage and no oscillations.
Do you have any idea about what I can do to to have a reliable startup at powerup ?
Thanks.
If you alter your diagram to look more like this it should work:
In your diagram, instead of a voltage divider and a capacitor you only have a single resistor. This won't work because the output of the Colpitts oscillator dips below 0.6 volts, and anything less than ~0.6V won't be picked up by an NPN transistor.
In the diagram, you can see that I've coupled the output of the oscillator with C2 to a voltage divider, with R1 and R2. The voltage divider adds a DC offset of 9V to the base of Q2. This way, the voltage at the base now ranges from 3V to 15 volts instead of -6V to 6V. Since the signal is always above 0.6V, Q2 passes it.
The output uses C1 to decouple the DC from the sine wave, leaving you with your original 6V amplitude sine wave, just with amplified current.
You can change the values of some of the components around if you want, but C2 should optimally be around 1/(2*pi*500KHz*R2), and C1 around 1/(2*pi*500KHz*R3).
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 .
Best Answer
Try reading this tutorial: -
Note the use of R3 to prevent the op-amp output being directly loaded by one of the capacitors. Theoretically you have to have some phase shift between output and first capacitor and therefore you need a resistor.
Note also that the gain in your circuit is far too high.
I'm not ruling out anything else too.