Electronic – RC Oscillator for high frequency not working

Inside the data sheet you will see the basic circuit of a CD4049. Here: -

If Vin is somewhere about halfway between Vcc and Vss, both transistors are conducting and the voltage at Vout will be neither 1 nor 0. In effect both transistors together act like a potential divider; an upper resistor and lower resistor.

This is the linear operating point of this digital circuit.

Next it should be noted that if Vin were taken slightly higher then Vout would fall to Vss much more rapidly than for the small change in Vin - this indicates that there is gain in the configuration and that it is an inverting type of gain.

Next, apply a high value resistor between output and input and the ouput will stabilize and some "halfway" voltage due to negative feedback just like an op-amp would.

Next apply an ac input to Vin via a small capacitor and resistor in series and you get an analogue AC amplifier.

Are you convinced that this type of digital gate can be made into an analogue amplifier? Here's and article showing how a CD4049 can be made into an op-amp. Below is a picture of a CD4049 being used as an AC amplifier: -

Here is the article.

All that remains to make a crystal oscillator is to provide the feedback resistor (maybe 1Mohm to 10Mohm), add a couple of capacitors and the crystal and you have an oscillator. Note that the combination of output impedance and capacitor connected to the output produce a signal that is more sinusoidal that square hence you can make an approximate sine wave oscillator.

Note also that it is this network (together with the input cap to ground) AND the crystal that produces 180 degrees phase shift.

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).