Unusual Colpitts

 

Is it your design?

 

Is it your design?

Yes, it is.

 

HI,
It looks like a variant of the popular Joule thief circuit.
E

 

What is the DC bias point?
There is a DC short between base and emitter.
And the B-C junction is forward biased.
Transistor in reverse operation?

 

What is the DC bias point?
There is a DC short between base and emitter.
And the B-C junction is biased in reverse.

With purpose to decrease β, Q1 is used in reverse mode.
Voltage on B-C is 0.7 V, current through R1 is (1.5 V-0.7 V)/R1 = 8 mA.
Because for reverse biased 2N2222 β=3, then Ib=2 mA, Ie=6 mA.
So DC bias point is 6 mA @ 0.7 V of emitter voltage.

 

With purpose to decrease β, Q1 is used in reverse mode.
Voltage on B-C is 0.7 V,

Yes - I have detected my typing error (B-C biasing) and have corrected my answer correspondingly (2:02 PM).

 

It has some interesting properties not always consistent with Br or Bf.
When you have very high Q CLC components, it doesn't take much to keep the ringing going.

I tried to push up f , at lower mA and tried different NPN's with low Br ( reverse Beta)

Essentially the R1 powers the inductor with an initial condition with Ve = 0 across the cap. In some cases with different transistors, the ringing decays as if no transistor was connected in a few ms. With very large L1, C1, the Vb is essentially flat , modulated by the output Ve voltage to reach > 0.7V.

Like most Colpitts oscillators it just needs a small nudge of current for each cycle to sustain a nice sine wave from peaking of Vbe (or Vce in this reverse case.)

The caps are C0G and DCR= 0.8 Ohm on L1.

There is no buffering by this transistor. It simply injects some emitter current out of phase with the cap. in the LC tank to keep it running.