Help me identify and analyze this oscillator

[Msakhy]

Can anyone identify this OSCILLATOR, and hence explain to me how I would go about analyzing it.

How do I get the above OSCILLATOR to oscillate at 20MHz with a 20V supply assuming 5pF stray capacitance at each transistor terminal? What if we use a FET (2N3819) instead of a BJT.

P.S. Note the inductor is tapped and not necessarily at the center!

 

[zeeshanzia84]

Re: Oscillator????

Hartley Oscillator - b/c of it's Inductive divider....split inductance.

The frequency of oscillator should be:

f = 1/(2*pi*√(L*C))

if you NEGLECT the transistor capacitances.


Otherwise, you have to replace the transistor with its small signal model and calculate the loop gain. Finally, apply the Barkhausen Criterion to obtain an expression for the exact frequency of oscillation.

HOPE THIS HELPS!

 

[Msakhy]

Re: Oscillator????

Confused!!! How did you arrive at the above formula??? Should it not depend on two inductors at least, and what role do the other capacitors play then - assuming that the capacitor in the formular is C3 in my schematic?

 

[E-design]

Re: Oscillator????

For frequency calculation the split inductor is treated as a single inductor by adding the values of the sections.

 

[Msakhy]

Re: Oscillator????

Enough said... How do I bias this circuit (i.e. determine R1, R2 and R3) if the output is @ the collector? How do I determine C1 and C2, and what role do they play in the whole configuration.

 

[zeeshanzia84]

Re: Oscillator????

L is the sum of the two inductors. The others capacitor is placed just to provide positive feedback with a difference of phase.