27MHz colpitts oscillator qestion

[mvs sarma]

i really mant that i still learn (not literal student going to collge )

 

[ccurtis]

Studying your circuit some more, I've come to the realization that the frequency pull from the calculated LC value is due more to the input resistance at the emitter of the transistor, Re, than on the feedback capacitor, although both are influential. All of the ac component of the collector current is forced through the feedback capacitor since the impedance of the LC tank is infinite. The higher Re is, the lesser the influence the feedback capacitor has on the LC tank. Re can easily be increased by lowering the base current, accomplished by increasing the value of R1. Also, since all of the ac component of the collector current must flow through the feedback capacitor, the feedback capacitor value can be quite small, as you have there, and more than enough feedback can be always be assured. The current gain of the common base configuration is very nearly one to begin with, requiring very little feedback. Of course, this is assuming ideal components, as you find in simulations. Is also assumes that you bias the transistor more toward one-half the supply voltage so that the transistor is operating in its linear region, connecting a 10K resistor from the base to ground. With the base voltage at nearly 9 volts, the circuit is highly non-linear and analysis of the circuit is complex, though the general ideas apply.