LC and Oscillators

[walid]

Hi
I noticed in many books that LC tank is connected to the base of the transistor, while connected to the collector in a real designs. Why this confusion?
thanks

[Hero999]

You forgot the schematic.

[KevinIV]

Walid,

         A parallel LC filter circuit has a roll-off of 40db/decade outside the resonant frequency. An oscillator requires high gain, but it's selectivity can be independant of the gain circuit. The final design depends on the characteristics of the circuits and the components used.

[walid]

Hi KevinIV,
Have you ever design an oscillator?
if yes, please tell me how with the related math
thanks

[Hero999]

How can he provide such an in-depth answer without the schematic? That's impressive.  

What sort of LC oscillator are you talking about?

There are many configurations. The only thing you hint at, is you're using a BJT.

Here are some examples:




http://www.sentex.ca/~mec1995/tutorial/xtor/xtor7/xtor7.html

In all circuits, the base is connected to the LC tank in some way, perhaps that should give you a clue.

If you want a decent answer, post the schematic.

[KevinIV]

Another aspect with these oscillators is the series capacitor inductor in parallel with the inductor. Not being a parallel LC means that the resonant frequency is not 1/(2pi x sqrt LC). The capacitive reactance does not equal the inductive reactance as it does in a resonant parallel LC. However, the reactance of the inductors will be equal at any frequency with a center-tapped transformer.