yacine

hi, please I want to know how to calculate the capacity that found between the trans and coll of transistor in the colpitts oscillator
sometimes the oscillator is using like an rf reciever
which freq we can take in our calculation
greeting

Hero999

It's a shame because there was a great site containing this sotr of formula but I don't know if it's still with us.
http://www.electronics-tutorials.com...scillators.htm

__________________
I also post at the following sites:
http://www.stop-microsoft.org http://www.heated-debates.com
Screen name: Aloone_Jonez

RadioRon

Your request is not clear so I do not exactly understand. The colpitts oscillator configuration includes a crsytal to ground from the base of the transistor and also a capacitor from base to emitter. These are the two critical parts and determining the value of that capacitor is probably the most important issue. The best value for this capacitor as well as the base bias resistor, the emitter resistor and the emitter capacitor are often optimized though experiement as they are sensitive to the inductance and resistance of the crystal. But most people don't find the best values, they just find what is good enough to function. Some good values to start with are:

the base to emitter cap = 47 pF
base bias resistor = 100K assuming 10MHz crystal (use larger R for lower frequency)
emitter resistor = 1K (tune for desired collector current)
emitter capacitor: use formula R(emitter) x C = 0.5/freq (freq in MHz), C in uF, R in ohms. Typical value is 100 pF for 10Mhz crystal

The best values are the ones that maximize the voltage swing across the crystal.

__________________
RadioRon

Hero999

What about the LC colpitts oscillator?

That's what I thought he was talking about

I didn't think that Colpitts is normally used for crystals, I thought Pierce is normally used for crystals.

__________________
I also post at the following sites:
http://www.stop-microsoft.org http://www.heated-debates.com
Screen name: Aloone_Jonez

RadioRon

I believe that colpitts is commonly used with crystals and most of the configurations below 20 MHz don't require inductors. It is an emitter follower configuration. The crystal provides the resonating inductance and the collector circuit need not be tuned as you usually take output from the emitter. However, if you want to pull harmonics from the oscillator, tuning the collector is done, with output taken from the collector. I've also seen harmonic colpitts oscillators that use an inductor from emitter to ground and take output from the emitter.

I have seen confusion in the past with the names of various oscillator configurations. Are we talking about different oscillator configurations?

__________________
RadioRon

RadioRon

Oh, I see your point. What if he is asking about LC, not crystal. Right, now I get it. Well, in that case I refer to the ARRL handbook for 2001, page 14.13 where they suggest that the parallel colpitts, in this case using a FET, be configured using an LC tank circuit which is then top coupled to the gate of the FET. In this case, make the feedback capacitors both with Xc of 45 ohms at the frequency of oscillation. Set the top coupling cap Xc to about 100 ohms, and then choose the tank circuit L and C to resonate with your choice of Q, presumably fairly high Q like 50 or so. The intent is that the tank be lightly coupled to the oscillator.

__________________
RadioRon

yacine

hi, thank your help but for clear what I mean it I send you with this Thread two schematic one of colpitts oscillator and other I design it by my self
my objective by this question is using them like an RF receiver
schematics
- you can see an capacity in first one between coll and trans of transistor
that what I search about in calculation please simple way
- second one I do not know about ITs work or not there is capacity between coll and base of T what is your openion about it and ITs value
thank you

Attached Files

a.pdf (14.2 KB, 14 views)

Russlk

I think Yacine wants to make a super-regenerative receiver. Perhaps he should Google that term.

__________________
see my website: www.geocities.com/russlk