Colpits Oscillator

Hello
i'm just design a Colpits oscillator in order to understand it but i have some doubts. In attach the simulation of the circuit but i can't adjust the feedback correctly, as you can see the transistor goes everytime in saturation and i can't find a way to avoid it.

It runs ok for me. Pure Sine Wave.

Ramussons said:

It runs ok for me. Pure Sine Wave.

Click to expand...

But the base current got a peak due to the transistor saturation. Is possible to let the transistor work in the active region?

Quasar999 said:

But the base current got a peak due to the transistor saturation. Is possible to let the transistor work in the active region?

Click to expand...

Why?.

I suspect you're missing the point of an LC oscillator - it's like a bell, you 'hit' it and it rings.

You could reduce the bias, but then you've got the issue of it not starting reliably.

So that's the normal behavoiur of a lc oscillator? Ther bias is correct but the amplitude of the current seems too much with no control.

Yes - for an oscillator to give a sine wave you need an accurate and exact amount of feedback - too little it won't oscillate, too much and it will be distorted (and even a square wave). Even worse, the gain varies with temperature, supply voltage etc. This is why wein bridge oscillators have some kind of control system in the feedback network to compensate.

An 'easier' solution is to use an LC tank, and allow that to create the sine wave - you can then have much higher gain, and the tank circuit takes care of reducing distortion.

If I understand your simulator display correctly, the frequency is about 4 kHz, so this is an audio frequency oscillator, as would be expected from the values of L and C in the tuned circuit.

In the real world it would be unusual to use an oscillator like this at such a low frequency, the inductor is big and expensive and the capacitors would need to be electrolytic types which are not the best for having a stable well defined capacitance.

For a well behaved audio oscillator, the Wien Bridge circuit is often used.
The frequency determining components are resistors and lower value capacitors made from stable materials.
The maintaining amplifier gain is stabilised usually by a thermistor or in some cases by a very small incandescent lamp.

For a radio frequency oscillator, the Colpits circuit is often used in one of its many variants.
As previously mentioned, the gain of the maintaining amplifier is often a compromise between low gain for a harmonic free output and a high gain for reliable starting.
A cleaner harmonic free output can be obtained by inductively coupling to the tuned circuit inductor, rather than a direct connection to emitter or collector of the maintaining amplifier.

JimB

Quasar999 said:

Ther bias is correct but the amplitude of the current seems too much with no control.

Click to expand...

R4 provides the negative feedback to control the current. the technical term in the case of R4 is "degenerative feedback"

unclejed613 said:

R4 provides the negative feedback to control the current. the technical term in the case of R4 is "degenerative feedback"

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Correct. But there's only precise value for the oscillator to start (i think is due to the Barkausen Criterion) but over that you'll have full saturation of the transistor any cycle. No good regutation si possible, maybe with a variable capacitor but the temperature will move the equilibrium.