Electronics > RF, Microwave, Ham Radio

Colpitts oscillator biasing

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init:
Hey guys, bit of a newbie question here. I'm looking at designing a colpitts oscillator using common emitter/source topology similar to this. I understand how to tune the tank circuit to create the desired resonance frequency but I am unsure how to go about biasing the circuit. From what I've read, the idea is to set the biasing conditions of the circuit such that "the oscillator works". What sort of biasing conditions does the circuit have to be set at to sustain oscillation?

MrSlack:
This is basically a common emitter amplifier with a frequency dependent feedback network. So there are two bits to this. With respect to the amplifier, setting the base voltage (defining operating point) and setting the negative feedback to maintain stability. If I'm honest I've usually set this up empirically with some trimmers. Define the quiescent current at Ic/Ie (500uA is pretty good starting point), pick the emitter resistor to set the base voltage to about 1/2 vcc (Ve+0.6) then crank up the gain by changing the Rc/Re ratio so it oscillates but doesn't saturate the transistor. 99% success rate, not much mathematics which tends to be a little too ideal when it comes to building these in real life.

However this is a textbook circuit by the looks rather than a real one so I'd do some research on other designs first. They can be a lot simpler.

init:
Thanks for the reply, I too have been fiddling with the biasing values in LTSpice until I get oscillation but I recognise this is a bit lazy and I would like some mathematical justification.

--- Quote from: MrSlack on April 20, 2016, 06:13:47 am --- then crank up the gain by changing the Rc/Re ratio so it oscillates but doesn't saturate the transistor

--- End quote ---

I've heard from someone a lot better than me at this that the reason the oscillations occur is because the transistor saturates. Is this true?

nugglix:
Hi!

--- Quote from: init on April 20, 2016, 06:31:42 am ---I've heard from someone a lot better than me at this that the reason the oscillations occur is because the transistor saturates. Is this true?

--- End quote ---

No.
The oscillation starts way earlier.
If you inspect the start of the oscillation (via LTSpice) you will find that the signal amplitude
is rising in a exponential manner.
And this happens way before the transistor saturates.
So the saturation is clearly not the cause of the oscillation.

The oscillation purely depends on the positive feedback of the signal.
So you feed back a signal which is larger then the original, which in turn
gets amplified more and this is fed back...

Given the above it is clear that the transistor will reach the saturation region when
the signal becomes too large and no counter-measures are implemented.

The resulting signal will be heavily distorted when the transistor reaches saturation,
which results in a lot of harmonics.
I'm not sure if you want this to happen.

The oscillation is started from the inherent noise of the system which gets amplified,
frequency filtered and fed back into the amplifier.
If you simulate oscillators in LTSpice, it might help to activcate the startup option, so
that the power is ramped up like in the real world. This helps a lot in most cases.
The parts in LTSpice are very clean when it comes to noise.

Just my 2 mOhm :)

Cheers
Guido

MrSlack:
Agree with setting startup condition. Solves most oscillator starting conditions instantly! You can also inject some noise into LTspice easily with the random() function. Occasionally I introduce some 2-3mV noise into the supply - helps to isolate where I'm going to see noise floor being pushed up.

Also a practical tip: Make sure every stage in any RF/oscillator circuits you build is decoupled from all the others cleanly. You'll be surprised where unintended oscillators appear with dire consequences. You can do this at a basic level with a small resistor in series with the stage and a decoupling capacitor across the stage if you're a cheap ass like myself but something with better rejection like a small DC inductor would be a better idea.