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Colpitts Oscillators

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I'm in the process of making colpitts oscillators as part of a theremin project. The question I have is this, why do most example circuits (bjt) have a low value emitter resistor in the range of 470r to 2.2k? Is it for stability, low drift, precision etc. or is it overkill.

I'm committed to 9v battery sound effect projects so low power drain is requisite.
 

That resistor in the emitter is usually for stability (higher Ic = higher Ie that will cause a higher drop voltage on that Re that will cause Vbe to go down what causes Ic to drop, stabilizing the circuit) and it is often used with a by-pass capacitor to not screw up the gain.
At least that's what I remember from eletronic classes.
 
With no emitter resistor, you have:
* greater gain
* greater sensitivity to small bias changes
* larger voltage swings of the output
* greater current flow through the transistor circuit.
* trickier to adjust so the circuit operates as desired

The last item is related to stability, as post #2 describes.
 
Look at the datasheet of a transistor. There are graphs that show how a "typical one" works at a certain temperature and certain current but the printed text lists the ranges of current gain and base-emitter voltage because each transistor is different even if they have the same part number. An emitter resistor reduces much of those changes between different transistors.

I design a transistor circuit so that each and every one of thousands work exactly the same with no adjustments, they use an emitter resistor. If you design a circuit without an emitter resistor then one might work if you adjust its biasing and its temperature and never change its supply voltage. The next one you make will need more adjustment changes.
 

Audioguru said:
If you design a circuit without an emitter resistor
Click to expand...

It's the other way round, I want to increase the value of the emitter resistor but I'm just concerned that none of the typical examples use a higher value. So, I'm wondering if there's a reason for that.
 

You can make it larger, but quickly run into the point of diminishing returns as regards further increasing stability.

An oversized emitter resistor then becomes just wasteful voltage dropper, reducing the available supply voltage across both the transistor and collector load.

Just enough to keep the operating current within reasonable bounds over a sufficient range of supply voltages, temperature, and individual device characteristics, is all you need. Its really a judgement call rather than something that has to be set within very narrow limits.
 
The LC (or LCC) tank gets less current if your emitter resistor is a very high value. You need a certain amount of current through the transistor, to drive the oscillator. A small current might work in a theremin, depending on your L & C value.

Perhaps the C value is very small, influenced by the musician's hands? Then current can be small, but then L value needs to be large. You'll adjust resistors to create the desired sounds from the LC tank.

One characteristic of an instrument's tones is attack. The amount of current available, makes a difference in how slowly or rapidly oscillations get going. So the emitter resistor could come into play here.
 
Are you trying to re-invent the wheel? Can't you simply copy one of the many Theremin projects on the internet? here is one:
 

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