Could some electronics guru here please help ? What is the best way to select a transistor for an oscillator ? Consider a Colpitts oscillator running at 5 MHz. The following are the analysis steps. 1. Select a value for the LC tank capacitor C1 = C2 = C 2. Select a load e.g., R=50 Ohm 3. To start oscillations, gmR > C2/C1. In this case gmR > 1 4. Using the oscillation frequency and the LC tank capacitance value, the inductor value is obtained. 5. Having determined gm, find the collector current using gm = Ic/VT where VT = 0.025 6. From the VCC value, the collector current limiting resistor is VCC/Ic 7. The base bias resistors are then obtained using the fact that the maximum base bias current is 0.1 times the collector current. Then the question is: how to select a transistor ? All hints/suggestions are helpful. Thanks in advance

1. Equal capacitors?? But C = C1*C2 / (C1+C2)? 2. Note this ignores component losses: inductor, capacitors, and transistor too. 3. To follow on from that, the reflected impedance of the base (assuming a common emitter configuration) needs to be similarly high, otherwise the tank goes thud, or insufficient signal gets to the base. Mind that, in addition to being a resonant tank and phase shift network, the CLC is also an impedance matching network. The capacitor ratio gives the voltage ratio, and therefore the impedance ratio squared, but that impedance is only valid if the capacitor impedance itself is low enough. Which is also to say, the tank Q is greater than the voltage ratio as well. 4. I would start with Zo = sqrt(L/C) and Fo = 1 / (2*pi*sqrt(L*C)) instead, where Zo is the resonant impedance, which will be Q times the equivalent total load impedance. 5. Yup, for a BJT of course; and for anything else, whatever it is. This is minimum, of course, and probably 2-3...

How to select transistor for oscillator ?

Sep 11, 2018 45 Replies

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<698839253X6D445TD 7 years ago

John Larkin wrote

It is probably one of those fortune telling machines that you have to rotate and then you see the future in it. google does not know that number, you should tell them.

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makolber 7 years ago

2N107 my first transistor m

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John Larkin 7 years ago

OK, you don't know.

John Larkin Highland Technology, Inc lunatic fringe electronics

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DecadentLinuxUserNumeroUno 7 years ago

wrote in news:pndu81$1l3q$ snipped-for-privacy@gioia.aioe.org:

It is either a diode vacuum tube or a vacuum tube triode. Used in transmitter output sections.

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<698839253X6D445TD 7 years ago

What you probably meant by CCRO is a dielectric resonator based oscillator

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The link you pointed to says ceramic-resonator-band-pass-filter

CCRO filter stands for Closed Circuit Reverse Osmosis filter something entirely different.

Yes dielectric resonators are the same as the 'pucks' in the picture I showed earlier:

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People can tune those (increase frequency) by removing something of the material. or add something to decrease frequency.

Those are not very stable compared to a crystal controlled PLL.

dielectric resonators!

Anyways, now we know what we talk about.

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Joseph Gwinn 7 years ago

On Sep 13, 2018, John Larkin wrote (in article):

I?d say HV spark gap.

It does not look like it world work as a T/R switch - wrong shape, although I suppose it could be between two pillars transverse to a rectangular waveguide (shorting the long walls).

Joe Gwinn

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