Questions about colpitts oscillator design


Posted on Feb 6, 2014

amplifier input and output impedance, load and source resistance are strongly mutually dependent in the CC circuit. That`s a reason, why `impedance matching` doesn`t achieve results representative for the circuit operation. Analyzing the circuit as CB with virtual ground, as suggested by LvW, gives a better decoupling of both amplifier ports. Impe


Questions about colpitts oscillator design
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dance matching isn`t a bad design idea, when the amplifier power gain is small and must be well utilized to get the oscillator working at all. But it must consider the actual frequency dependent transistor parameters. - amplifier input and output impedance, load and source resistance are strongly mutually dependent in the CC circuit. That`s a reason, why "impedance matching" doesn`t achieve results representative for the circuit operation. Analyzing the circuit as CB with virtual ground, as suggested by LvW, gives a better decoupling of both amplifier ports. Impedance matching isn`t a bad design idea, when the amplifier power gain is small and must be well utilized to get the oscillator working at all. But it must consider the actual frequency dependent transistor parameters. For an aid to understanding the operation of these LC tank oscillators, I suggest you watch my Youtube videos which show an animation of Colpitts and Clapp oscillators. It`s a pity that samy uses symbols without sufficient explanation, but I think he didn`t make any error as far as RE is concerned (but I am surprised about his comment "yes, you are right"). According to his calculation I suppose the symbol "RB" stands for the resulting DYNAMIC input resistance of the whole amplifier stage and the symbol "RE(total)=RE+re" is just an intermediate step for the sum of two resistances. It`s true, that the original circuit also covers the clapp oscillator...




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