[snip]
Colpitts and Clapp circuits can be analysed as negative resistance
oscillators, and the capacitive divider is shown as part of the
negative resistance amplifer, even though it is part of the tuned
circuit. If the dotted line - where tuned circuit meets "reflection
amplifier" - is drawn to the left of the capacitive divider, there
would be equal and opposite reactances either side of the line. But
the oscillator frequency is not the same as the resonant frequency of
the series LC components to the left alone, because the capacitive
divider contributes significant reactance.
I am glad to hear you say that. I have always had a problem with the
concept of the negative resistance connected to a tank circuit. My
problem is that the transistor by itself cannot create the negative
resistance, it needs the impedance transformation created by the tank
circuit to create the negative resistance. So it really is all about
where you "put the dotted line" as you say. The negative resistance
may be one concept for the thought process but there is no actual
physical way that you can disconnect the tank circuit from the
transistor, and connect a network analyzer, and measure a negative
resistance. The tank circuit must also be attached to create the
negative resistance because it creates the impedance transformation.
So the tank circuit and transistor TOGETHER create the negative
resistance, but this is just another way of saying that you connect an
amplifier to a tank circuit and the amplifier makes up for the losses
of the tank and for the output power so it oscillates. To me, the
amplifier concept is much more straight forward compared to the
negative resistance concept.
Mark