Dion said:
Can you elaborate a little. Try to talk about the current. For example:
current goes down from +15V through the center tap, goes left and
down T3 and into TR1, if you can continue in this manner until you
fully describe a full cycle. This way I will know whats happening in
the electrons level, rather than talk about phase shift, inverting
configurations, and other terms which don't mean much to me.
Here is a shot. However, one of the more knowlegable folks may wish to put
forth a better description.
Capacitors are plates that are some given distance apart. They don't allow
electrons to flow across them, but act like they do; turns out that how fast
the electrons accumulate is linearly related to how quickly the voltage
changes across the plates. The constant that relates the rate of change of
voltage to the current is called the capacitance C of the capacitor.
Inductors are devices that want to maintain the flow of electrons through
them at a constant rate. If the rate of flow of electrons changes, the
voltage across the inductor will change linearly with the change. The
constant for a particular device is called the inductance L.
If an inductor and capacitor are in parallel, and a DC voltage is applied
across the two, then current (electrons) will flow through the inductor
faster and faster until the resistance of the wires limits them.
However, if an AC voltage is put across them, then an odd thing happens. One
side is increasing in voltage while the other is decreasing in voltage. If
the speed of this change (the frequency) is just right, no AC current will
flow through the inductor/capacitor combination. However, current will flow
back and forth between the capacitor and the inductor. This situation has
been compared to a spring and weight in dynamics. The AC voltage excites the
system, and the current flows back and forth like a spring moving back and
forth.
Oddly enough, however, if the AC voltage fluctuations are lower or higher
than that 'perfect' value, AC current will flow through the system. You've
probably heard of the concept of resonance. With a spring and weight, you
can 'hit' it, and it will usually oscillate at a frequency that is related
to the force in the spring and the mass of the weight. For AC, if the ac is
lower frequency, then the inductor doesn't oppose the current as much, and
the capacitor opposes it more, but the sum doesn't equal 0, so current flows
through rather than circulating. Same if its higher, although in that case
the inductor opposes the current more, and the capacitor less. Same thing,
though, the total will be less than the maximum at that 'perfect' frequency.
Set that aside for a moment.
Now, the gain of a common emitter amplifier (which is how the transistors
are hooked up in your circuit) is related to the resistance at the collector
(among other things). Larger resistance means larger gain, up to a certain
point. Resistance means the ability to block electron flow; its the same as
the AC resistance described above. Given random fluctuations, there are lots
of different frequencies in both the transistors initially. However, because
of the fact that fluctuations at the particular 'perfect' frequency are
blocked, this increases the gain of the amplifier for fluctuations at that
frequency, and decreases the gain at all other frequencies. Thus, the
natural fluctuations at that frequency are amplified into oscillations at
that frequency.
I don't usually think about these things in this way, so my descriptions are
based on half remembered physics lectures in the early 80s.
Regards,
Bob Monsen