In <a231f34f-d2dd-4f2c-a7f0-9c2c45e59...@i24g2000prf.googlegroups.com>,
Both resemble variants of the Hartley oscillator, with the capacitor
across the tapped inductor removed.
A blocking oscillator has the oscillation feedback path through a
resistor and capacitor in parallel. During the half-cycle where the
transistor conducts, positive feedbck is through the capacitor. Once the
capacitor has become excessively charged to maintain enough feedback to
keep the transistor saturated, this half-cycle ends. This half-cycle is
usually the shorter one. During the other half-cycle, the transistor is
off until the capacitor is discharged sufficiently by its paralleled
resistor to allow current to flow through the base of the transistor.
In a ringing choke oscillator, the oscillation feedback (assuming a
bipolar transistor) is through a resistor. The "transistor-on" half cycle
is usually the longer one. That half-cycle ends when either the tapped
inductor saturates or the transistor comes out of saturation. The
transistor-off half cycle's onset reinforces itself with the feedback
winding's voltage changing in a direction to reduce the transistor's
collector current. In fact, once collector current starts decreasing, the
transistor usually quickly slams off, and a high voltage pulse can occur.
If a ringing choke oscillator is not carefully designed, the transistor
may be destroyed by breakdown from high voltage pulses resulting from
suddenly interrupting current flowing through the inductor.
The transistor-off half cycle ends when the transistor's collector
current has decreased to zero and has become steadily zero. There may be
a delay for stray capacitance charged by the high voltage pulse to
discharge before the feedback winding produces voltage in the forward bias
direction, but that is usually short.
- Don Klipstein (
[email protected])
