(1) Stability depends on several
factors
built
into
this
circuit. The higher the Q of
the tank circuit the better
the
stability.
Bias stability is attained by
the
action
of
R2
in
conjunction with R1 and C1.
Resistor R1 and capacitor C1
provide
a
self-regulating
base bias.
This regulating
action is identical with that
obtained
with
grid-leak
biasing of an electron-tube
oscillator.
Like grid-leak
bias, base-leak bias builds
up and adjusts itself to
insure
a
constant
output
signal amplitude.
(2) Since an LC tank insures a
good
sinusoidal
this oscillator and other LC
types
can
be
biased
to
operate Class C.
Class C
operation is preferred for
Figure 2-5.
Tickler-coil oscillator.
its higher efficiency.
(3) Modified versions of this type may have the tuning tank placed in the
input circuit or use different transistor configurations.
b. Hartley. As in the case for electron-tube oscillators, the dc power supplied
can be series fed or shunt fed. Recall that a series feed passes dc through the
tank circuit, whereas a shunt feed does not. The tickler circuit shown in figure
2-5 is series fed.
The Hartley oscillator circuit shown in A of figure 2-6 is
shunt fed; note that no dc is applied to the tank circuit.
The RF choke (RFC)
blocks the signal from the power supply.
Capacitor CO blocks dc and effectively
passes RF. Either series or shunt feed can be used for LC oscillators.
(1) Whether series or shunt fed, the ac circuit for a Hartley oscillator is
that shown in B of figure 2-6.
Capacitors CO, CB (for biasing), and
bypass CE are effective RF shorts.
The tank circuit is common to the
input and output circuits.
This type oscillator is characterized by the
connection made to the tank's inductance. When a single coil is tapped,
there is inductive coupling and auto-transformer action. Phase inversion
from collector (point c) to base (point b) can be explained by the fact
that the signals at opposite ends of the coil are 180 out of phase with
reference to the emitter (point e).
However, inductive coupling is not
necessary. The tank inductance can be made up of two separate coils, one
in the base circuit (b and e) and the other in the collector circuit (c to
e).
17
Previous Page
