2-18
When a voltage is applied to the circuit, current from the battery flows through coil L1 and to the
emitter through RE. Current then flows from the emitter to the collector and back to the battery. The surge
of current through coil L1 induces a voltage in coil L2 to start oscillations within the tank circuit.
When current first starts to flow through coil L1, the bottom of L1 is negative with respect to the top
of L2. The voltage induced into coil L2 makes the top of L2 positive. As the top of L2 becomes positive,
the positive potential is coupled to the base of Q1 by capacitor C1. A positive potential on the base results
in an increase of the forward bias of Q1 and causes collector current to increase. The increased collector
current also increases the emitter current flowing through coil L1. Increased current through L1 results in
more energy being supplied to the tank circuit, which, in turn, increases the positive potential at the top of
the tank (L2) and increases the forward bias of Q1. This action continues until the rate of current change
through coil L1 can no longer increase. The current through coil L1 and the transistor cannot continue
increasing indefinitely, or the coil and transistor will burn up. The circuit must be designed, by proper
selection of the transistor and associated parts, so that some point is reached when the current can no
longer continue to increase. At this point C2 has charged to the potential across L1 and L2. This is shown
as the heavy dot on the base waveform. As the current through L1 decreases, the voltage induced in L2
decreases. The positive potential across the tank begins to decrease and C2 starts discharging through L1
and L2. This action maintains current flow through the tapped coil and causes a decrease in the forward
bias of Q1. In turn, this decrease in the forward bias of Q1 causes the collector and emitter current to
decrease. At the instant the potential across the tank circuit decreases to 0, the energy of the tank circuit is
contained in the magnetic field of the coil. The oscillator has completed a half cycle of operation.
Next, the magnetic field around L2 collapses as the current from C2 stops. The action of the
collapsing magnetic field causes the top of L2 to become negative at this instant. The negative charge
causes capacitor C2 to begin to charge in the opposite direction. This negative potential is coupled to the
base of Q1, opposing its forward bias. Most transistor oscillators are operated class A; therefore, the
positive and negative signals applied to the base of Q1 will not cause it to go into saturation or cutoff.
When the tank circuit reaches its maximum negative value, the collector and the emitter currents will still
be present but at a minimum value. The magnetic field will have collapsed and the oscillator will have
completed 3/4 cycle.
At this point C2 begins to discharge, decreasing the negative potential at the top of L2 (potential will
swing in the positive direction). As the negative potential applied to the base of Q1 decreases, the
opposition to the forward bias also decreases. This, in effect, causes the forward bias to begin increasing,
resulting in increased emitter current flowing through L1. The increase in current through L1 causes
additional energy to be fed to the tank circuit to replace lost energy. If the energy lost in the tank is
replaced with an equal or larger amount of energy, oscillations will be sustained. The oscillator has now
completed 1 cycle and will continue to repeat it over and over again.
Shunt-Fed Hartley Oscillator
A version of a SHUNT-FED HARTLEY OSCILLATOR is shown in figure 2-14. The parts in this
circuit perform the same basic functions as do their counterparts in the series-fed Hartley oscillator. The
difference between the series-fed and the shunt-fed circuit is that dc does not flow through the tank
circuit. The shunt-fed circuit operation is essentially the same as the series-fed Hartley oscillator. When
voltage is applied to the circuit, Q1 starts conducting. As the collector current of Q1 increases, the change
(increase) is coupled through capacitor C3 to the tank circuit, causing it to oscillate. C3 also acts as an
isolation capacitor to prevent dc from flowing through the feedback coil. The oscillations at the collector
will be coupled through C3 (feedback) to supply energy lost within the tank.
