Hartley Oscillator |
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The Hartley Oscillator
The basic LC Oscillator circuit we looked at in the previous tutorial has no means of controlling the amplitude
of the oscillations. If the electromagnetic coupling between L and L2 is too small
there would be insufficient feedback and the oscillations would eventually die away to zero. Likewise if the feedback was too strong
the oscillations would continue to increase in amplitude until they were limited by the circuit conditions producing distortion.
It is possible to feed back exactly the right amount of voltage for constant amplitude oscillations. If we feed back
more than is necessary the amplitude of the oscillations can be controlled by biasing the amplifier in such a way that if the
oscillations increase in amplitude, the bias is increased and the gain of the amplifier is reduced. If the amplitude of the oscillations
decreases the bias decreases and the gain of the amplifier increases, thus increasing the feedback. In this way the amplitude of the
oscillations are kept constant and this is known as Automatic Base Bias.
One big advantage of automatic base bias is that the oscillator can be made more efficient by providing a Class-B bias
or even a Class-C bias as the Collector current flows during only part of the cycle and the quiescent Collector current is very small.
Then this "self-tuning" Base oscillator circuit forms the basic configuration for the Hartley Oscillator circuit.
In the Hartley Oscillator the tuned LC circuit is connected between the Collector and the Base
of the transistor amplifier and as far as the oscillatory voltage is concerned, the emitter is connected to a tapping point on the
tuned circuit coil. The feedback of the tuned tank circuit is taken from the centre tap of the inductor coil or two separate coils
in series which are in parallel with a variable capacitor, C. An Hartley Oscillator can be made from any
configuration that uses either a single tapped coil (similar to an Autotransformer) or a pair of series connected coils in parallel
with a single capacitor as shown below.
Basic Hartley Oscillator Circuit
When the circuit is oscillating, the voltage at point X (collector), relative to point
Y (emitter), is 180o out-of-phase with the voltage at point Z (base)
relative to point Y. At the frequency of oscillation, the impedance of the Collector load is resistive and
an increase in Base voltage causes a decrease in the Collector voltage. Then there is a 180o phase change in the voltage
between the Base and Collector and this along with the original 180o phase shift in the feedback loop provides the correct
phase relationship of positive feedback for oscillations to be maintained.
The amount of feedback depends upon the position of the "tapping point" of the inductor. If this is moved nearer to
the collector the amount of feedback is increased, but the output taken between the Collector and earth is reduced and vice versa.
Resistors, R1 and R2 provide the usual stabilizing DC bias for the transistor in
the normal manner while the capacitors act as DC-blocking capacitors.
In this Hartley Oscillator circuit, the DC Collector current flows through part of the coil and for
this reason the circuit is said to be "Series-fed" with the frequency of oscillation of the Hartley Oscillator being given as.
Note: LT is the total inductance if two separate coils are used.
The frequency of oscillations can be adjusted by varying the "tuning" capacitor, C or by
varying the position of the iron-dust core inside the coil (inductive tuning) giving an output over a wide range of frequencies making
it very easy to tune. Also the Hartley Oscillator produces an output amplitude which is constant over the entire frequency range.
As well as the Series-fed Hartley Oscillator above, it is also possible to connect the tuned tank circuit across the
amplifier as a shunt-fed oscillator as shown below.
Shunt-fed Hartley Oscillator Circuit
In the Shunt-fed Hartley Oscillator both the AC and DC components of the Collector current have separate paths
around the circuit. Since the DC component is blocked by the capacitor, C2 no DC flows through the inductive
coil, L and less power is wasted in the tuned circuit. The Radio Frequency Coil (RFC), L2
is an RF choke which has a high reactance at the frequency of oscillations so that most of the RF current is applied to the
LC tuning tank circuit via capacitor, C2 as the DC component passes through
L2 to the power supply. A resistor could be used in place of the RFC coil, L2
but the efficiency would be less.
Example No1
A Hartley Oscillator circuit having two inductors of 0.5mH each is tuned to resonate with a
capacitor which can be varied from 100pF to 500pF. Determine the upper and lower frequencies of oscillation and the oscillators bandwidth.
The frequency of oscillations for a Hartley Oscillator is given as:
The circuit consists of two inductive coils in series, so the total inductance is given as:
Upper Frequency
Lower Frequency
Oscillator Bandwidth
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