LC feedback are used in oscillators that require higher frequency and higher Q factor. Transistors
are used as amplifiers because of the limitation (lower unitygain frequency) of Opamps. There are
four types of LC oscillators, Armstrong, Clapp, Hartley, Colpitts.
The benefits of LC oscillators are phase stability and low susceptibility to noise. LC oscillators have
a higher Q factor than Relaxation oscillators. However, they have lower tuning range and higher
cost compare to Relaxtion oscillators.
A LC oscillator in which dc power is supplied to the transistor through the tank circuit, or a portion
of the tank circuit, is said to be SERIES FED. A LC oscillator which receives its dc power for the
transistor through a path that is separate and parallel to the tank circuit is said to be PARALLEL
FED OR SHUNT FED. All the LC oscillators for this article can be constructed either way, series
or shunt fed.
Armstrong Oscillator
The armstrong oscillator uses transformer coupling to feed back a portion of the amplifier output to
the amplifier input. It is used less often then the other LC oscillators because of the transformer
size
and cost.
Fig 1
Fig 2
Fig 3
The circuit in Fig3 has all three requirements for an oscillator: (1) amplification, (2) a frequencydetermining
device, and (3) regenerative feedback. The oscillator in this schematic drawing is a
tunedbase oscillator, because the fdd is in the base circuit. If the fdd were in the collector circuit,
it
would be a tunedcollector oscillator.
When VCC is applied to the circuit. a small amount of base current flows through R2 which sets the
forward bias on Q1. This forward bias causes collector current to flow from ground through Q1,
R1, and L1 to +VCC. The
current through L1 develops a magnetic field which induces a voltage into the tank circuit.
The voltage is positive at the top of L2 and C1. At this time, two actions occur. First, resonant tank
capacitor C1 charges to this voltage; the tank circuit now has stored energy. Second, coupling
capacitor C2 couples the positive signal to the base of Q1.
With a positive signal on its base, Q1 conducts harder. With Q1 conducting harder, more current
flows through L1, a larger voltage is induced into L2, and a larger positive signal is coupled back
to
the base of Q1. While this is taking place, the frequencydetermining device is storing more energy
and C1 is charging to the voltage induced into L2.
The transistor will continue to increase in conduction until it reaches saturation. At saturation, the
collector current of Q1 is at a maximum value and cannot increase any further. With a steady
current through L1, the magnetic fields are not moving and no voltage is induced into the secondary.
With no external voltage applied, C1 acts as a voltage source and discharges. As the voltage across
C1 decreases, its energy is transferred to the magnetic field of L2. Now, let's look at C2.
The coupling capacitor, C2, has charged to approximately the same voltage as C1. As C1
discharges, C2 discharges. The primary discharge path for C2 is through R2 (shown by the dashed
arrow). As C2 discharges, the voltage drop across R2 opposes the forward bias on Q1 and
collector current begins to decrease. This is caused by the decreasing positive potential at the base
of Q1.
A decrease in collector current allows the magnetic field of L1 to collapse. The collapsing field of
L1 induces a negative voltage into the secondary which is coupled through C2 and makes the base
of Q1 more negative. This, again, is regenerative action; it continues until Q1 is driven into cutoff.
When Q1 is cut off, the tank circuit continues to flywheel, or oscillate. The flywheel effect not only
produces a sinewave signal, but it aids in keeping Q1 cut off. Without feedback, the oscillations of
L2 and C1 would dampen out after several cycles.
To ensure that the amplitude of the signal remains constant, regenerative
feedback is supplied to the tank once each cycle, as follows: As the voltage across C1 reaches
maximum negative, C1 begins discharging toward 0 volts. Q1 is still below cutoff. C1 continues to
discharge through 0 volts and becomes positively charged.
The tank circuit voltage is again coupled to the base of Q1, so the base voltage becomes positive
and allows collector current to flow. The collector
current creates a magnetic field in L1, which is coupled into the tank. This feedback action replaces
any lost energy in the tank circuit and drives Q1 toward saturation. After saturation is reached, the
transistor is again driven into cutoff.
The operation of the Armstrong oscillator is basically this: Power applied to the transistor allows
energy to be applied to the tank circuit causing it to oscillate. Once every cycle, the transistor
conducts for a short period of time (Class C operation) and returns enough energy to the tank to
ensure a constant amplitude output signal.
Class C operation has high efficiency and low loading characteristics. The longer Q1 is cut off, the
less the loading on the frequencydetermining device.
Figure 4 shows a tunedbase Armstrong oscillator as you will probably see it. R3 has been added
to improve temperature stability. Bypass capacitor C3 prevents degeneration. C4 is an output
coupling capacitor, and impedancematching transformer T2 provides a method of coupling the
output signal. T2 is usually a loosely coupled rf transformer which reduces undesired reflected
impedance from the load back to the oscillator.
Fig 4
For circuits in the 400 to 2000 MHz range, modern oscillators tend to use transmissionline
resonators and capacitive feedback of the Colpitts or Clapp type.
Colpitts Oscillator
The colpitts oscillator uses a pair of tapped capacitors and an inductor to produce the regenerative
feedback nesessary for oscillation. The amplifier output is developed across C1, and feedback
voltage is developed across C2. The voltage across C2 is 180 degrees out of phase with the
voltage across C1, thus feedback is regenerative. .
Resistors, R1 and R2 provide the usual stabilizing DC bias for the transistor in the normal manner
while the capacitor acts as a DCblocking capacitors.
Fig 5
The frequency of oscillations for a Colpitts Oscillator is determined by the resonant frequency of the
LC tank circuit and is given as:
where CT is the capacitance of C1 and C2 connected in series and is given as:
The configuration of the transistor amplifier is of a Common Emitter Amplifier with the output signal
180 degrees out of phase with regards to the input signal.
Both the Armstrong and the Hartley oscillators have a tendency to be unstable in frequency because
of junction capacitance. In comparison, the colpitts oscillator has fairly good frequency stability,
is
easy to tune, and can be used for a wide range of frequencies. The large value of split capacitance
is in parallel with the junctions and minimizes the effect on frequency stability.
.Clapp Oscillator
The Clapp oscillator is simply a Colpitts oscillator with an extra capacitor in series with the coil.
It is
labeled as C3 in Fig 6. The function of C3 is to reduce the effects of junction capacitance on
operating frequency.
C1 is in parallel with the Miller input capacitance, Cin(M).
C2 is in parallel with the Miller output capacitance, Cout(M).
C3 is always much lower in value than either C1 or C2, so it becomes the dominant capacitor in
any frequency calculation. The reason we still need C1 and C2 is to provide the phase shift needed
for regenerative feedback. C3 has not replaced C1 and C2. It is simply there to determine the
operating frequency. Since C1 and C2 are eliminated from the frequency calculation, junction
capacitance has little or no effect on operating frequency.
Advantages of Clapp Oscillators
Because there is no load on the inductor a high "Q" circuit results with a high L/C ratio
and of
course much less circulating current. This aids drift reduction. Because larger inductances are
required, stray inductances do not have as much impact as perhaps in other circuits.
The additional capacitor offers more accurate and stable frequency compared to Hartley and
Colpitts oscillators. It has the added advantage of a higher loaded Q factor than the Colpitts.
Fig 6
Fig 7
A Clapp circuit is often preferred over a Colpitts circuit for constructing a variable frequency
oscillator (VFO). In a Colpitts VFO, the voltage divider contains the variable capacitor (either C1
or C2). This causes the feedback voltage to be variable as well, sometimes making the Colpitts
circuit less likely to achieve oscillation over a portion of the desired frequency range. This problem
is avoided in the Clapp circuit by using fixed capacitors in the voltage divider and a variable
capacitor (C0) in series with the inductor.
Hartley Oscillator
The hartley oscillator is similar to the colpitts oscillator. It uses a pair of tapped inductors and
capacitor to produce the regenerative feedback nesessary for oscillation. It has the advantage of
having its output amplitude remain constant (amplitude stability) over a tunable frequency
range.However, it generates harmonics frequencies which prevents it from producing pure sine
waves.
The hartley oscillator is an improvement over the Armstrong oscillator. Although its frequency
stability is not the best possible of all the oscillators, the Hartley oscillator can generate a wide
range
of frequencies and is very easy to tune. The Hartley will operate Class C with selfbias for ordinary
operation. It will operate Class A when the output waveform must be of a constant voltage level or
of a linear waveshape.
Advantages of Hartley Oscillators
The frequency is simply varied by the net value of C in the tank circuit.
The output amplitude remains constant when tuned over the frequency range.
The feedback ratio of L1 to L2 (figure 1) remains constant.
Disadvantages of Hartley Oscillators
The output is rich in harmonic content and therefore not suitable where a pure sine wave is required.
SeriesFed
One version of a SERIESFED hartley oscillator is shown in figure 8. The tank circuit consists of
the tapped coil (L1 and L2) and capacitor C2. The feedback circuit is from the tank circuit to the
base of Q1 through the coupling capacitor C1. Coupling capacitor C1 prevents the low dc
resistance of L2 from placing a short across the emittertobase junction and resistor RE. Capacitor
C3 bypasses the sinewave signal around the battery, and resistor RE is used
for temperature stabilization to prevent thermal runaway. Degeneration is prevented by CE in
parallel with RE. The amount of bias is determined by the values of RB, the emittertobase
resistance, the small amount of dc resistance of coil L1, and the resistance of RE.
Fig. 8
Shunt Fed
A version of a SHUNTFED hartley oscillator is shown in figure 9. The parts in this circuit perform
the same basic functions as do their counterparts in the seriesfed Hartley oscillator. The difference
between the seriesfed and the shuntfed circuit is that dc does not flow through the tank circuit.
The shuntfed circuit operation is essentially the same as the seriesfed 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.
Fig 9
Application: Doorbell
