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Armstrong Oscillator

An Armstrong Oscillator is a type of oscillator that uses a tickler coil which provides feedback from tank circuit. The Armstrong Oscillator is used to produce a sine-wave output of constant amplitude and of fairly constant frequency within the rf range.

Inductor L1 operates as a tickler coil, providing feedback from the output circuit [collector] to the input circuit [base] of the transistor. Inductor L1 is coupled to transformer T1, by mutual inductive coupling. between the tickler and LC tuned circuit. For the circuit to oscillator, the feedback is regenerative.

The transistor is identified as a NPN transistor, and could be almost any NPN transistor. The circuit is a basic common emitter configuration. The exact transistor part number would depend on the frequency of operation of the oscillator, and the value of the voltage used as Vcc. The important criteria is that the transistor provides amplification in the frequency range of oscillation. A 2N2222 NPN Transistor offers operation up to 300MHz.

Resistor Rc is used to bias the collector of the transistor, a small loss is incurred over inductor L1, applying Vcc to the collector circuit. Resistor Rb is the base bias resistor. Resistor Re is the emitter bias resistor, also called self-bias. The capacitors Cb and Ce are used to by-pass the bias resistors for non-DC voltages. The resistors only set the DC bias, for AC voltages the resistors are shunted out of the circuit.

The circuit to the right shows a series-fed armstrong oscillator. The tank circuit is form by variable capacitor C1 and the primary of T1. DC voltage is applied to the tank circuit, making the circuit a series-fed oscillator. Vcc is applied to the bottom side of the tuned circuit and is returned via T1, the collector of Q1 and emitter resistor Re [than back to Vcc].

The tank circuit is the frequency determining component of the oscillator. The frequency of oscillation is determined by the resonant frequency [Fr] which is 1 / (2 x 2.1415927 x [LC]1/2). Trimmer capacitor C1 provides adjustments to the frequency by varying the capacitance in the tuned circuit. See Trimmer Capacitors Styles,
Companies producing Trimmer Capacitors.

The shunt-fed armstrong oscillator [shown in the lower side-bar] differs slightly, in that the DC voltage is no longer applied to the tank circuit. Vcc is still connected to resistor RC, but bypasses the tank circuit and connected directly to the collector of the transistor. In addition, only AC flows into the tank circuit, capacitor C2 blocks any DC voltage from the tank circuit. The tank circuit is now grounded to provide a return path for the output of the transistor.

Tuned-Base Armstrong Oscillator

An oscillator that has the frequency determining components in the base of the transistor circuit, and that uses a tickler coil which provides feedback from the tuned circuit.

Tuned Circuit Armstrong Oscillator circuit schematic
Tuned Circuit Armstrong Oscillator

Resistor R1 sets the amount of current through L1, the higher the adjusted value of R1 the more current flows through the L1 winding. So the higher R1 becomes the more feedback is presented or coupled into the C1, L2 tank circuit. Trimmer R1 is adjusted so that current through L1 is sufficient to sustain oscillations in the tank circuit. In other words the resistor adjusts the gain of the feedback, the higher the resistance value the higher the amplitude of the feedback signal. Also refer to Companies producing Trimmer Resistors. Or Common Resistor Trimmer packages in common usage.

Resistor R2 sets the DC bias voltage at the base of the transistor. With power applied a small amount of base current [Vcc/R2] will flow through R2 and forward bias the base of the transistor. While the coupling capacitor C2 blocks any DC voltage from reaching the tuned circuit. Because C2 is used as a blocking capacitor, this style of circuit is a shunt-fed oscillator. The DC bias to the collector is set by the parallel combination of R1 and L1. Note that L1 is not part of the tuned circuit, so the DC current flowing through L1 does not make the circuit a series-fed oscillator.

With the base of the transistor forward biased, the transistor is turned on and collector current flows from Vcc through the resultant resistance of R1 and L1 to the collector of the transistor. A magnetic field is developed across the windings of T1 and induces a voltage into the tank circuit. The transformer is inverting the signal so the top side of C1 becomes positive with respect to the grounded end. Capacitor C1 charges up to the voltage potential, dropped across L1.

With the larger voltage now on the base of the transistor, Q1 begins to conduct harder and returns a larger voltage to the feedback loop. The more current through L1 induces a larger voltage to L2, which C2 charges up to. The amplitude feedback continues until the transistor reaches saturation and and the current peaks out. Once saturation is reach no further change in current is realized and no further voltage is induced from the primary to the secondary of the feedback transformer.

With no feedback source, capacitor C1 discharges through L2 which than induces a magnetic field in L2. As C1 begins to discharge, so does C2. However capacitor C2 discharges through R2, decreasing the base voltage which in turn forces the collector current to fall. 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 sine-wave signal, but it aids in keeping Q1 cut off. Without feedback, the oscillations of L2 and C1 would dampen out after several cycles. [See Damped Oscillations]

The oscillations continue because regenerative feedback is still being supplied to the tank circuit. 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 again creates a magnetic field across the transformer and into the tank circuit. The transistors sees the increase in base voltage, turns on more fully and approaches saturation. Once saturation is reached the above process repeats and the transistor is driven into cut-off.

While the tank circuit is oscillating, L2 acts as the primary of the transformer and L1 acts as the secondary. The signal from the tank is, therefore, coupled through T1 to coupling capacitor C4, and the output voltage across L4 is a sine wave. That is a sine wave at the resonant frequency of the tank circuit.

The common emitter circuit produces a signal that is 180 degrees out of phase with the input signal. To compensate for the phase inversion the feedback transformer inverts the signal again so the the feedback is regenerative. The feedback offsets the damping in the tuned circuit and provides unit gain to the amplifier.

Emitter resistor R3 improves temperature stability. The by-pass capacitor C3 prevents degeneration of the signal. Capacitor C4 is the output coupling capacitor to T2 which is used as an impedance matching transformer. Transformer T2 is a loosely coupled RF transformer, used to reduce any reflected impedance from the load back to the oscillator.

Circuit Configurations

Any transistor configuration can be used to make an Armstrong oscillator, after all the transistor is just being used as the amplifier. So any circuit configuration that still allows the transistor to be an amplifier is acceptable. So a common-base [CB], common-emitter [CE], or common-collector [CC] circuit could be used. Of course the feed-back coil changes location along with the connections of the transistor. The Shunt-Fed Armstrong oscillator, in the upper right side-bar, is an example of a tuned-collector circuit. A Tuned-base circuit is used as the prime example above. However both circuits are common-emitter circuits.

Related topics;
Shunt-Fed Crystal Controlled Armstrong Oscillator circuit.
Types of Oscillator Vendors.

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