Armstrong Oscillator Circuit Working and Application

An Armstrong oscillator, Colpitts, Clapp, Hartley, and crystal-controlled oscillators are several types of resonant LC feedback oscillators (LC electronic oscillator). An Armstrong oscillator (also known as Meissner oscillator) is actually an LC feedback oscillator which uses capacitors and inductors in its feedback network. The Armstrong oscillator circuit can be built from a transistor, an operational amplifier, a tube, or some other active (amplifying) devices. Generally, the oscillators consist of three basic parts:

  • An amplifier This will usually be a voltage amplifier and may be biased in class A, B or C.
  • A wave shaping network This consists of passive components such as filter circuits that are responsible for the wave shaping and the frequency of the wave produced.
  • A POSITIVE feedback path A Part of the output signal is fed back to the amplifier input in such a way that the feedback signal is regenerated and re-amplified. This signal again fed back to maintain a constant output signal without the need for any external input signal.

The below given two conditions for the oscillation. Every oscillator must satisfy these conditions in order to make proper oscillations.

  • The oscillations should be taken place at one particular frequency. The oscillation frequency f is determined by the tank circuit (L and C) and is approximately given by
Oscillation Frequency
Oscillation Frequency
  • The amplitude of the oscillations should be constant.

Armstrong Oscillator Circuit and Its Working

The Armstrong oscillator is used to produce a sinusoidal wave output of constant amplitude and of fairly constant frequency within the given RF range. It is generally used as a local oscillator in receivers, can be used as a source in signal generators and as a radio-frequency oscillator in the medium- and high-frequency range.

The identifying characteristics of the Armstrong oscillator

  • It uses an LC tuned circuit to establish the frequency of oscillation.
  • Feedback is accomplished by mutual inductive coupling between the tickler coil and the LC tuned circuit.
  • Its frequency is fairly stable, and the output amplitude is relatively constant.
Armstrong Oscillator Circuit and Its Working
Armstrong Oscillator Circuit and Its Working

The above figure shows a typical Armstrong circuit using an NPN BJT transistor. The inductor L2 is called as Trickler Coil, this will provide feedback (regeneration) to the input of the BJT by coupling with L1 individually. Some of the signals in the output circuit is inductively coupled to the input circuit by L2. The base circuit of transistor contains a parallel tuned tank circuit with L1 and C1. This tank circuit determines the oscillation frequency of the oscillator circuit.

Here C1 is a variable capacitor to change the frequency of oscillation. The resistor Rb provides foe=r the correct amount of bias current. DC bias current flows from ground to emitter via Re, out of the base, via Rb and then back to the positive. The value of Rb and Re determines the amount of bias current (generally Rb> Re). The resistor Re provides emitter stabilization to prevent thermal runaway and capacitor CE is the emitter bypass capacitor.

Armstrong Oscillator Circuit and Its Working
Armstrong Oscillator Circuit and Its Working

From the above circuit-fig (a), the amount of DC biased current is determined by the value of the resistor Rb. The capacitor C in series with the base (B) is a DC blocking capacitor. This will block the DC bias current from flowing into L1 but it allows the signal coming from L1-C1 to pass to the Base.Fig (b) shows the DC output emitter-collector current.

Here the transistor is in forwards biased in its emitter-base circuit. Then, the emitter-collector current will flow through it. So from the above circuits fig (a&b), the signal current occurs when the circuit is oscillating. So if the oscillations were stopped, that means of by opening the tickler coil, then we would only have the DC currents just described.

The above Fig (b) shows the DC output emitter-collector current. Here the transistor is in forwards biased in its emitter-base circuit. Then, the emitter-collector current will flow through it. So from the above circuits fig (a&b), the signal current occurs when the circuit is oscillating. So if the oscillations were stopped, that means of by opening the tickler coil, then we would only have the DC currents just described.

Armstrong Oscillator Circuit and Its Working
Armstrong Oscillator Circuit and Its Working

The above schematic shows where the signals would flow in this oscillator. Assume that the oscillator is meant to produce a sine wave on 1MHz. This will be a sine wave of varying the DC, not the AC. Because most of the active devices do not work on the AC. When the Armstrong oscillator is turned on, L1 and C1 start producing oscillations on 1MHz. This oscillation would normally fall down due to losses in the tank circuit (L1 &C1). The oscillating voltage across L1 and C1 is superimposed on the top of the DC bias current in the base circuit. So a 1MHz signal current flow in the base circuit as shown above (in green line).

Here the current through the resistor Re is negligible (the capacitive resistance of CE at 1MHz would be 1/10th the value of RE). Now, this 1MHz signal in the base circuit causes a 1MHz signal in the collector circuit (aqua blue). The capacitor across the battery bypasses the signal around supply. The amplified signal flows in the tickler coil. The tickler coil (L2) is inductively coupled to L1 and L3 simultaneously. So we can take amplified output signal from L3.

Advantages and Disadvantages

  • The main advantage is that, the construction of Armstrong-type tube oscillators using a tuning capacitor where one side is earthed. It produces a stable frequency and stably amplified output waveform.
  • The main disadvantage of this circuit is that the resulting electromagnetic vibrations may contain interfering harmonics very light, which are undesirable in most cases.

Applications of Armstrong Oscillator

  • It is used to generate the sinusoidal output signals with a very high frequency.
  • It is generally used as a local oscillator in receivers.
  • It is used in the radio and mobile communications.
  • Used as a source in signal generators and as a radio-frequency oscillator in the medium- and high-frequency range.

Thus, this is all about An Armstrong Oscillators and its applications. We hope that you have got a better understanding of this concept. Furthermore, any doubts regarding this concept or to implement electrical and electronics projects,  Please give your valuable suggestions by commenting in the comment section below. Here is a question for you, What are the conditions for Oscillation?

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