Tuned-base Armstrong oscillator

Posted on Feb 6, 2014

The circuit in view (D) has all three requirements for an oscillator: (1) amplification, (2) a frequency-determining device, and (3) regenerative feedback. The oscillator in this schematic drawing is a tuned-base oscillator, because the fdd is in the base circuit. If the fdd were in the collector circuit, it would be a tuned-collector oscillator.

Tuned-base Armstrong oscillator
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The circuit in view (D) is basically an Armstrong oscillator. Refer to figure 2-10, view (D), for the following discussion of the circuit operation of the Armstrong 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 frequency-determining 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...

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