ARMSTRONG OSCILLATOR

2-15 flows through L1, and less energy is coupled to the tank circuit; therefore, feedback amplitude decreases. R1 is normally adjusted so that the L1 current is adequate to sustain tank oscillations. View (D) shows the complete oscillator circuit. Connecting the feedback network through coupling capacitor C2 to the base of Q1 forms a "closed loop" for feedback (shown by the solid arrows). Let's verify that the feedback is regenerative. Assume a positive signal on the base of Q1. The transistor conducts heavily when forward biased. This current flow through L1 and R1 causes the voltage across L1 to increase. The voltage increase is inductively coupled to L2 and inverted. This action ensures that the voltage is positive at the base end of L2 and C1 and in phase with the base voltage. The positive signal is now coupled through C2 to the base of Q1. The regenerative feedback offsets the damping in the frequency-determining network and has sufficient amplitude to provide unity circuit gain. 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. 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 V_CC 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 +V_CC. 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 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 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. 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