The Armstrong oscillatorA common-emitter or common-source amplifier can be made to oscillate by couplingthe output back to the input through a transformer that reverses the phase of thefed-back signal. The phase at a transformer output can be inverted by reversing the sec-ondary terminals.The schematic diagram of Fig. 25-2 shows a common-source amplifier whose draincircuit is coupled to the gate circuit via a transformer. In practice, getting oscillation iseasy. If the circuit won’t oscillate with the transformer secondary hooked up one way,you can just switch the wires.The Hartley circuit45925-2An Armstrong oscillator.The frequency of this oscillator is controlled by means of a capacitor across eitherthe primary or the secondary winding of the transformer. The inductance of the wind-ing, along with the capacitance, forms a resonant circuit. The formula for determiningthe LC resonant frequency is in chapter 17. If you’ve forgotten it, now is a good time toreview it.The oscillator of Fig. 25-2 is known as an Armstrong oscillator. A bipolar transistorcan be used in place of the JFET. It would need to be biased, using a resistive volt-age-divider network, like a class-A amplifier.The Hartley circuitA method of obtaining controlled feedback at RF is shown in Fig. 25-3. At A, an NPNbipolar transistor is used; at B, an N-channel JFET is employed. The PNP and P-chan-nel circuits are identical, but the power supply is negative instead of positive.