Cathode follower oscillator

The Cathode follower oscillator is an electronic oscillator circuit in which the oscillation frequency is determined by a tuned circuit consisting of capacitors and inductors, that is, an LC oscillator. The circuit is also known as differential amplifier oscillator, emitter follower oscillator, source-coupled oscillator or Peltz oscillator. This oscillator uses one connection to get a signal from the LC-circuit and feeds an amplified signal back. The amplifier is a long-tail pair of two triodes, two bipolar transistors or two junction FETs.

Operation
In the Cathode follower oscillator schematic, the long tail amplifier is connected to a tap of the LC-circuit inductance for a light load on the LC-circuit. A grid-leak couples the grid of the left triode to the LC-circuit. The left triode uses common anode circuit which has high input impedance, low output impedance and no voltage amplification. The long-tail resistor couples the two triodes. The right triode uses common grid circuit which has low input impedance, high output impedance and no current amplification. A capacitor at the anode resistor of the right triode couples the amplified signal back to the LC-circuit. At low frequency, both triodes have a phase shift of zero degree.

The Peltz oscillator schematic uses pnp transistors. The LC-circuit L1, C1 is connected to the base of the left transistor and to the collector of the right transistor. Both transistors are coupled through the long-tail resistor R1. Oscillation frequency is around 10 MHz. The circuit is optimized for low supply voltage.

The source-coupled oscillator uses two junction FET and a LC-circuit to produce a sine wave signal. L1 and C1 are the LC-circuit, J1 is a common drain amplifier, J2 is a common gate amplifier. The grid-leak C2, R2 connects the LC-circuit to J1 input. J1 output is directly connected to J2 input. The long-tail resistor R1 works as a primitive current source. If the current through J1 increases, the current through J2 decreases. The LC-circuit is directly connected to J2 output. The circuit is optimized for low supply voltage.