Typical block diagram of a negative resistance electronic oscillator circuit, which uses a one-port component with negative differential resistance such as a magnetron tube, tunnel diode, or Gunn diode to generate a sine wave signal.  It consists of a resonant circuit, such as a tuned circuit, cavity resonator, or quartz crystal, connected in series with the negative resistance.  A DC bias voltage Vs which moves the operating point into the negative resistance region, provides power for the circuit.  The negative resistance cancels the positive resistance in the resonant circuit, creating a circuit without damping, which generates spontaneous electrical oscillations at the resonant frequency of the tuned circuit.  Negative resistance oscillators like this are often used at microwave frequencies.
Circuits with "N" shaped (voltage controlled) negative resistance devices usually use this series connection.  Circuits with "S" shaped (current controlled) negative resistance devices are often connected in parallel.
Own work
Chetvorno