Esaki Oscillator

An Esaki oscillator is a very simple oscillating circuit that uses a single transistor (connected backward), a resistor, and a capacitor to create a repeating waveform, often a sawtooth wave or a flashing LED. It works by having the capacitor slowly charge through the resistor until its voltage exceeds the transistor's reverse breakdown voltage, which triggers a sudden discharge that completes one cycle. The frequency is determined by the values of the resistor and capacitor, with larger values resulting in a lower frequency. 

How it works

• Charging phase: The capacitor charges slowly through the resistor. The voltage across the capacitor, and therefore the voltage seen by the transistor's emitter-collector junction, steadily increases.
• Breakdown phase: When the capacitor's voltage reaches the transistor's reverse breakdown voltage, the junction suddenly conducts current. In a simple circuit with an LED, this discharge is enough to flash the LED.
• Discharging phase: The capacitor discharges through the now-conducting transistor, and the voltage across it drops.
• Cycle repeats: Once the voltage drops below the breakdown point, the transistor stops conducting, and the capacitor begins charging again, restarting the cycle. 

Key components and setup 

• Transistor: An NPN transistor is used in reverse, with the emitter connected to the positive terminal of the power source and the collector connected to the negative terminal (ground). The base is left unconnected.
• Power source: A DC power source of at least 12 volts is typically needed to ensure the reverse breakdown voltage can be reached.
• Resistor and Capacitor: These are connected in series with the power source. The values of the resistor (
  

  $R$
  ) and capacitor (
  

  $C$
  ) determine the charging time and, therefore, the oscillation frequency. 

Adjusting the output

• Frequency: The oscillation frequency is inversely proportional to both the resistance and capacitance values. To decrease the frequency, increase either the resistor or capacitor value; to increase the frequency, decrease either value.
• Waveform: The resistor value can also be adjusted to change the shape of the output waveform, with a higher resistor value increasing the charging time.

Esaki Oscillator circuits using BC 547 transistors 3.3k ohm resistors and various capacitors which govern the frequency of an LED turning on/off. The circuits were powered by 12.8 V DC.

Capacitor        Actual Capacitance     LED flash frequency

100uF                    110uF                                1s

470uF                    472uF                                4s

1000uF                  957uF                                8.5s

100uF

470uF

1000uF