A tunnel diode oscillator is a type of oscillator circuit that uses a tunnel diode as the active component to generate an oscillating signal. Tunnel diodes are unique semiconductor devices that exhibit a negative resistance region in their current-voltage characteristic curve. This negative resistance property allows them to generate microwave and RF signals efficiently, making them suitable for oscillator applications.
Here's a description of the working of a tunnel diode oscillator:
Tunnel Diode Basics: A tunnel diode is a two-terminal device with a heavily doped p-n junction. Unlike regular diodes, tunnel diodes exploit quantum tunneling, a phenomenon where electrons can pass through energy barriers in the diode's structure. This results in the diode having a region in its current-voltage curve where the current decreases as the voltage increases, leading to negative resistance behavior.
Biasing the Tunnel Diode: To operate the tunnel diode as an oscillator, it needs to be biased in the negative resistance region of its I-V curve. This is typically achieved by applying a DC bias voltage across the diode. The exact biasing arrangement depends on the specific circuit design.
Feedback Network: The tunnel diode is connected in a feedback network, which can be an LC tank circuit or a resonant circuit. This feedback network sets the frequency of oscillation. The negative resistance of the tunnel diode compensates for the losses in the feedback network, sustaining the oscillations.
Start-up: Initially, when power is applied to the circuit, noise and perturbations in the system might trigger the tunnel diode to produce a small signal. This signal then propagates through the feedback network, gets amplified by the tunnel diode's negative resistance, and returns to the input. The feedback reinforces the signal, causing it to grow.
Oscillation: As the signal circulates through the feedback loop, it continues to strengthen due to the negative resistance effect of the tunnel diode. Eventually, the signal reaches a stable amplitude, and the oscillator settles into a steady-state condition. At this point, a continuous sinusoidal output waveform is present at the output of the oscillator circuit.
Output Filtering: The output of the tunnel diode oscillator often contains harmonics and spurious frequencies. To obtain a clean output signal at the desired frequency, additional filtering may be applied using band-pass filters or other techniques.
Load Termination: The output of the tunnel diode oscillator is connected to a load, which can be an antenna, amplifier, or any other circuit that uses the generated signal.
It's important to note that tunnel diode oscillators are most commonly used in high-frequency and microwave applications due to the tunnel diode's unique characteristics. They have been extensively used in early microwave communication systems and radar applications. However, as technology has progressed, other oscillator types, like transistor-based oscillators, have become more prevalent due to their better stability, ease of integration, and lower power consumption.