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Chaos in a Diode observing period-doubling and chaotic behavior in a driven diode circuit Chris Osborn Daniel Richman The Diode • Passive (not self-powered) circuit component • Restricts current flow to one direction • Modern diodes are based on semiconductor p-n junctions – Situation when diode is not connected to a circuit (a voltage): Electron and hole recombination in vicinity of junction, depletion of mobile charges; remaining negative charges in p-type and positive charges in n-type cause potential difference across junction (field points from n-type to p-type) – Connection methods: • Forward bias—positive emf terminal to p-type, negative to n-type Holes and electrons are pushed toward non-conducting center and oppose built-in potential barrier; current tunnels through the barrier • Reverse bias: negative emf terminal to p-type, positive to n-type Reinforces built-in potential barrier; current is blocked The Circuit Characteristics (why nonlinearity) • Diode has threshold voltage required for conduction – Forward voltage drop must reach this value for conduction to occur • Diode has residual conduction during reverse-bias part of cycle (forward bias means diode “on”, reverse bias means diode “off”) – Exponential decay of current—recovery time – Frequency of cycle is faster than recovery time Affects starting condition for next forward-bias part of cycle Experimental Procedures • Function generator output: sine – – Frequency is circuit’s resonance frequency (73.7 kHz) Amplitude adjustable from millivolts to tens of volts • • Inductor set to ten mH Oscilloscope reads generator voltage and diode voltage 1. 2. 3. 4. Fix the driving frequency Start at a low voltage amplitude Increase amplitude in steps of tenths Observe multiplying curves on diode voltage display Observing the Circuit What are we looking at? Diode voltage versus time (# of periods is # of distinct peaks) Diode voltage versus driving voltage (# of periods is # of loops) Chaos Experimental Data [Diode voltage versus driving voltage bifurcation graph] brief chaos between red and blue chaos between purple and gold Simulation xn+1 = r xn (1 – xn) • produces a sequence of terms whose long-time convergence is the bifurcation diagram • xo = 0.7, r = {2.5 : 0.015 : 4.0} Original computation in Matlab for the longtime convergence of xn