Physics · Ch 14 — Electronic Devices
Junction Diode as a Rectifier
Junction Diode as a Rectifier
The rectifying idea. Because a p-n junction diode conducts easily when forward biased but blocks current almost completely when reverse biased (Sections 9.10-9.11), connecting a diode in series with an AC source and a load resistor lets current flow through the load only during the half-cycles for which the diode happens to be forward biased, and blocks it during the half-cycles for which it is reverse biased -- converting an alternating input, which reverses direction every half-cycle, into an output current that never reverses direction. This process is called RECTIFICATION, and a circuit built to do it is called a rectifier.
Half-wave rectification. The simplest possible rectifier uses a SINGLE diode in series with the load. During the half of each AC cycle for which the diode is forward biased, it conducts and the output follows the shape of the input; during the other half-cycle, for which it is reverse biased, it blocks current almost entirely and the output stays at (or very near) zero. The result is a series of one-directional current pulses -- one pulse per full input cycle -- separated by gaps of zero output: a PULSATING direct current, correctly one-directional throughout, but far from the smooth, steady output of an ideal DC source. Since this design uses only ONE of the two half-cycles of every input period, it is called half-WAVE rectification, and its output has the SAME repetition frequency as the input AC supply. …
What this figure shows. A step-down transformer's secondary winding is drawn connected in series with a single p-n junction diode (symbol: a triangle pointing toward a short bar, representing the direction of easy, forward conduction) and a load resistor , the three forming a single loop. Above the circuit, the AC INPUT waveform across the transformer secondary is drawn as a smooth, symmetric sine wave, with equal positive and negative half-cycles. Below the circuit, the OUTPUT waveform across is drawn directly underneath, time-aligned with the input: during each POSITIVE half-cycle of the input, the output trace follows the same rounded hump shape as the input (the diode is forward biased and conducts); during each NEGATIVE half-cycle, the output trace is drawn as a flat, zero line (the diode is reverse biased and blocks current almost entirely). The resulting output is therefore a series of one-directional humps separated by flat gaps -- a PULSATING DC waveform a …