When voltage is applied with a plus sign to the emitter of a pnp type transistor, and a minus sign to the collector, the collector pn junction is closed. When a small voltage is applied with a minus sign to the base, the emitter junction opens, and holes enter the base from the emitter, creating an emitter current. The base is very thin, so most holes pass through it and go into the collector, creating a collector current. When the voltage at the emitter pn junction changes, the currents of the emitter, collector and base change. The ratio of collector current to base current for transistors can reach values up to 500.
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