when we connect a diode with a source it operates as ahalf wave rectifier so the output voltage is a half wave which it is the positive part this is because the diode is forward biased in the negative part there is output voltage is zero since the diode is reverser biased so the circuit is open and there is no cuurrent flow. note that the input voltage must be greater than 0.7V if the diode is silicone type and 0.3V if it is Ge.
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