jinyong,
The output polarity in the diagram is reversed. This is the circuit for a negative supply. Operation is as follows:
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On the positive half cycle, D1 is forward biased and C1 charges to the peak. Node 1 positive wrt node 2. D2 is reverse biased.
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On the negative half cycle, VI and the voltage on C1 are in series-aiding, with a total voltage of 2Vpeak. D2 is forward biased, and C2 is charged to 2Vpeak.
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To get a positive supply, reverse the diode directions.
Regards,
Kral