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I am reading about half-wave rectifier and there are two questions below. Hope anyone could help me out.

Q1: the text said that the diode-connected PMOS body is tied to source. However, in the figure, body is tied to the drain.

Is this a mistake?

Q2: Why connect the PMOS transistor in parallel with NMOS transistor improve the circuit performance?

enter image description here

enter image description here

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  • \$\begingroup\$ Whatever the body diode is connected to becomes the source. \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 9 '16 at 17:26
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Q1: In this circuit the transistor is sometimes reversed, i.e. source becomes drain and vice versa. Still, I would say it is a mistake. The circuit however looks OK.

Q2: This question is already answered in the text. The PMOS does not suffer from a threshold increase due to body effect. Moreover, the terminal with the arrow ("source") is P+ and the well connected to the other terminal is N doped, so you get a body diode for free, which helps as well.

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  • \$\begingroup\$ Thank you. For the question 1, normally S and D are symmetric. However, for the diode-connected transistor, the two terminals tied together should be gate and drain. So is the drain fixed here? The remaining terminal is source and there is no symmetric? For the question 2, why don't we use pMOS only in the rectifier? I mean that we use only pMOS instead of connecting nMOS and pMOS in paralllel. \$\endgroup\$ – anhnha Oct 9 '16 at 19:50
  • \$\begingroup\$ 1: As soon as the transistor is reversed it is off, so technically they are reversed, practically not. 2: For higher voltages (and therefore higher currents) the NMOS will kick in and improve the performance. NMOS transistors have a better performance. A PMOS alone would have to be made much bigger. \$\endgroup\$ – Mario Oct 9 '16 at 19:59

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