circuit analysiscircuit-designcmosintegrated-circuit
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?
The two MOSFETs are drawn incorrectly in the question's picture. That is why you are confused. This is how they should be i.e. sources tied together: -
I'm not aware that there is a decent non-isolated gate-drive version.
The Source and Bulk do not have to be connected.
In power devices, and especially in discrete transistors, the S & B are built very close together and shorted. This improves the breakdown voltage performance of the transistor.
In an IC, in some CMOS processes, the B of NMOS devices is always substrate (ground), and so in structures such as NOR gates which have 2 NMOS in series, the 2nd NMOS doesn't have the S=B.
Generally performance (gm, current) is better with S=B, but some technologies don't allow the B to be separated from the substrate for NMOS devices. PMOS devices in an IC generally can have separate S & B connections.
If you connected the B of PMOS to GND, you would have a parasitic diode from S to B (GND), and so your supply would be shorted (unless you wanted to run on a very low supply voltage of << 0.6 V). Some very low voltage circuits do use this technique.
A MOSFET connected as a diode will generally have worse performance than a PN junction in terms of the 'sharpness' of the curve. However, FETs with low threshold voltage (say 0.4 V or lower) will turn on at a lower voltage than a diode will, and this can be useful in low voltage circuits. For the same reason that the B is always substrate in some CMOS ICs, there isn't the flexibility to use a PN junction as a diode in all circuit configurations. If the PN junction of a PMOS is used (P = Source, N = Bulk), then there are some additional parasitics that need to be considered that make this not useful generally.
Best Answer
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.