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can the circuit be used to divide voltage?

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cicer

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hello,

attached please find a circuit of divide voltage,which is made up of two pmos.

i want to know whether is it used to divide voltage?

thanks


barry
 

gevy

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Yes, this circuit is a voltage divider by 2, if each transistor to place in a separate nwell and their sizes are equal.
 

lavitaebelle

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I am not sure this even works because top transistor gate is connected to source to vdd. VGS=0. Hence I think there will be no current flow.

The circuit may work if you connect top transistor gate to the out terminal instead of vdd.Also connect gate of bottom transistor to gnd.

If the bottom one is an nmos then connecting both gates to out and playing with sizes may give voltage division, but is highly dependant on supply,process and temperature.
 

jayc

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lavitaebelle is right. The circuit as you have drawn it will have no DC path from Vdd to Gnd since both transistors are in cutoff, so the output will be floating.

As lavitaebelle also mentioned, you can diode connecting both transistors to get a voltage divider. However, you must be aware of the body effect. Certainly, you can get the output to be half the supply, but to do it with two identical devices, you need to connect the source of each transistor to their respective bulks. Otherwise, you can adjust the sizing until you get the output you want.

However, note that this is by no means a good way to get a DC voltage reference. Not only does the output change with supply, process, and temperature variation, it wastes a lot of power (you're basically putting two relatively small resistors from Vdd to Gnd).
 

cicer

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thanks for every friends!
I think that the two pmos be cutoff,but there be leak current in the circuit, and the driver ability is weakly.
but i am not surely,is the opinion right?
 

kenlino

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cicer,you are right. and I also agree with gevy's viewpoint.
the two Pmos are cutoff,and the leak current is weakly,almost zero,and the impedance of these pmos are very large(infinity),if they have the same sizes,this circuit would be a voltage divider by 2.
other way,we can regard cotoff pmos as capacitor(Cgd),then the result is the same.
but this circut can not work accurately.
 

cicer

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thanks, I think I understand the circuit better. it is not available in fact.
 

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