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MosCap realized by NMOS and by PMOS

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TiwstedNeurons

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MosCap

Hi,Everyone

We can use mosfet as a capacitor by connecting the s/d/bulk together as one node and gate as another node. Then,here is a question: what is the difference of the moscap realized by NMOS and the moscap realized by PMOS? thx
 

Re: MosCap

Hi,
I think in many standard CMOS processes, using NMOS transistor as an MOSCAP is impossible, since bulk of all NMOS transistors are connected together and ti the lowest voltage (usually ground) in the circuit. Hence almost always we use PMOS transistors as MOSCAPs where their bulk terminal is accessible. However if you use more expensive processes with more than one well (e.g. two-well process) you may use NMOS ones as MOSCAPs, BUT I really don't think that it's worthy.

Regards,
EZT
 
Re: MosCap

EZT,

Not neccessary. NMOS can also configured as n-cap which is NMOS plus a n-well.
 
MosCap

cap A-| |-GND use mos cap ,I have try 2 way to realized. in P-sub

1.use NMOS A connecting poly gate GND connecting S/D/B.
2.use PMOS without n-well A connecting poly gate GND connecting S/D.
 

Re: MosCap

hi,

usually the pmos cap has a higher capacitance than nmos cap (for the same area) and both have higher capacitance than a mimcap for instance). however they are not as precise as a mimcap (but they save area).
anyway when you connect them make sure that the pcap has the gate to a lower voltage than the B, D & S; and the ncap has the B, D & S connections at a lower voltage than the gate.

rgds,
M
 
MosCap

pmos cap is formed between gate and channel so, both drain and source r connected to gnd.
nmmos cap is formed between gate and substrate so, both r connected to gnd.
the difference between these is that the capacitance of pmos cap is more than nmos cap.
and more importantly the capacitance of these capacitors vary with gate voltage. that is they have different capacitances in different operating regions. so pmos cap needs less voltage to operate in inversion region which is stable where as nmos cap needs more voltage for the same. so the application of moscaps vary with voltage at that particular point.
 

Re: MosCap

use mos as cap, the mos must be work at triode to guarantee the series resistor in chanel be smallest. certainly, since the pmos at n-well, the parasitic cap is larger to 1/3 cap value(maybe), so nmos may be a good selection. if the gate voltage is too low to create chanel, maybe use nmos in n-well, nwell connect ground.
 

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