Can i use MOS capacitor to implement compensation?

[walker5678]

I am designing an Op amp, and i want to choose the MOS capacitor to implement the compensation capacitor, because it has larger unit capacitance than the metal capacitor. But i am not sure is it good, and whether there was any concern.


Best regards,
walker

 

[pfd001]

Of course you can use a mos capacitor for compensate capacitor

 

[wdd]

you can use it
but have an eye on the PM range~

 

[mpkp123]

Yes, ofcourse u can use a MOSCAP there is no harm in using it.

 

[walker5678]

If use PMOS as capacitor, then one terminal of the capacitor is the gate, and the other terminal is nwell. As the attached file show, is there a parasitic resistor R?

 

[basunaik]

I am designing an Op amp, and i want to choose the MOS capacitor to implement the compensation capacitor, because it has larger unit capacitance than the metal capacitor. But i am not sure is it good, and whether there was any concern.


Best regards,
walker

Hi,

I don't think there is any harm in using MOS capacitor for compensation. If it is affecting in any way do let me know. Because I never faced any problem.

Bye

 

[Alan_Nesta]

Of course there is harm in mos compensation capacitor
because its c-v, more nonlinearity is inside your op

If your op is only for dc using, it's harmless, but if for ac use like audio, vedio, you can get bad thd in it

 

[chang830]

Of course there is harm in mos compensation capacitor
because its c-v, more nonlinearity is inside your op

If your op is only for dc using, it's harmless, but if for ac use like audio, vedio, you can get bad thd in it

Yes, the c-v is more nolinear. But I think it is OK if you can perform the stability analysis with all PVT corners combined.

 

[Alan_Nesta]

I means not stability, but the ac characteristic just like thd of audio applications

Added after 25 seconds:

I mean not stability, but the ac characteristic just like thd of audio applications

 

[walker5678]

Yes, the C_V characteristics of MOS capacitor makes it not suitable for AC signal. I have consulted some experienced guy, and know that normally if we want to use MOS capacitor in AC application, we need to add a process step to implant P+ to the channel region under the gate oxide. So the capacitance will be constant.

 

[Alan_Nesta]

I want to know if any foundry have this process?
and have this model?

 

[walker5678]

As i know, this is not a standard process step. For example, i am using HHNEC .35 CMOS process, and it only has metal capacitor model. So if you do need MOS capacitor, maybe you can deal with the foundry.

 

[fendy]

Yes.of course you can.

 

[jamesmau]

Yes,you can. you must make capacitor Vgs>Vth that in strong inversion. The source and drain of MOS capacitor is connected.

 

[meet_ganesan]

compensation can be achieved by using passive components like resistor,capacitor,inductor.this improves stability of a closed loop feedback system by increasing characteristics namely phase margin,G.M ...etc hence
we can use MOS capacitor for compensation...........

 

[manissri]

i dont think is there any harm using the mos cap as compansation provided that the ac analysis should be done at all pvt

 

[mists]

also should check power up sequence

 

[xiongshoufen]

it is not good,

please refer to razavi book page 33

 

[zxcynosure]

yes, you can. and make sure that one port of cap(connect to bulk) should be connected to Vdd or GND.

 

[tandr]

But the Mos cap 's value woul change with the voltage , Why not 2 plate cap ?