How to make OPAMP capacitor layout smaller ?

[jacksimonbirch]

OPAMP capacitor question

Hi,

I'm designing a simple opamp which has a 5pF capacitor. I'm using the layout library that is available in the gpdk446 library. The result of the 5pF capacitor is really "BIG", bigger than the whole circuit 100 times or more. I know that the capacitor layout can be smaller using different design like poly-poly2, though gpdk446 doesn't have poly2. So, I kinda stuck.

Is there anyway to make the capaciot layout smaller using the library I have?

thanks

 

[sunking]

Re: OPAMP capacitor question

You can use mos cap.
This is to say, short SD as one terminal, gate as another terminal. Make sure that |Vgs| is much less |Vt|.

 

[kyrandia]

Re: OPAMP capacitor question

Use N-well Nmos as the cap

 

[tlihu]

Re: OPAMP capacitor question

Use N-well Nmos as the cap

There is no turn-on issue if using n-well nmos cap. However, it is said that this kind of device is not well-modeled. Right?

 

[jordan76]

Re: OPAMP capacitor question

Hi

What is the meaning of N-well Nmos? To my common sense, there will normally be Pmos in N-well. Even if it is triple well, it is actuall still so.

Please correct me if I am wrong. Thanks.

regards,
jordan76

 

[jordan76]

Re: OPAMP capacitor question

You can use mos cap.
This is to say, short SD as one terminal, gate as another terminal. Make sure that |Vgs| is much less |Vt|.

Hi sunking,

I am quite confused by your last sentence. True, mos cap is usually used with SD shorted as one terminal and Gate as the other terminal, but
|Vgs| is often VDD(usually larger than |Vth|).

Please correct me if I am wrong. Thanks.

regards,
jordan76

 

[Hughes]

Re: OPAMP capacitor question

Hi

What is the meaning of N-well Nmos? To my common sense, there will normally be Pmos in N-well. Even if it is triple well, it is actuall still so.

Please correct me if I am wrong. Thanks.

regards,
jordan76

Normally mosfet in nwell is p-type. But we still make nmos in nwell using n+ type S/D implantation.

 

[piao]

OPAMP capacitor question

huges,

Can you explain the detail about the capacitance during accumulation and such process, and could you tell me how to connect the 4 terminal while using such a Nwell NMOS?

I am confused about such a device.

Thanks

 

[Hughes]

Re: OPAMP capacitor question

The layout view and cross-section view are show in the attached figure. Since S/D and channel are all n-type, the B terminal need not be drawn explicitly.

 

[selvaraja]

Re: OPAMP capacitor question

you can use mim capasistor.

 

[tlihu]

Re: OPAMP capacitor question

you can use mim capasistor.

What's mim cap? There is MMC but unit capacitance is relatively small than mos cap.

 

[Hughes]

OPAMP capacitor question

MiM is initial for Metal-Insulator-Metal. Maybe MMC is Metal-Metal Capacitor. Then they are the same thing. The unit capacitance of MiM cap in mixed-signal technology is around 1fF/um². Although this value is smaller than that of MOS cap, it is much larger than that of capacitor formed by routing metal layers. MiM cap has minimum parasitic resistance. But it requires Mixed-Signal process option as PiP cap does.

 

[sunking]

Re: OPAMP capacitor question

You can refere to "CMOS analog circuit design"
Page 83 to 85

The book can be download in this board.

 

[sunking]

Re: OPAMP capacitor question

In the CMOS analog circuit design, Page 44 to Page47 introduce more cap which you can use

 

[rtpq]

Re: OPAMP capacitor question

Sometimes you need to use PMOS because bulk of normal NMOS have to be connected to ground always.

 

[tlihu]

Re: OPAMP capacitor question

Sometimes you need to use PMOS because bulk of normal NMOS have to be connected to ground always.

Yes, for AC-coupled cap, I'll choose PMOS cap.

But, my question is: do we need to consider the polarity of the PMOS cap, i.e., top and bottom plates, when connecting.

 

[Hughes]

OPAMP capacitor question

In my opinion, we need to consider the polarity of the PMOS cap. The G terminal should be connected to relative lower voltage.