Large Capacitors and Resistors in 180nm

[fcfusion]

Hi guys. I'm designing a Class D amplifier. I have almost no Layout experience and I would like some experienced Layout designers to clear a few doubts.

1) I need to use about 8 resistors with 1MΩ each. Won't these resistors occupy a very large area? How high will be the stray capacitance for each resistor?

2)I need to use four 4pF capacitors. Are these too large? How much area do they occupy?

I only need aproximate values, so I can have an idea of how much will they screw the performance. I'm using 0.18um technology.

I've been told that their sizes are not significant when compared with the rest of the circuit which includes some power devices, but I have my doubts.

Another thing I almost forgot: using transistor's in triode region as resistors is a decent option? Is the linearity good or bad?
Thanks

 

[diemilio]

Yeap, those are usually pretty large values...

a 4 pF cap occupies about 65x65 um^2 (using MIMCAPs) or about 30x30 um^2 (using MOSCAPs). Each 1 MOhm resistors (using RPHRIPOLY) will take a little bit more than 30x30 um^2

Hope this helps,

diemilio

 

[fcfusion]

Thanks man.

How about using transistors in triode region as resistors? How good is the linearity? Would I save a lot of area if I used those instead of resistors?

 

[diemilio]

Well... in linear region you get:

rds ≈ L/(W*k'*(Vgs-Vth)) so you can estimate how much area is it going to take for a certain resistive value (kn ≈ 172.5 uA/V^2; kp ≈ 35.5 uA/V^2 for TSMC 0.18 which I don't know if this is your process)

Just remember that this only holds if Vds << Vgs-Vth so you have to guarantee that this condition is met for all operating points! The linearity of the resistor will depend on how much smaller Vds will be as compared to Vgs-Vth

diemilio