CMOS Amplifier (Class A)

[justsunny]

Dear All,

I am new to the CMOS design. Any help would be highly appreciated.

I am designing the class A CMOS amplifier in 180nm (Cadence). I have attached the picture. I need your help to start the design.

(1). What are the design considerations ?
(2). How do I select/design VGG2 ?
(3). How do u select/design Vss ?
(4). What smulations do I need to do in Cadence ?

 

[doibongvui]

Hi
1/To figure out VGS2 you need to know your spec like power dissipation and from this you will know the current ===> VGS2 with W/L to be chosen
2/ You should know gain of this amp from spec ===> I (current) > bias

 

[justsunny]

1/ ok,if power dissipation is 10 mW and Vdd = 1.8 V

P = VI
I = P / V = 10(mw) / 1.8 (v)

and, I = 1/2 * M * (W/L) * ( Vgs2 - Vt )2 , Vt=.7V

Is this the right process to get Vgs2 ?

2/ If the gain is 5dB then how do I calculate oter parameters ?

I am new in Analog IC design.

Thanks.

 

[diemilio]

You need to make some assumptions to get all the values (like for example setting L to a certain number and sticking to it even for M1. You also need your gain equation which will be given by vout/vin = -gm1*rds1||rds2||RL. gm1 is gonna be a function of Id1 (which should be equal to IQ to center Vout at midrail), W and L of M1. By setting Vout to 0 you have Vds1 (DC) which allows you to set a reasonable Vdast number (depending on linearity specs) which will finally determine Vgs1. As you add more and more constraints to your design your number of equations increases and allows you to select the appropriate values of Vgs2, W1, W2, etc to meet those specs.

Hope this helps,

diemilio

 

[justsunny]

" By setting Vout to 0 you have Vds1 (DC) which allows you to set a reasonable Vdast number (depending on linearity specs) which will finally determine Vgs1"

What would be the equation for this ?

Thanks.

 

[lete]

Hi justsunny,

We assume that M1 and M2 operate in saturation mode. You can use the below equations to calculate your circuit:

For M2:
Id = 0.5*k*(W/L)*(Vdd - Vgg2 -Vt)^2

For M1:
Vdssat = Vin - Vt
gm1~k*(W/L)*Vdssat
ro1 ~ 1/(lamda*Id)
Gain = gm1 * (ro1//ro2//RL)

Regards,