[Moved]: Sizing an OpAmp with gm/Id - help needed

Hi there,


I have to size the simple two-stage opamp (see picture) using the gm/Id methodology. But, every source which I checked, describes one transistor... I understand firstly u have to choose working points on these grapes, but how does it work for a system of transistors(like an opamp).

Honestly, I don't have time for going throw every point, maybe someone here have a perfect and easy solution how to size a two-stage opamp with gm/id?




really need help,

Best,
Alex

Re: Newbie question > Sizing an OpAmp with gm/Id - help needed

You need a specification at the beggining.
But let's say: GBW = 10MHz, Cload = 10p, A > 60dB.

1. Cc = 0.22*Cload (this is the value for 60 degree of PM)
2. GBW = gm1/(2*pi*Cc), so gm1 calculated from here
3. Take for example gm1/Id1 = 10 (this means 200mV of ovedrive for input pair). From here you have a bias current.
3a. W1/L1 = gm1^2/(2*K*Id1)
4. gm6/gm1 = 4*Cload/Cc (the second pole p2 will be three times larger than pole p1, approximately)
5. Id6 = gm6/10;
5a. W6/L6 = gm1^2/(2*K*Id1)
6. W4/L4 = (W6/L6)*I6/I1;
7. W5/L5 = 2*I5/(K*0.2*0.2) = 4*I5/(K*0.2*0.2); e.g. 0.2V overdrive voltage
8. The ratio of S7/S5 = B and the ratio of S6/S4 = 2*B; Reason is equal current density for
Q4 & Q6 (Q3 and Q4 should have the same Vds in DC). From this ratio you can now find W7/L7

Re: Newbie question > Sizing an OpAmp with gm/Id - help needed

lilke84 said:

You need a specification at the beggining.
But let's say: GBW = 10MHz, Cload = 10p, A > 60dB.

1. Cc = 0.22*Cload (this is the value for 60 degree of PM)
2. GBW = gm1/(2*pi*Cc), so gm1 calculated from here
3. Take for example gm1/Id1 = 10 (this means 200mV of ovedrive for input pair). From here you have a bias current.
3a. W1/L1 = gm1^2/(2*K*Id1)

Click to expand...

thank you for positing it!

I have many things to ask, but lets start from simple questions:

3rd step: W1/L1 = gm1^2/(2*K*Id1)
we know, gm/Id is 10, means we can write W1/L1 = gm1^2/(2*K*10*gm1)?

Also, Do you have a paper behind this approach?

Have a nice evening,
Alex

@lilke can u explain on what basis u gave those formula.. do u have any paper for this. It will really helps