Designing Differential amplifier with differential output.

[imrankhanPNU]

Hello!
I have done calculations for designing a differential amplifier (DA) with differential output. I simulate (using cadence ) the DA with the calculated values and perform the DC analysis of the circuit to inquire all the transistors are operating in saturation. However, I did not get all the transistors in the saturation region. I tried to tune the system (means to bring it to saturation region) but tuning one set of transistor results in the detuning of other transistors and vice versa.

if someone can review my calculations and suggest me correction. I will be thanked for my help.

Design Parameters:

GBW= 5MHz CL= 10pF, Vdd=5V

UnCox=151u A/V^2;

UpCox=26u A/V^2;

Av= 100

Vtn=0.85V Vtp=-1V

Slew Rate (SR) :5V/uSec

ICMR+= 4V ; ICMR- =1.5V (input Common mode range)

Step 1 _ Calculating Id:

Id=SR*CL 5V/uSec * 10pF= 50uA

Step 2 - Calculating (W/L) for M3 and M4:

Vds1>=Vmax-Vthn=4-0.85=3.15V (let consider it as 3.5V)

Vds3=Vdd-Vds1=5-3.5=1.5V

For M3 to be in saturation:

|Vds3|>=|Vgs3|-|Vthp|

1.5V >= |Vgs3| - 1V

|Vgs3|<= 2.5V (I take |Vgs3| : 2V )

Id3=Id/2 = 25uA

(W/L)3= 2* Id3 / UpCox * (|vgs3| - |Vthp| = 2

Step 3 - Calculating (W/L) for M1 and M2:

gm1=GBW * 2 * pi * CL

= 5M * 2 * 3.14 * 10pF

=314u

Id1=Id/2=25uA

(W/L)=gm1^2 / 2 * Id1 * unCox

(W/L) =13.06

Step 4 _ Calculating (W/L) for M5 and M6:

To keep M1 in saturation under ICMR- (1.5V):

Vin>=Vgs1+Vdsat5

Vgs1=1V (I calculate it as we know Id1=25uA , W1/L1=13.06 )

Putting Vin=1.5 (ICMR-) & Vgs1= 1V

Vdsat5<=0.5V

Id5=Id

(W/L) =2 * Id5 / UnCox * (Vdsat) ^ 2= 2.64

Finally, transistor dimensions are:

(W/L)1,2 = 1.9u / 1u

(W/L)3,4 = 13.06u / 1u

(W/L)5,6 = 2.64u / 1u

 

[Dominik Przyborowski]

You need feedback and common mode feedback to make it working.

 

[frankrose]

You didn't calculate with Av, you have just set L=1um.
If you want to design on theoretical base I think it will be much better if you involve gain calculation of an amplifier.

 

[imrankhanPNU]

You need feedback and common mode feedback to make it working.

Thanks for your suggestion, My ultimate goal is to design a Fully Differential Amplifier. The first step is a differential amplifier with differential output, then CMFB design.
So my approach is to design & test the differential amp with differential output then will go for the CM detector and CMFB loop. Please comment, Is this approach is correct?