• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Problem in CMOS differential amplifier design.

Alex_bam

New Member
Hello,
It's my first attempt to design CMOS Differential amplifier in 0.35um technology using a cadence tool. I search threads related to my problem in this forum but could not able to solve my problem. So I will explain my approach in deriving dimensions of MOSFET's used in it. Moreover, I have also attached my simulation picture for your reference.

I get help from the following reference for calculations:
https://medium.com/@vishnuanugrahith/designing-of-a-cmos-differential-amplifier-68d3d90baae4

Design Parameters:
GBW
= 10M CL= 10pF, Vdd=5V
UnCox=190u A/V^2; UpCox=55u A/V^2;
Vtn=0.8V Vtp=-1V
Slew Rate(SR) :10V/uSec
Vcmax=4.5V ; Vcmin=1.5V (input common mode range)


Step 1 _ Calculating Id:
Id=SR*CL 10V/uSec * 10pF= 100uA

Step 2 - Calculating (W/L) for M3 and M4:
Vds1=Vmax-vthn=4.5-0.8=3.7V
Vds3=Vdd-Vds1=5-3.7=1.3V
Id3=Id/2=50uA
(W/L)= 2*Id3/UpCox*vds3=1.075

Step 3 - Calculating (W/L) for M1 and M2:
gm1=GBW*2*pi*CL
= 10M*2*3.14*10pF
=6.27*10^-4
Id1=Id/2
(W/L)=gm1^2 / 2*Id1*unCox
=20.75

Step 4 _ Calculating (W/L) for M5 and M6:
Vx=(Vmin-Vtn)-Vdsat1
Vdsat1= (2*Id1/UnCox * (W/L)1 )^1/2= 0.16
Vx=0.54
Id5=Id
(W/L) =2*Id5/UnCox * Vx^2= 3.6

Finally, transistor dimensions are:
(W/L)1,2
= 1.07u / 1u
(W/L)3,4 = 20.75u / 1u
(W/L)5,6 = 3.6u / 1u

Differential input Signal:
Freq= 1 Mhz; Amp= 500 mV; Sine DC level= 2V

Differential gain:
= gm2 (ron_n2 || ron_p4)
= 418.5u( 71.73K || 17.04K )
=5.7

1_ I am not getting the output signal according to my calculated gain? I am making mistake in my calculations?
2_ Could anyone comment on my design and mathematical calculations?
3- I am always struggling to find the un*Cox value of MOSFET. after searching in different threads in this and other forums. One way is to find it from betaeff=un*Cox*W/L. But un*Cox changes with changing W/L. So I am always doubtful regarding un*Cox value. Can anybody know the accurate method to find un*Cox value of MOSFET especially using Cadence?

Thanks.
 

Attachments

Latest threads

EE World Online Articles

Loading
Top