Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Source follower design issue for cascoded two stage op-amp

Status
Not open for further replies.

Junus2012

Advanced Member level 5
Joined
Jan 9, 2012
Messages
1,552
Helped
47
Reputation
98
Reaction score
53
Trophy points
1,328
Location
Italy
Activity points
15,235
Dear friends,

I am designing the circuit shown in the figure below, In my first design I didn't use the source floower transistors MT1 and MT2, thus I gout a high current at the output stage (1.3mA). It was explaned by Allen holberg that the use of the source follower is to shift the dc level to the output stage so VSG6= VSD4, comparing without using the source follower VSG6= VSD4+VSD4C... this mean that the source follower shifted the value by the amount of VSD4C... it is illustrated in the graph as well.

my question, how we design the source follower in this circuit to get shift by that value, up to my knowledge that the shift of the source amplifier is equal to VSD+Vth.

So please help me on designing the aspect ratio of the source follower that satisfy this condition


Thank you very much


Holberg_circuit.PNG
 

No. The shift of the source follower MT1 of course is VSG (VSG1T in this weird terminology).

Dear Erikl,
the shift of VSG you said is the same as I expressed VSG = VSD(sat)+Vth,

secondly why you dont like this topology

Thanks
 

Ok, I see what you mean.


Did I say that? I wrote weird terminology!

I misundrestood the wired :D

back to my problem, the shift by the Vth+VSD(sat) is too big I think, more than of what needed to connect the M6 to M4 gate virtually
 

Why? The center voltage vo1 should be around VDD/2 (to achieve high swing), so you need that high shift 'cause VSG6 shouldn't get too much overdrive voltage. You can adjust Id6 by its W/L(6) ratio - I think this is meant by the Id6 vs. VSG6 diagram.
 

Status
Not open for further replies.

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top