Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronic 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.

Register Log in

Input DC offset result output dc value reach vdd

SDRookie

Junior Member level 1
Joined
Aug 23, 2017
Messages
16
Helped
Reputation
0
Reaction score
0
Trophy points
1
Activity points
151
Hi,

I'm designing a fully differential amplifier. My block include a amplifier 30dB gain open loop and a common mode feedback circuit with 20dB gain.

when the input have dc offset (0.1V), it saturate the output, so one output port of differential amplifier's voltage reach VDD, which is turned off.

I think the common mode feedback should correct the dc output voltage, but it didn't work. Could someone tell me how should I prevent this happen? should i increase gain of common mode fb block?
 

dick_freebird

Advanced Member level 5
Joined
Mar 4, 2008
Messages
6,682
Helped
1967
Reputation
3,936
Reaction score
1,789
Trophy points
1,393
Location
USA
Activity points
53,596
Show your simulation circuit testbench. It should resemble the
datasheet application figure for a similar function IC part.

Jam the outputs together and sweep them rail-rail with a vdc
source. Look at the output of your CMFB block. Does it "switch"
at about the right VCM and does it switch in a direction that
would oppose?

30dB is not great for voltage gain - not even A=100. Not even
acceptably accurate for many A=10 closed loop applications.
Aim higher.

But an imposed input offset -should- rail out any amplifier worth
more than an empty package. CMFB may be satisfied with one
output railed high and one railed low.
 

FvM

Super Moderator
Joined
Jan 22, 2008
Messages
46,602
Helped
13824
Reputation
27,896
Reaction score
12,497
Trophy points
1,393
Location
Bochum, Germany
Activity points
272,171
It's not the purpose of CMFB to correct input offset. Use differential feedback.
 

SDRookie

Junior Member level 1
Joined
Aug 23, 2017
Messages
16
Helped
Reputation
0
Reaction score
0
Trophy points
1
Activity points
151
I planed to use open loop, that's why I only set gain to be 30dB. Will the close loop give me higher linearity?
 

dick_freebird

Advanced Member level 5
Joined
Mar 4, 2008
Messages
6,682
Helped
1967
Reputation
3,936
Reaction score
1,789
Trophy points
1,393
Location
USA
Activity points
53,596
Closed loop in an application-realistic circuit is going to
put everything where it should normally be (or else you
have work to do). You don't get much usable info from
an amplifier that's got railed outputs and wound up gain
stage(s).
 

sutapanaki

Advanced Member level 4
Joined
Nov 2, 2001
Messages
1,005
Helped
384
Reputation
766
Reaction score
339
Trophy points
1,363
Location
US
Activity points
8,581
A gain of 30dB, which is about 40V/V will produce diff output of abut 4V for an offset of 0.1V. If your supply voltage is below that value or about that value, the amplifier will saturate to the rails. Offset is a kind of differential signal, producing differential output and thus is not affected by the CMFB.
 

SDRookie

Junior Member level 1
Joined
Aug 23, 2017
Messages
16
Helped
Reputation
0
Reaction score
0
Trophy points
1
Activity points
151
Closed loop in an application-realistic circuit is going to
put everything where it should normally be (or else you
have work to do). You don't get much usable info from
an amplifier that's got railed outputs and wound up gain
stage(s).
My input voltage is pretty small, so 30dB will not distort the signal. My first design was close loop, but the bandwidth is not enough because of the feedback and phase margin. I thought the open loop don't need to consider about the phase margin, which could lead me have more bandwidth. Do I need to consider the phase margin issue in open loop circuit when there is no differential feedback?
 

sutapanaki

Advanced Member level 4
Joined
Nov 2, 2001
Messages
1,005
Helped
384
Reputation
766
Reaction score
339
Trophy points
1,363
Location
US
Activity points
8,581
My input voltage is pretty small, so 30dB will not distort the signal. My first design was close loop, but the bandwidth is not enough because of the feedback and phase margin. I thought the open loop don't need to consider about the phase margin, which could lead me have more bandwidth. Do I need to consider the phase margin issue in open loop circuit when there is no differential feedback?
No, you don't
 
Toggle Sidebar

Welcome to EDABoard.com

Sponsor

Sponsor

Design Fast


X
Top