+ Post New Thread
Results 1 to 4 of 4
  1. #1
    Advanced Member level 4
    Points: 5,915, Level: 18
    Achievements:
    7 years registered

    Join Date
    Jan 2012
    Location
    Italy
    Posts
    1,042
    Helped
    41 / 41
    Points
    5,915
    Level
    18

    Rail to rail input folded operational amplifier

    Dear friends,

    in the circuit shown below, the output cascode stage of the folded cascode amplifier has two miror, one up and one down, and there is no current source, how the current is then being defined to 2 Ib? we usually have in the output cascode one mirror and the other side is current source

    Thank you

    Click image for larger version. 

Name:	foolded.PNG 
Views:	13 
Size:	175.1 KB 
ID:	156402

    •   AltAdvertisement

        
       

  2. #2
    Super Moderator
    Points: 52,015, Level: 55
    Achievements:
    7 years registered
    erikl's Avatar
    Join Date
    Sep 2008
    Location
    Germany
    Posts
    8,101
    Helped
    2675 / 2675
    Points
    52,015
    Level
    55

    Re: Rail to rail input folded operational amplifier

    Quote Originally Posted by Junus2012 View Post
    ... how the current is then being defined to 2 Ib?
    By W/L ratio (=1) between (P3,P4):(P1,P2) and (N3,N4):(N1,N2) resp. Don't forget: V2 & V3 must be supplied externally. Also, the common drain voltage of P5,N5 (≈VDD/2) supplies all the gates of N3,N4 & P3,P4, hence this design isn't suited for high VDD voltages, cf. Fig. 3 of the attached document.


    1 members found this post helpful.

    •   AltAdvertisement

        
       

  3. #3
    Advanced Member level 4
    Points: 5,915, Level: 18
    Achievements:
    7 years registered

    Join Date
    Jan 2012
    Location
    Italy
    Posts
    1,042
    Helped
    41 / 41
    Points
    5,915
    Level
    18

    Re: Rail to rail input folded operational amplifier

    Quote Originally Posted by erikl View Post
    By W/L ratio (=1) between (P3,P4):(P1,P2) and (N3,N4):(N1,N2) resp. Don't forget: V2 & V3 must be supplied externally. Also, the common drain voltage of P5,N5 (≈VDD/2) supplies all the gates of N3,N4 & P3,P4, hence this design isn't suited for high VDD voltages, cf. Fig. 3 of the attached document.
    Dear Erikl,

    Thank you for your reply

    Do you practically prefer to deal with such circuit ? or to use the traditional folded one

    my second question, does it worth to have a constant gm amplifier? because it makes the design more complicated and consumes more power, or you think it is not that mandatory



    •   AltAdvertisement

        
       

  4. #4
    Super Moderator
    Points: 52,015, Level: 55
    Achievements:
    7 years registered
    erikl's Avatar
    Join Date
    Sep 2008
    Location
    Germany
    Posts
    8,101
    Helped
    2675 / 2675
    Points
    52,015
    Level
    55

    Re: Rail to rail input folded operational amplifier

    Quote Originally Posted by Junus2012 View Post
    Do you practically prefer to deal with such circuit ? or to use the traditional folded one
    Depends. I used it for supply voltages of 1.0±0.2V and it worked well, as the Vth voltages of the process suited fine. With higher VDD this simple (but admittedly very elegant, because of its perfect symmetry) topology probably isn't well suited, because the circuit unnecessarily will end up with too much current consumption. Actually you should have a process with Vthp+Vthn ≈ VDD .

    Quote Originally Posted by Junus2012 View Post
    does it worth to have a constant gm amplifier? because it makes the design more complicated and consumes more power, or you think it is not that mandatory
    Again depends. If you are bound to get rather flat open loop gain over your ICMR, you need constant gm architecture. If the application allows for scaling down the gain considerably by neg. feedback (closed loop gain « open loop gain), you can achieve the necessary gain-over-ICMR flatness by this feedback. In this case you don't need constant gm topology.


    1 members found this post helpful.

--[[ ]]--