How to choose error amplifier for a LDO?

Discussion in 'General Electronics Chat' started by anhnha, Apr 3, 2014.

  1. anhnha

    Thread Starter Active Member

    Apr 19, 2012
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    Hi.

    With LDO (Low Dropout Voltage regulator) with input voltage ranges from 2 to 3.3V and output voltage is 1.8V.
    From these specs, how should I choose error amplifier?
    I am confused about how the input range relates to the error amplifier.
     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Are you building your own LDO?

    The input range is how much voltage your circuit has for power. Including the error amp.
     
  3. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Simply make sure that your error amp will work in linear region for 2V also. And will have as much gain as he can.
     
  4. anhnha

    Thread Starter Active Member

    Apr 19, 2012
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    Hi.
    Thank you.
    Yes.
    My problem is here:
    Vin: 2 to 3.3V
    Vout: 1.8V

    I am designing the error amplifier for this LDO.
    For Vin from 2 to 2.9V: Vout = 1.8V
    For Vin from 2.9 to 3.3V Vout follows exactly Vin: Vout = Vin

    I need to increase the input range in which Vout = 1.8V. What specs of error amplifier that I need to change?
     
  5. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    We need to see a schematic of the complete circuit. There are way too many possibilities to discuss without more information.

    ak
     
  6. anhnha

    Thread Starter Active Member

    Apr 19, 2012
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    Hi. This is the circuit.

    [​IMG]

    BTW, how can I choose Vbias1, Vbias2 and Vbias3 so that all transistors are in saturation?
     
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  7. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Ok I see a folded-cascode amplifier + PMOS as a CS amp.
    Vbias1 will set the tail current for the long tail pair (M1,M2)
    Vbias3 will set the cascode and you can set this current to I_tail_current.
    Vbias2 - set as 2*Vgs for example.
    http://www.eit.uni-kl.de/koenig/deutsch/TESYS_Lutgen_09.pdf
     
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  8. anhnha

    Thread Starter Active Member

    Apr 19, 2012
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    I set Vbias1, Vbias2 and Vbias3 above the threshold voltages of M5, M8, M9, M10 and M11 a little and these transistors are in saturation.

    However, some other transistors are not in saturation. How can I solve that?
    Could you answer the question in post #4?
     
  9. anhnha

    Thread Starter Active Member

    Apr 19, 2012
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    My voltage gain is about 40dB. I need over 60dB. How can I increase that?
    From the formula, I need to increase gm1, gm2. However, when I increase width of M1, M2 in a wide range, the voltage gain increase only 1dB.
     
  10. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Can you show me your open loop gain expression. And RL for PMOS is equal to?
    Because PMOS gain is equal to gm*RL
     
  11. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    I'm not so sure but I think that the gain from Vref/Vg = gm1 * Rout
    where
    Rout = rds7||((gm9*rds9) * rds11||rds2) ≈ rds7

    And PMOS gain is gmp*RL.
    So as you can see to increase the gain replace M6 and M7 with cascode current mirror.
     
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  12. anhnha

    Thread Starter Active Member

    Apr 19, 2012
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    Here is some cascode current mirror topologies I found. Which one should I use?
    Also, could you tell me how to replace M6 and M7 by cascode current mirror?

    [​IMG]
     
  13. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    I don't know, try them all in simulation.
    Also don't forget that I'm not a IC CMOS designer (I'm not even a EE).
    And I'm not interested to go into this subject. So I don't help you with choosing the right W/L.

    Simply delete M6 and M7 from the circuit. And place current mirror from fig12 for example.
     
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  14. anhnha

    Thread Starter Active Member

    Apr 19, 2012
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    Thank you.

    Ha, I knew that. :)
    However, you know much more than I do. And I need your help in understanding circuits in general.

    Oh, I thought if we need to replace each transistor M6 and M7 by a cascode current mirror.

    I tried the circuit but I didn't sure.
    I simulated figure 12 and 13 and the figure 13 gives the largest gain.
    My gain increased from 40dB to 50dB now.
    However, that is still not enough.
     
  15. Jony130

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    And your gm and lambda is ?
     
  16. anhnha

    Thread Starter Active Member

    Apr 19, 2012
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    M1:
    region: subthreshold
    gm = 2.97*10^(-7)S
    M2: subthreshold
    gm = 4.21*10^-5 S

    lamda = 0.6667
     
    Last edited: Apr 9, 2014
  17. Jony130

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    Feb 17, 2009
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    Can you tell me where you have connected M2 gate and M1 gate?
    M1 - to the AC source ?
    M2 - gate to ground ?
     
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