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    OTA 2 stage (folded cascode + common source)

    Good evening, I was designing a 2-stage OTA consisting of a folded cascode, as a first stage, followed by a common source, as a second stage. Almost all transistors, except one, work in saturation, then, between the first stage and the second, I inserted a common drain to perform a level-shifter, so that both transistors of the common source amplifier work in saturation. But the MOS of the pull-down network reaches the nominal value of the power supply and therefore the pull-up one does not turn on. How could fix it?

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  2. #2
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    Re: OTA 2 stage (folded cascode + common source)

    To better understand the problem, I post the electronic circuit that must be integrated on an IC

    Click image for larger version. 

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ID:	156715Click image for larger version. 

Name:	img.jpg 
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ID:	156715



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  3. #3
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    Re: OTA 2 stage (folded cascode + common source)

    M3 and M4 are not cascoded. They should be. Otherwise it won't work well.
    Don't bias NMOS devices with PMOS diodes, it is a bad idea, they don't vary with process together.
    Source followers add extra pole, which is bad for stability, they limit slew rate, and why did you connected them to NMOS sources? The voltage amplification is low there.
    Stacked diodes are also bad idea, they reduce the voltage headroom.
    What do you call pull-down/up network? I have totally esle knowledge about pull-up/down devices.
    Learn a bit more about folded cascode OPAmp and MOS transistor operation rather, your circuit is bleeding on many points.
    "Try SCE to AUX." /John Aaron/


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    Re: OTA 2 stage (folded cascode + common source)

    Quote Originally Posted by frankrose View Post
    What do you call pull-down/up network? I have totally esle knowledge about pull-up/down devices.
    by pull-up network I meant everything above the output terminal, and pull-down the other way around. Anyway thanks for the reply



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    Re: OTA 2 stage (folded cascode + common source)

    Quote Originally Posted by frankrose View Post
    M3 and M4 are not cascoded. They should be.
    Those transistors I need only to bring the current to the second stage, in fact I connected them to a mirror



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    Re: OTA 2 stage (folded cascode + common source)

    It is not really a folded cascode amplifier, the way you've drawn the circuit. The transistors at the drains of M3, M4, should be PMOS, not NMOS. With NMOS, there is nothing that controls the common-mode voltage at the drains of M2,M4.


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    Re: OTA 2 stage (folded cascode + common source)

    Quote Originally Posted by sutapanaki View Post
    It is not really a folded cascode amplifier, the way you've drawn the circuit. The transistors at the drains of M3, M4, should be PMOS, not NMOS. With NMOS, there is nothing that controls the common-mode voltage at the drains of M2,M4.
    in deed i fixed my circuit. thx for reply



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