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2 stage op amp question

Discussion in 'General Electronics Discussion' started by oreee, Sep 22, 2014.

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  1. oreee

    oreee

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    Sep 22, 2014
    Hi

    I'm currently learning about op amps but i'm not quite sure about what the pmos's role is in this specific 2 stage amplifier. To my understanding, for pmos to work it needs to have a more negative gate voltage(Vgs) than its threshold voltage? Is that the case here?

    Q: Could someone please explain what the pmos's are doing at the input stage? (i know its a current mirror and what a current mirror does) but I'm not sure how the pmos's work in this case.

    thanks
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  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Welcome to our forum.

    These transistors form a MOS current mirror. PMOS is used as the sources are connected to Vdd, therefore the gate is at a lower potential than the source, as you expect. The current mirror is used here as a load to the input NMOS transistors. It has a high differential resistance and therefore allows high AC gain.
     
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    If you're asking how PMOS transistors work as a current mirror, the answer is that they work pretty much like PNP transistors do, but with no gate current.

    If you're asking why a current mirror is used for the collector loads in a differential input stage, it's as Harald said - it works in conjunction with the current sink at the "long tail" to provide high gain and good linearity. This is explained in many places on the net. Just Google long tailed pair with current mirror.
     
  4. oreee

    oreee

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    Sep 22, 2014
    Hey guys, thanks for the explanation.

    So that means NMOS current mirror also works the same way?
     
  5. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Yes, an NMOS current mirror works pretty much like an NPN current mirror except with no gate current. The VGS(th) voltage of a MOSFET is typically more than 1V, so the input side of a MOSFET-based current mirror generally drops more voltage than a BJT-based current mirror, but they work in a similar way.
     
  6. oreee

    oreee

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    Sep 22, 2014
    Cool!

    So assuming that this is a low voltage operational amplifier (1.8v max @ output) and say I wanted to design a higher voltage operational amplifier of... 100v swing. What are some design considerations that I could look into? Not that i'm going to attempt to make one but would like to know anyways, also I read that if more gain is needed = more stages are used? Would that be easily stabilised or would it be a difficult task? Could a high voltage operational amplifier be done using two stages?

    thanks! (sorry if some of my questions are dumb, haha)
     
  7. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    I don't know what considerations become important at higher voltages - apart from the breakdown voltage ratings of the components, of course!

    Yes, more stages are needed if you need more gain, and that does make it more difficult to stabilise because it reduces the phase margin. But that's as much as I can tell you. Adam has written an article on op-amp stability at https://www.electronicspoint.com/resources/op-amp-stability.24/ which may help you.

    Any kind of op-amp can be done in two stages, but performance will be limited.
     
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