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larger output voltage buffer

Discussion in 'General Electronics Discussion' started by camphor1122, Oct 7, 2014.

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  1. camphor1122


    Aug 12, 2014
    Hey guys,

    Suppose I had a simple two stage operational amplifier like the one below:


    The mosfets are low voltage (2v vgs(max), 2v vds(max))

    It is then configured as a buffer, but now it needs to be able to output up to 12 volts
    What would be the simplest way of changing the above circuit as little as possible to achieve this? assuming noise, linearity, speed, etc aren't important (lol)

    Would it be possible to use a voltage divider at the input, to scale down the voltage by some fixed factor that would put it in a working range (e.g 12v input, scale it down by 6 so input becomes 2v)
    Uprate the op-amp output stage transistors and add some sort of circuitry in between to bias the uprated transistors properly?

    Sorry if my question is a bit silly.

    Any input is appreciated!

  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    How would this help? You still don't achieve 12V output voltage by this.

    For max. +12V output voltage you need transistors that can withstand at least that voltage. Since your transistors are rated at 2V Vds (max), you cannot use the circuit above 2V. Any output > 2V will destroy the bottom right NMOS output transistor. Any voltage less than Vdd-2V will destroy the top right PMOS output transistor. You will have to use transistors suitable for Vds > 12V.
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