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Is it possible to build an op amp with BJTs instead of MOSFETs?

  1. Sep 18, 2011 #1
    I am trying to understand how some of the standard ICs work in terms of simple components I understand, i.e. transistors, resistors, capacitors, and inductors (although I doubt there are any inductors in an IC.)

    I was trying to build and op amp from transistors, resistors, etc, and I noticed that all of these ICs are constructed with MOSFETs. In hindsight, that should have been obvious from the start.

    In any case, is there anyway I can use my cheap BJTs instead of buying 10 MOSFETs?
     
  2. jcsd
  3. Sep 18, 2011 #2
    Most op-amps are built with BJTs!!! Where did you get that idea that they are built from mostly FETs?

    There are very very important factors that you need BJTs to build op-amps. BJTs has much lower low frequency voltage noise, low 1/f noise than all FETs. There is no way out of this. I am currently working on electronics using in guitar. I have been looking for low noise op-amps and the noise in audio range is staggering between BJT and FET.

    Also BJT is so much more predictable that the most precision amps has to be BJT. MOSFET is good for wide band op-amp where it work from 1MHz to 100s of MHz. That you stay away from low freq noise and people usually don't care about precision. If you really want precision, you can piggyback a precision op amp in an easy design to get the stability at low frequency.
     
  4. Sep 18, 2011 #3
    Yes, and they do it all the time. The input transistors are simply connected in a common-collector configuration so that the input impedance is high.
     
  5. Sep 18, 2011 #4

    cmb

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    Have you tried JFET opamps, like TL072? I've been looking at them recently as I needed to make a very high impedance sensor pre-amp.
     
  6. Sep 18, 2011 #5
    I just looked at it, If noise is not a problem and you need low bias current, JFET is the way to go.

    It all depends what is the requirement, thats the reason there are so so many op amps made by so so many manufacturers. If you post more of the requirements, I might be able to help you. I just spent 4 days looking at a lot a lot of op amps for my needs.
     
  7. Sep 18, 2011 #6

    cmb

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    Thanks. I've got a feedback amp circuit working on discretes, so looks like I've got a solution now.
     
  8. Sep 18, 2011 #7
    I did a lot of discrete op amp design in my days because there weren't any in the market at the time. I think it is a good practice to do discrete op amp design so you really learn transistors.

    I designed some quasi op amps with GaAs transistors and dual gate MESFET to get sub-nano seconds rise and fall time in the late 80s and early 90s where it was unheard of in IC op amps. We had no choice but to do that. Also very high voltage op amps of +/-750 volts supply voltages. And later we even had +/- 2.5KV op amps to replace expensive bi-polar lens power supplies using optical drive and cascade MOSFETs.
     
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