Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

2 Stage JFET amplifier

  1. Dec 9, 2007 #1
    Hey guys,

    I am curious on how to set up a 2 stage JFET amplifier. I've reviewed some analysis on 2 stage BJT amplifiers as well as 2 stage BJT/JFET amplifiers but I cannot find anything on how to use 2 JFETs. Any help would be greatly appreciated. Thank you!
  2. jcsd
  3. Dec 9, 2007 #2


    User Avatar
    Gold Member

    Any reason why you need a 2 stage JFET? Are you looking explanations on how to get equations on input impedance, output impedance, gain, etc. You shouldn't bother yourself with input impedance as FETs aren't lacking in that category.

    Generally in a multistage amplifier, you'd use a FET as an input stage, For example, in a three stage amp, your first stage should have high input impedance, so you'd used a FET. The middle stage can used for gain. Its not that wise to use a FET solely for high gain. Remember the issue with FETs and transconductance. The middle stage can be a common emitter. Finally, you'd want low output impedance. Which BJT configuration has low output impedance?
  4. Dec 9, 2007 #3
    thanks a lot for the reply!

    I realize JFETs aren't good compared to BJTs for voltage gain. I am just practicing different configurations, in this case 2 gain stages, and would like to know how to set up the circuit and get the equations. I do understand JFETs but need some help in this scenario. Thanks again!
  5. Dec 10, 2007 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I have to disagree with most of what ranger said...

    It's extremely hard to guarantee stability in three-stage amplifiers (if they're used in feedback loops), so don't build them. You can easily obtain a gain of ~2500 per stage with FETs, and there's rarely a need for gains higher than 2500^2. Most designs can actually be realized with a single stage, in fact. If you do use two stages, you should generally make most of your gain in the first stage -- this will permit your output swing and dynamic range to be larger.

    Depending upon your application, you may or may not want low output impedance. The actual choice of output stage would depend upon the nature of the load (resistive, capacitve, etc.).

    You'd need to tell us quite a bit more about your specific requirements for us to be able to provide you any more specific help.

    - Warren
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook