how to use MOSFET in it's linear region

Discussion in 'General Electronics Chat' started by Pan Tong, Jun 8, 2015.

  1. Pan Tong

    Thread Starter Member

    Apr 26, 2015
    50
    0
    Hello, i want to have a control circuit like this:
    upload_2015-6-8_22-34-0.png
    i want to control the voltage drop across the R, for Q1, what should i use? BJT or MOSFET which is better? if use MOSEFT, how should i chose a mosfet, it will work in it's linear region, but what i know is the linear region of MOSFET is very nerrow... what parameter should i pay attention to?i used mosfet before as a switch, in that case, i will choose a low Ron and low Qg/Cds, in linear mode, should i choose high Qg one so the linear range will be wider ?


    thanks and regards
     
  2. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,515
    1,246
    Without telling us the intended voltage, current, power, or frequency of the intend circuit, how can you expect an answer? Also, the linear region of a MOSFET is not very narrow.

    ak
     
  3. crutschow

    Expert

    Mar 14, 2008
    12,977
    3,220
    Do you want to adjust the voltage drop across R (voltage control) or the current through R (current control)?

    Either way there are techniques to linearize the control, such as using an op amp in a feedback loop with a transistor (either MOSFET or BJT).
    Without feedback it's difficult to obtain linear control in that circuit.
     
  4. Pan Tong

    Thread Starter Member

    Apr 26, 2015
    50
    0
    Hi, yes I want to control (adjust ) the current pass through R,2-5A, using feedback to opamp,would you please give me some example circuits?

    Thank you very much for help
     
  5. john*michael

    Member

    Sep 18, 2014
    43
    5
    You can use a low Ron, but remember that in the linear region, you won't be there. Try putting a small sense resistor between the source and ground and use and op amp to compare it with a reference and drive the gate. Remember that you will be dissipating power in the MOSFET (Vds times current), which will show up as heat in the MOSFET or Transistor.
     
  6. kyka

    New Member

    Jun 7, 2015
    22
    1
    aae1.jpg
    Here's a schematic that can help. The current through resistor R2 is simple V1/R1. Since 5 amps is a lot of current you must use a power transistor, like the good old 2n3055. However, that BJT has a low gain and, for that reason, a darlington configuration must be employed. The voltage source V1 can be any voltage source, for example a simple voltage divider. Hope it helped.
     
  7. ian field

    Distinguished Member

    Oct 27, 2012
    4,413
    782
    Put a resistor to sense current in the source lead and use the voltage developed across it to bias a bipolar transistor in common emitter. The collector terminates the bottom of a voltage divider that provides gate bias for the MOSFET. You can bootstrap the gate bias with a high value resistor to maintain a good input resistance.
     
  8. Pan Tong

    Thread Starter Member

    Apr 26, 2015
    50
    0
    Hello All:
    Thank you very much for all the suggestions, I will try first and let you know the result.

    best regards
     
  9. Pan Tong

    Thread Starter Member

    Apr 26, 2015
    50
    0
    hi
    I did a simulation using LTSpice, like this :
    upload_2015-6-9_11-0-48.png

    but when I try to change the C1 to 200nf, the simulator give a warning of :
    upload_2015-6-9_11-2-22.png

    anybody know why?
     
  10. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,281
    1,232
    I didn't have your op amp and transistor, but it works ok with some I have.
     
    • try.asc
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  11. Pan Tong

    Thread Starter Member

    Apr 26, 2015
    50
    0
    Hi, Ronv:
    thank you for your reply, I attached my opamp and transistor library in my last post, I upload them again, would you please help me try again? my problem happen when I change C1 to 200nf...

    thank you for help.
     
  12. Pan Tong

    Thread Starter Member

    Apr 26, 2015
    50
    0
    Sorry Ronv:
    or, would you please share with me your simulation circuit?

    thanks
     
  13. Pan Tong

    Thread Starter Member

    Apr 26, 2015
    50
    0
    Hi, Ian:
    thank you very much for your reply,i am very interested on your idea, is it possible you can do a simple drawing for me?
     
  14. Pan Tong

    Thread Starter Member

    Apr 26, 2015
    50
    0
    sorry Ronv:
    i didn't see your circuit this morning :p
    i had tried your circuit, yes, it works fine with changing C1, and the problem is the opamp: OPA2251 vs LM358.

    Thanks and regards
     
  15. ian field

    Distinguished Member

    Oct 27, 2012
    4,413
    782
    I'm a little busy to look for a schematic app that will output "universal" files.

    The idea is too simple to need a diagram - decide what mid-point drain current you want to flow and calculate a resistor to drop 0.7V at that current. That resistor now becomes the current sense between source and GND. Connect a common emitter NPN transistor, E to GND and B to source of the MOSFET, its best to put a resistor between base and source to soak up any spikes - about 15R is sufficient. An actual voltage divider probably isn't necessary to bias the gate - just connect at least 47k from collector to Vcc, current increase in the source resistor will now tend to bias on the bipolar transistor so the collector voltage falls - that is your nfb that stabilises the operating point. You need to string a high resistor between the bipolar collector and the MOSFET gate otherwise the collector will shunt the input.

    So far with just the transistor and a few resistors, you have both AC & DC nfb so not much gain - you need an AC shunt (small electrolytic) on the nfb path. You could try from collector to GND, but that may not be fully effective, put a few k from the collector to the cap, then a high resistor from the cap to gate.

    The transistor, "few k resistor" and cap develop your average bias to hold the MOSFET drain at its mid-point, the high resistor from cap to gate gives you a high resistance input.
     
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