How does this current source work?

Discussion in 'Analog & Mixed-Signal Design' started by sp2821, Jul 24, 2017.

  1. sp2821

    Thread Starter New Member

    Jan 8, 2014
    21
    0
    Hi,

    I was reading an application note from ADI (AN-968) about current sources and saw a circuit below.
    The note only had basic description of the circuit and I'm wondering how the circuit works (especially D1, D2, Q1 and Q2)
    Why are there 2 diodes and how do R2 and R3 help preventing thermal runaway?

    Thank you!

    upload_2017-7-24_17-19-19.png
     
  2. dl324

    AAC Fanatic!

    Mar 30, 2015
    7,242
    1,682
    The diodes bias the transistors so they're always on.

    The app note describes how R2, R3, and the sense resistor prevent thermal runaway.
     
  3. KeepItSimpleStupid

    Distinguished Member

    Mar 4, 2014
    2,879
    505
    D1 tracks the BE drop of Q2 and conversely for D2. They should be on the same heat sink.

    The emitter resistors are typically wire wound or metal oxide and they also act as current limiters/fuses as well as well as help with thermal run-away.

    How is this https://www.onsemi.com/pub/Collateral/AND8196-D.PDF part for coolness.
     
    RichardO and OBW0549 like this.
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