are current mirrors current limiting or constant current?

Discussion in 'General Electronics Chat' started by tainoindustries, Nov 28, 2014.

  1. tainoindustries

    Thread Starter New Member

    May 22, 2014
    Hey All,

    The book is awesome!

    I was wondering, in the first paragraph of ch. 4 page 14 (link below):

    It says "An often-used circuit applying the bipolar junction transistor is the so-called current mirror, which serves as a simple current regulator, supplying nearly constant current to a load over a wide range of load resistances."

    My confusion is, are they constant current or current limiting? I tried these circuits and from my experiments it looks like it is more of a current limiting device. Say for example I have 10mA going through R_load. If I increased the resistance of R_load, I noticed that the current kept decreasing.

    What I was expecting from a constant current device, is to constantly adjust the current through R_load so that it stays at 10mA as I increased the resistance. Is this the right assumption?

    On the other hand, if I decreased the resistance of R_load, it would never go passed 10mA which I expected it to be so.

    So to me a current mirror is a current limiting device.

    I could also be misunderstanding what current limiting and constant current devices are to begin with :).

    If anyone can clarify I would really appreciate it.

  2. crutschow


    Mar 14, 2008
    Practical current-limiting and constant-current devices are basically the same type of device. They both provide a constant current but real devices are limited by their voltage compliance range. It's just the context in where they are being used that causes the label to be one or the other.
    Thus, for example, if the maximum voltage (compliance) that the device can output is 10V and it is adjusted for a 10mA (constant or limit) current then it will provide that current to the load for any resistance value from 0 ohms to 1k ohms.
    So if you are concerned about limiting the current (such as in a power supply), then you would say this device limits the current to 10mA for any resistance below 1k ohm (current-limit).
    If you are concerned about delivering a constant current to a load, then you would say this device delivers a constant-current for any load up to 1k ohm (constant-current).

    Make sense?
  3. tainoindustries

    Thread Starter New Member

    May 22, 2014
    I didn't realize the concept behind them was based on the context of how they are being used. Makes perfect sense. Thank you very much.
  4. ian field

    Distinguished Member

    Oct 27, 2012
    A current mirror is exactly that - all the transistors who's base isn't connected to their collector; "mirror" the current flowing in the collector of the one that has.
  5. WBahn


    Mar 31, 2012
    You should see roughly 10mA until your load resistance is large enough that the voltage dropped across it is so much that the transistor starts saturating. This will happen when, roughly, the load resistance is Vcc/Icc. At that point, the transistor is no longer in the active region and you can't expect it to do much of anything beyond looking, for the most part, like a closed switch.

    In practice there are other effects that will come into play, namely the finite output resistance of the transistors, the matching between the two transistors, and thermal effects. The use of ballast resistors in the emitter paths can greatly alleviate these.
  6. WBahn


    Mar 31, 2012
    Except to the degree to which they don't. ;-P