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Biasing Transistors in the linear region? - Page 2 — Parallax Forums

Biasing Transistors in the linear region?

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  • jmgjmg Posts: 15,175
    edited 2012-11-15 15:20
    I was once trying to make a 2 terminal bi-directional current source with an NPN and PNP transistor pair.

    That reminds me of a similar two device, bi-directional current limit 'cheap fuse' and slip protector I worked up.

    A standard Bipolar does work backwards, just with lower Beta and lower Vceo, but I wanted symmetric current limit, of USB levels.
    Say 50-100mA
    Solution was a Dual-Device SOT23-6 so they thermally track, and run two junctions in parallel - with a single base resistor.
    The current will not split equally, and the shunt junction also lowers Beta, but that is not a bad thing.
    Those balance identically in each direction, and the final equivalent indicated Beta, is around 14
    Precision was not important, and it is very cheap :)

    Spice image attached :
    1024 x 640 - 37K
  • xanatosxanatos Posts: 1,120
    edited 2012-11-15 15:39
    ...it's only the fraction of current that passes through that 1.8 & 6.8 leg that is being monitored by the 358

    That is the crux of the matter. No matter what load you put on the output, even a variable load, the op-amp will drive the transistor in exactly the right amount to maintain the voltage at its two inputs equal to one another, and that means a constant voltage into the pair of feedback resistors at the emitter output junction.

    I found Duane's comment about the extra reverse emitter-base diode interesting. The base-emitter junction of a NPN bipolar transistor, when reverse biased, acts like a zener diode with a breakdown of about 7 volts. If there is a reactance on the output (long cable?), and the input suddenly drops to zero, there could be a transient reverse breakdown condition in the transistor. Not for long with a 250Ω load, though. Food for thought. A high breakdown diode in series with the emitter to the load, rather from emitter to base, would be an alternative way to forestall that breakdown I think.

    Much clearer in my digital-oriented brain now, thanks Tracy - and everybody - for helping me come up to speed on my analog studies in this instance. I plan to play a LOT with the 358 and bipolar transistors now that I have some small handle on them - I just received an order of 100 of each 2N3904, 2N3906 and LM358!

    Thanks again,

    Dave
  • Tracy AllenTracy Allen Posts: 6,664
    edited 2012-11-15 16:03
    jmg, interesting limiter. There was a discussion a while back about inverted transistors. Another useful characteristic they have is a very low saturation voltage.

    I still refer back to my dog-eared copy of Walter Jung's IC Array Cookbook. The NPN transistors in IC arrays (CA3046, CA3093) were rated for 35mA reverse current at their knee at 7V, with a dynamic resistance of 10Ω-- 100Ω. Here is what he says, confirming Duane's comment: "..operation in the Zener mode degrades the noise and gain of the transistor. Therefore transistors subjected to reverse Zener currents should not be considered for subsequent amplifier uses. Also, devices connected as amplifiers should be caefully protected agains possible EB voltages, which could cause Zener action. Protection of differential amplifiers against excessive input overvoltage is a case in point. Protection can consist of a simple pair of back-to-back signal diodes to limit Vin to ±Vf."
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