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  • Author Author: jc2048
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Transistors: Output Characteristic Curves

jc2048

This one is similar to what I did with the gain, except that this time I kept the base current constant and measured the collector current in relation to the collector-emitter voltage. I reworked the original test circuit, which is how this blog follows the last one so quickly. As before, I'm looking at an NPN bipolar junction transistor (a 2N3904) and the situation where the emitter is the common terminal between input and output.

 

Here's the circuit (minus decoupling caps):

 

image

 

I've moved the constant-current source to the base and chosen a resistor that will give me base currents in the area of a few tens of microamps. The collector I'm driving with a follower so that I can set the voltage without having to account for the voltage drop of the ammeter. All the measuring I'm doing is with meters, so there's no need for great precision and accuracy - as long as it holds everything steady, that's enough.

 

Here are the curves I get for various base currents.

 

image

 

If the transistor output was a perfect current sink, the output curves would be horizontal lines, each at a single value of collector current. As it is, whilst approximating a current sink, the lines slope and the transistor doesn't operate all the way down to a Vce of 0V. The situation at the bottom end is due to saturation - when the collector voltage gets down to the base voltage, the base-collector diode is no longer reverse biased and the increasing forward bias as the collector is moved lower starts to affect the way the transistor operates. The slope higher up is due to base width modulation (Early effect) - here's the Wikipedia entry if you want to read more:

https://en.wikipedia.org/wiki/Early_effect

 

For a fixed collector voltage, the proportionality between the base and collector currents is reasonably good. However, in a practical circuit, when a simple resistor is used to convert from the output current to an output voltage, there will be some distortion because of the variation in gain as the collector voltage varies. That distortion will be higher for a large signal than a small one.

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  • Good series.

    Was it waht you expected/remembered ??

  • in reply to mcb1

    Remembered? It's a long, long time ago and I'm not very clear now what lectures covered. This is from Integrated Electronics by Millman and Halkias which we used as a text, so I would definitely have been introduced to it all, but unfortunately it was largely book learning - there was very little practical work - which didn't really suit me (I learn best by doing, not looking at equations on a page).

     

    image

     

    Even though it was published in 1972, the curves they show are for a germanium PNP device. They've added a curve for the maximum power dissipation and a load line for a 500 ohm resistor to 10v at the collector.

     

    Since I never did proper transistor design after leaving university, I'm now left with the usual ragbag of practical rules-of-thumb for the odd occasions I need to use them and would certainly struggle to explain the physics. For example, I still don't quite get what's happening when the transistor enters saturation, from the solid-state physics point of view, though I know in practical terms that the transistor bottoms somewhere in the region of 50-150mV if you massively overdrive the base current. I've avoided saturation so far, because I'm not clear what exactly I should be measuring in terms of what the dependancies are.

     

    I don't know if the blogs are any use to others; they're proving very useful to me - a practical road to understanding - but I need to move on a bit quicker and tackle some proper design now.

  • in reply to jc2048

    the usual ragbag of practical rules-of-thumb for the odd occasions I need to use them

    I've always worked on that principle.

     

    Never was much for the theory, just enough to understand what was happening and why ...

    I don't know if the blogs are any use to others

    They are very interesting, so thanks for doing them.

     

    Mark

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  • in reply to jc2048

    the usual ragbag of practical rules-of-thumb for the odd occasions I need to use them

    I've always worked on that principle.

     

    Never was much for the theory, just enough to understand what was happening and why ...

    I don't know if the blogs are any use to others

    They are very interesting, so thanks for doing them.

     

    Mark

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