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Old 16th July 2009, 04:38 AM   #1
akis is offline akis  United Kingdom
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Default small transistor linearity

Hello

I was looking at the linearity of small transistors and noticed that very few, if any datasheets would show the hfe for small collector currents less than 1mA (with exceptions, like MPSA18).

So I set up a small test to measure at very low currents. I took care and the two resistors I used for measurements were 999.6 Ohm and 32900 Ohm, ie my multimeter should not affect the measured voltage.

The table below shows the results for a BC550C. As you can see it is very linear down to 34 uA - and I presume I could measure even lower.

Code:
hfe	  Ic (uA)	      Ib (uA)
528.1779   33.7135	      0.0638
530.3951   55.5222	      0.1047
530.1231   81.5326	      0.1538
528.8381   137.7551     0.2605
529.5837   228.0912     0.4307
530.3543   510.2041     0.9620
530.4975   808.3233     1.5237
527.1172   1332.5330   2.5280
It follows therefore that I could build a class A stage where I could let Ic swing as low as 34uA and still be perfectly linear with no distortion (note: swing down to 34uA, not bias at 34 uA).
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Old 16th July 2009, 05:20 AM   #2
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The data you have given shows that the current gain is almost constant over a wide range of currents. However, the base emitter voltage varies non-linearly by about 60 mv per decade of current. So if you are really driving the base through a finite resistance, the base current will not vary linearly with the source voltage. The specific details depend on the circuit.
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Old 16th July 2009, 06:31 AM   #3
akis is offline akis  United Kingdom
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Yes, I did not measure voltage applied to the base, just the currents. I will edit my post.

Edit: oh well, I cannot edit my post, it won't let me.
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