small transistor linearity

akis · 16th July 2009, 04:38 AM

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).

sawreyrw · 16th July 2009, 05:20 AM

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.

akis · 16th July 2009, 06:31 AM

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.

16th July 2009, 04:38 AM	#1
akis diyAudio Member Join Date: Jul 2008	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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16th July 2009, 05:20 AM	#2
sawreyrw diyAudio Member Join Date: Apr 2006 Location: Minnesota	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.

16th July 2009, 06:31 AM	#3
akis diyAudio Member Join Date: Jul 2008	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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