2
\$\begingroup\$

schematic

simulate this circuit – Schematic created using CircuitLab

enter image description here

I have read several times that in a BJT transistor the variation of VCE barely influences IC.

Looking at the curve graph of a BC550 I see that for an IB of 150 μA we get about 40 mA of IC regardless of whether VCE is 6 or 12 V.

I have assembled the circuit and I have verified that keeping IB constant at 150 μA and varying VCE between 6 and 12 V, IC varies between 30 and 64 mA.

Am I misinterpreting the graph of curves?

[Edit1] I have added the schematic of the circuit that I physically assembled, on the right is my Vc vs Ic measurement table

[Edit2] I have corrected in the table the value of 2v to the correct value 12ma, typing error

\$\endgroup\$
13
  • \$\begingroup\$ The curve is given for one part with a probably unspecified hfe. If the dc gain of your part is very different (range is from 110-800, can even vary with manufacturer) you end up on a steeper curve (like the ones at 400 uA). What hfe class do you have? \$\endgroup\$
    – datenheim
    Jan 22, 2023 at 22:38
  • \$\begingroup\$ For Ib 150uA and Vce=9v and Ic 50mA the hfe = 300 \$\endgroup\$
    – Mario
    Jan 22, 2023 at 23:15
  • 1
    \$\begingroup\$ @Mario Can you show us a schematic of the circuit you assembled and tell us your testing procedure? \$\endgroup\$
    – GodJihyo
    Jan 22, 2023 at 23:28
  • 1
    \$\begingroup\$ Also mind thermal drift: the value will creep up over time, particularly at high Vce, due to temperature rise. Preferably the measurement should be pulsed (usually < 300µs). \$\endgroup\$ Jan 23, 2023 at 0:40
  • 1
    \$\begingroup\$ Also all manufacturers carefully avoid to state anything about temperature dependance of all the parameters, whuch tells me, there lurks something ;) Repeat your measurement with short pulses and longer delays and you will find something nearer to the graph. AFAIR the curves get steeper with increasing temperature. \$\endgroup\$
    – datenheim
    Jan 23, 2023 at 8:06

1 Answer 1

1
\$\begingroup\$

After many tests I have discovered that only the two units I have of the BC550C behave in this strange way

I have subsequently tested with PN2222A and BC337 and both behave correctly as indicated by their curves

[Edit] New tests

I have tried with a new unit of BC550C and the result is the same, what I have discovered after many more tests is that the BC550C when we apply less than 100uA of base current the collector current fits the curve correctly but when we apply to the base more than 100uA the result is from the table that I show in the diagram

I have also proven that above 100ua when we increase Vce slightly increases the base current, this does not happen so accused with the 2N2222 or with the BC537

\$\endgroup\$
7
  • \$\begingroup\$ Do you know the manufacturer of the two misbehaving transistors? Maybe there is a small sign/icon on them? \$\endgroup\$
    – datenheim
    Jan 23, 2023 at 22:38
  • \$\begingroup\$ Mario - Hi, We're getting flags as this answer isn't clear and it seems to be asking more questions. Either: (a) You correctly wrote it as an answer, because it really does answer your original question, and the new, implied questions are unimportant. In that case, please clearly state that this is the final solution, and come back in 2 days to "accept" this answer (or another one, if a better one has been posted by then) to close the whole topic. (Self-answers can only be accepted when 48 hours has elapsed since asking the question.) \$\endgroup\$
    – SamGibson
    Jan 23, 2023 at 22:46
  • \$\begingroup\$ [continued] Or: (b) You mistakenly wrote this as an answer, when in fact you still want more responses. In which case, this is an update, not an answer, and must be "edited into" your original question e.g. click "Edit" under the question, add this new information at the bottom, explain what further help is needed, then delete this "answer". Which applies here, (a) or (b)? Thanks. (As the OP, you would only write an answer if you solved the problem on your own and the topic can be closed.) \$\endgroup\$
    – SamGibson
    Jan 23, 2023 at 22:46
  • 1
    \$\begingroup\$ @SamGibson I think it's more A than B :-), I'll accept the answer and open another question later if I get more BC550 and can perform more tests \$\endgroup\$
    – Mario
    Jan 24, 2023 at 6:47
  • 1
    \$\begingroup\$ @greybeard When I decrease Vce from 20 to 2 volts, the result is the one in the table that I show above \$\endgroup\$
    – Mario
    Jan 24, 2023 at 18:23

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.