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BC5478 (or BC547B) transistor: enter image description here VBB = 22mV, 0.03mA RE = 220ohms, there is an LED after the RE resistor.

As shown there is no current to the collector pin. The readings I get: collector shows 296mA Base shows 0.03mA with 22mV Emitter shows 0.03mA with 198mV

I keep expecting the emitter to show zero because no collector voltage (Vcc) is being supplied. << ok after further reading, with no supply to the collector, the BE junction acts like a diode.. So the follow on question is why the difference between the base & the emitter (22mV vs 198mV)?

I changed the configuration such that the load (resistor + LED) is prior to the collector and I have a 1K resistor on the base. Thus nothing on the emitter. Then it behaves the way I expected and the math works out too. So why does it not work with the load on the emitter and a 1K resistor on the collector?

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closed as unclear what you're asking by Phil Frost, Chetan Bhargava, Matt Young, Daniel Grillo, clabacchio Dec 5 '13 at 7:53

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    \$\begingroup\$ Please post the circuit you are using. \$\endgroup\$ – tcrosley Dec 5 '13 at 0:02
  • \$\begingroup\$ Have a look at this answer, and see if you still have questions: electronics.stackexchange.com/questions/51405/… \$\endgroup\$ – The Photon Dec 5 '13 at 0:12
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    \$\begingroup\$ To get to your specific question, I'll assume you're talking about a common-emitter configuration. 1. The collector voltage drops because the pull-up load is dropping more voltage as the collector current increases. 2. Before the collector voltage drops more than a couple 0.1's V below the base voltage, the transistor will go into saturation and the behavior will change, so you don't normally see this happen. If you force it to happen (say with a voltage source on the collector), you will forward bias the BC junction and get very high current flowing from base to collector. \$\endgroup\$ – The Photon Dec 5 '13 at 0:15
  • \$\begingroup\$ I suspect you mean BC547B not BC5478. B as in mid-range gain. \$\endgroup\$ – RedGrittyBrick Dec 5 '13 at 12:24
  • \$\begingroup\$ RedGrittyBrick: you might be right, fonts are too small for me to actually tell (from an arduino kit). \$\endgroup\$ – user1683519 Dec 5 '13 at 16:56
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As comments have indicated, context is crucial. For example, consider the following circuit:

schematic

simulate this circuit – Schematic created using CircuitLab

In this circuit, the base voltage VBB may increase but the collector voltage is VCC and thus will not change; The collector voltage is fixed.

However, the collector current will increase with an increase in VBB and this is the answer to your question.

To see this, modify the circuit slightly by adding a resistor in series with the collector:

schematic

simulate this circuit

Remember, as VBB increases, the collector current increases - this is crucial and fundamental.

Since the collector current \$I_C\$ is from right to left through RC, it follows from Ohm's Law and KVL that the collector voltage is

$$V_C = V_{CC} - I_C \cdot R_C$$

Thus, as \$I_C\$ increases, \$V_C\$ decreases.

But remember, as VBB increases, the collector current \$I_C\$ increases.

Putting the two together, it follows that:

as the base voltage increases, the collector voltage decreases for this type of circuit.

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  • \$\begingroup\$ thanks. I have resistors attached to all three pins of the transistor. It has been a very very long time (decades) since I took circuit analysis in college. That does bring back memories. As for my 2nd question, Proton, I'll have to reread your answer. I was forcing the situation. I think I had forced a situation where there was current from base to emitter, even with having the collector current at zero. \$\endgroup\$ – user1683519 Dec 5 '13 at 4:43
  • \$\begingroup\$ I changed the configuration such that the load is prior to the collector and I have a 1K resistor on the base. Thus nothing on the emitter. Then it behaves the way I expected and the math works out too. So why does it not work with the load (resistor + LED) on the emitter and a 1K resistor on the collector? \$\endgroup\$ – user1683519 Dec 5 '13 at 22:03
  • \$\begingroup\$ @user1683519, with the emitter grounded (is that what you mean by "nothing on the emitter"?) and the load in series with the collector, an increase in base voltage will result in an increase in current through the load and thus, a possibly much larger voltage across the load. With the load in series with the emitter, an increase in base voltage results in a slightly smaller increase in the voltage across the load. The reason for this is beyond the scope of a comment. You should ask it in another question. Make sure to include before and after schematics. \$\endgroup\$ – Alfred Centauri Dec 5 '13 at 22:12

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