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I came across this question in my book, it says:

what will happen to the output voltage if we increase the base current in the figure below?

enter image description here

What confused me in the figure is that the output voltage is the voltage on the load Rc, and I used to think of it to be the voltage across the emitter-collector junction, like this: enter image description here

In the second figure I would say that increasing the base current will decrease the output voltage, but I don't know what to say in the first case in Figure one. What is exactly the output voltage in Common Emitter configuration? And what is the difference between the first and the second figures? And what will happen if we increased the base current in the first and second figures?

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  • \$\begingroup\$ Output voltage is where it is said that the output is, not where you'd like it to be ;) \$\endgroup\$ – Todor Simeonov May 31 '17 at 21:00
  • \$\begingroup\$ And what do you think will happen with output voltage (e.g. the voltage on Rc)? Just follow the things you know. \$\endgroup\$ – Todor Simeonov May 31 '17 at 21:01
  • \$\begingroup\$ Oh, Ok, thank you, and in this case I think the output voltage will increase. \$\endgroup\$ – Asmaa May 31 '17 at 21:03
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Collector current, which flows from \$V_{\text{CC}}\$ to ground (i.e. the emitter), is proportional to base current so increasing the base current also increases the collector current. The voltage across \$R_C\$ is \$I_CR_C\$, and it is also the output voltage in the first figure. So in this case the output voltage increases.

The second figure simply defines \$V_{\text{out}}\$ differently. It is \$V_{\text{out}} = V_{\text{CC}} - I_CR_C\$, so since \$I_CR_C\$ increases with increasing base current we can say that \$V_{\text{out}}\$ decreases in that case.

The second figure is the more typical definition of \$V_{\text{out}}\$, since typically voltages are referenced to ground. But the two circuits are basically equivalent (they are both common emitters) -- it's just a difference in the definition of the output voltage.

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I would say that the magnitude will increase, but if the positive is (as is conventional) on the top, the voltage will go down (start off negative and become more negative).

However this is probably just the answer they want and therefore what will get you the mark. In fact it is quite possible for Vce to increase with increasing base current. This will happen when the transistor is in saturation and generally when Ib > Ic. You can easily demonstrate this with a normal small signal transistor and a couple of resistors.

So the real-life answer is "it depends" with an extra layer of uncertainty caused by their failure to mark + and - on the measurement points for Vout. Your teacher may not know this.

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Your book gives a very bad defined question.

As long as they do not specify which is positive terminal of Vout it can either be Vout=Rc×Ic or Vout=-Rc×Ic.

Given rising Ic first voltage increases while the second one gets more negative and hence decreases.

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  • \$\begingroup\$ A voltage getting more negative doesn't decrease, it increases, decrease means get closer to zero, not approaching the negative infinte. \$\endgroup\$ – Marcelo Espinoza Vargas Jun 1 '17 at 1:43
  • \$\begingroup\$ @Marcelo Espinoza Vargas What you say applies to absolute value of voltage only. Numbers are lined like -2 < 0 < 3, no chance to change this! \$\endgroup\$ – carloc Jun 1 '17 at 5:03

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