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A text is telling that a transistor can amplify power and gives common base amplifier as an example. Here is what it mentions:

enter image description here

But what is the use of the transistor here? Below I draw the same circuit with and without a transistor and plot the dissipated powers at resistors:

enter image description here (left-click to see bigger view)

I dont understand, what is the difference?

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When Zbe(f) =0 It forces a lower input Ze(f) so there is no power gain, just level shift on Vc.

But if you add Rb such that Ic/Ib< Hfe you will have high Ze and thus lower input power, thus power gain. Power gain depends on Vcc, Ic, Rb,Rc , f and GBW of Q.

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The book is indeed a little bit confusing because the text says

The output circuit carries the same current, however, but typically will deliver it to load of thousands of ohms

but the diagram shows not load resitor at all.

Imagine a (high) load resistor inserted in the output circuit between Vcc and collector. The voltage change across that load will be much higher than the voltage change across the source; since the current will be (about) the same there will be power amplification.

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  • \$\begingroup\$ But for that you don't need a transistor. This is my confusion. See my second circuit and plot. Again same current passes and more voltage in load resistor hence more power. What is the extra from transistor here? Both circuits amplify power. \$\endgroup\$ – floppy380 Oct 27 '17 at 10:50
  • \$\begingroup\$ I'm not sure, but maybe your biasing is not correct. Try smaller Vbb. If you do it right there will be a big difference between the circuit with transistor and the one without. \$\endgroup\$ – Curd Oct 27 '17 at 12:19
  • \$\begingroup\$ I found the mistake. I labeled Vout for both outputs thats why they were same and deluding,. \$\endgroup\$ – floppy380 Oct 27 '17 at 13:22

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