I have this task: When Vb changes, shall Vc change -4 time as much. Can you deside Re so this will happend?

can somebody please help me?

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  • 3
    \$\begingroup\$ What steps have you taken to figure it out for yourself? \$\endgroup\$
    – JRE
    Nov 26 '15 at 16:03
  • \$\begingroup\$ Are you sure you mean "-4 times as much?" \$\endgroup\$
    – JRE
    Nov 26 '15 at 16:03
  • \$\begingroup\$ It will never happen as V1 is 0V. You might want to study a bit more on the common emitter amplifier and then all will be clear. \$\endgroup\$ Nov 26 '15 at 16:06
  • \$\begingroup\$ English is not my native language, but it is what the task says.. I have not done much, because i dont now where to start. I am looking for an equation to find the value, but so far i havent find any. \$\endgroup\$
    – oystein
    Nov 26 '15 at 16:09
  • \$\begingroup\$ equations are: Ic = Ib * hfe and: V=IR \$\endgroup\$
    – Icy
    Nov 26 '15 at 16:12

Here's a hint.

Imagine V1 is 2 V. The transistor's emitter is 0.7 V lower than the base voltage, so it is at 1.3 V. A current I = 1.3/Re flows. This same current flows through the collector (ignoring beta effects). So the collector voltage is 12 V - I*1k. Now, if V1 increases to 3 V, the emitter will increase to 2.3 V, and I will increase to I_new = 2.3/Re, and the collector will decrease to 12 - I_new*1k.

The gain is the change in collector voltage divided by the change in emitter voltage.


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