Common collector

136

02:55:07

It’s said in common collector configuration, the voltage between base and emitter should be constantly around 0.7v, but why did it change here when I change that resistor?

published 5 years ago

Robert_Kidd

5 years ago

As you alter the emitter resistor the current through it changes and, therefore, the voltage across it. You are measuring the base emitter voltage which will always the difference between the emitter voltage and the 1V supply you have applied to the base. Nominally around 0.7V, yes, but it will vary somewhat depending on exactly what components you put around it.

Robert_Kidd

5 years ago

In a real circuit you would not connect a voltage source directly to the transistor base. Insert a 10k resistor between the base and the 1V supply, then vary the emitter resistor and see what happens to base emitter voltage.

d00b00

5 years ago

Thanks and very excited to see your comments! I understand there exist gaps between reality and theory, but I wonder how could an engineer calculate and decide what components to use in this situation? Do they have any practical method or formula for this?

Robert_Kidd

5 years ago

In basic terms, the emitter current equals base current plus collector current. Collector current is base current multiplied by the transistor current gain (default value for gain in EC transistor is 100). You already know base emitter voltage is around 0.7V.

d00b00

5 years ago

Wow, it works! I use Ic = 100Ib, and Ube = 0.703v, with some math works on paper, finally I got Ib = 5.83 ua! Can’t believe it!

d00b00

5 years ago

When the power supply on the left is 1v, the resistor on the left is 100ohms and the resistor on the right is 500ohms.

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