# VOLTAGE GAIN AND VOUT RESISTENCE OF TRANSISTOR IN AC CIRCUIT

#### sxy

##### Member
HI
Why the voltage gain of transistor in AC circuit is: -hfe*RL/hie without Re resistor;
and with Re resistor is: -hfe*RL/Rin?
and why the Rout-Resistence of Vout is infinite?
Thanks a lot

#### Dick Cappels

##### Active Member
Re is approximately 26/ (emitter current in milliamps). When Ie is 2 milliamps Re is bout 13 Ohms.

In the ideal transistor the collector is and ideal current source and has infinite resistance. For many purposes this model works fine when there is a collector load (because the load becomes the output impdeance).

#### Ramussons

##### Active Member
Re is approximately 26/ (emitter current in milliamps). When Ie is 2 milliamps Re is bout 13 Ohms.
Really? That's something new ( no sarcasm intended ). Any rerefernce please.

#### Dick Cappels

##### Active Member
The part about Re was in one of my old textbooks or lectures from the 1970's I can't remember the names of the book, author, or lecturer this many years later. have used that definition for Re for about a decade and a half of analog design using transistors in benign environments and it has worked well for me over the years.

Here's and idea - try it out in a small common emitter amplifier with the emitter resistor heavily bypassed. You will need a collector load.

#### Ramussons

##### Active Member
Thanks for the leads. I'm quite sure that a lot of us would not have been aware of this.

##### Active Member
Partial derivation of re here in GE transistor manual, attached.

Then there is this -

Regards, Dana.

#### Attachments

• GE - Transistor Manual 1964.pdf
13.2 MB · Views: 19
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