VOLTAGE GAIN AND VOUT RESISTENCE OF TRANSISTOR IN AC CIRCUIT

[sxy]

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]

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]

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]

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.

 

[Dick Cappels]

If it is of any help, I believe I once derived the relationship based on material in the old Analog Devices application note. https://www.analog.com/media/en/training-seminars/tutorials/mt-079.pdf

 

[Ramussons]

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

 

[danadak]

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

Then there is this -

how to derive the ac emitter resistance of transistor amplifier biasing?

I went through many textbooks and websites and I found out the derivation for AC resistance for diode which is \$r_d=26mV/I_{F}\$ where as for ac emitter resistance it is \$r_e=25mV/I_E\$ for which I

electronics.stackexchange.com

https://uotechnology.edu.iq/dep/coe/lectures/dr-THAMER/ELECTRONIC%20I/Chapter%20Three.pdf

Regards, Dana.