CALCULATING THE INTERNAL BASE-EMITTER RESISTANCE OF A TRANSISTOR. by David Cline In the field of electronics engineering, the internal base-emitter resistance of a transistor is usually not significant enough to insert into design calculations. However, in some cases this small amount of resistance can have a noticeable affect on circuit operation - as is the case with a Common Collector amplifier, or unity gain amplifier. As we shall see, the internal base-emitter resistance plays a key role in determining the actual gain of a Common Collector amplifier. Before we go into base-emitter resistance calculations, I want to examine the impact that the base-emitter resistance will have on the Common Collector Amplifier. For a unity gain amplifier, you would expect the voltage gain to be "one" as calculated from the output voltage divided by input voltage or the emitter voltage divided by the base voltage. AV = Vout / Vin = 1 However, in reality, the voltage gain is slightly less than one (typically around 95%). At first I thought that this was attributed to the base-emitter threshold voltage (typically around .7 volts between the emitter and base) which does cause a voltage offset between emitter and base. Then I realized that the voltage offset simply means that the base voltage will just ride a little higher than the emitter voltage. Variations in the output emitter voltage should still precisely follow, in the same magnitude, as variations in the input voltage at the base, no matter what their respective quiescent voltages are. For instance, if the input is increased by 1 volt, then the output should be increased by 1 volt. However, the output will always swing slightly less than the input. What is really going on is that part of the unity gain is being eaten up by some intrinsic resistance within the transistor. This resistance is called the base-emitter resistance, which we shall designate as Rbe.