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5/15/2017 769894785 1/4 BJT Amplifier Gain and the Active Region Consider this simple BJT circuit: VCC Q: Oh, goody—you’re going to waste my time with another of these pointless academic problems. Why can’t you discuss a circuit that actually does something? vI RC vO RB A: Actually, this circuit is a fundamental electronic device! To see what this circuit does, plot the output voltage vO as a function of the input vI. BJT in cutoff vO VCC BJT in active mode BJT in saturation vI VCC 5/15/2017 769894785 2/4 Note that: vI vO 0 VCC VCC 0 Mode Cutoff Saturation Why, this device is not useless at all! It is clearly a: ____________________ Digital devices made with BJTs typically work in either the cutoff of saturation regions. So, what good is the BJT Active Mode ?? Sir, it appears to me that the active region is just a useless BJT mode between cutoff and saturation. 5/15/2017 769894785 3/4 Actually, we will find that the active mode is extremely useful! To see why, take the derivative of the above circuit’s transfer function (i.e., d VO d VI ): VO VCC d VO d VI VI VCC We note that in cutoff and saturation: d VO 0 d VI while in the active mode: d VO 1 d VI Q: I’ve got better things to do than listen to some egghead professor mumble about derivatives. Are these results even remotely important? 5/15/2017 769894785 4/4 A: Since in cutoff and saturation d VO d VI 0 , a small change in input voltage VI will result in almost no change in output voltage VO . Contrast this with the active region, where d VO d VI 1 . This means that a small change in input voltage VI results in a large change in the output voltageVO ! I see. A small voltage change results in a big voltage change—it’s voltage gain! The active mode turns out to be—excellent. Whereas the important BJT regions for digital devices are saturation and cutoff, bipolar junction transistors in linear (i.e., analog) devices are typically biased to the active region. This is especially true for BJT amplifier. Almost all of the transistors in EECS 412 will be in the active region—this is where we get amplifier gain !