Because voltage gain is close to one, the circuit has substantial power gain. For this reason, an emitter follower has current gain. Accordingly, less power is required from the input to drive the output if there were a straight-through connection rather than the emitter follower circuit. The answer is that its value lies in the fact that its input impedance is far greater than its output impedance. Because there is no voltage amplification in this circuit, one may wonder if it has a purpose. In an emitter follower amplifier circuit, the output in addition to being inverted, is just a little less than one because the emitter voltage is pegged at the diode drop of about 0.6 V below the base. If there is an even number of stages, the overall output is not inverted. This is not at all a disadvantage, provided it is taken into account. And second, the output is the inverse of the input. The circuit is characterized by two important attributes: The gain is close to unity at all times, regardless of changes in bias, individual manufacturing variations, thermal effects, loading in a subsequent stage or value of any collector resistor. Because the emitter voltage is a function of the input difference, the device is usually called an emitter follower. In the common collector arrangement, the input voltage is applied between base and collector, and the output voltage comes from the differential that exists between emitter and collector circuits.
In other words, the transistor is a robust voltage buffer. Moreover, the output impedance is low, so it is not destabilized by the following circuit if that happens to be a heavy load.
being virtually invisible, to the previous circuit. What’s good about it is its high input impedance, which has the effect of not loading, i.e. In the old world of vacuum tubes, they were common cathode, common grid and common plate.) This particular classification derives from the external circuit configuration.Īn emitter follower circuit, also known as a common-collector amplifier, is the quintessential negative feedback device. (For field-effect transistors, the analogous circuit configurations are common source, common gate and common drain. Transistors are created by either stacking an n on top of a p on top of an n, or p over n over p.There are three bipolar junction transistor amplifier topologies: common emitter, common base and common collector. A semiconductor material with extra electrons is called an n-type ( n for negative because electrons have a negative charge) and a material with electrons removed is called a p-type (for positive). Some of those layers have extra electrons added to them (a process called "doping"), and others have electrons removed (doped with "holes" - the absence of electrons). Transistors are built by stacking three different layers of semiconductor material together. Using the diode (or resistance) test function on a multimeter, you can measure across the BE and BC terminals to check for the presence of those "diodes".) Transistor Structure and Operation (This model is useful if you need to test a transistor. There's a whole lot of weird quantum physics level stuff controlling the interactions between the three terminals. Don't base your understanding of a transistor's operation on that model (and definitely don't try to replicate it on a breadboard, it won't work). The diode representation is a good place to start, but it's far from accurate. The diode connecting base to emitter is the important one here it matches the direction of the arrow on the schematic symbol, and shows you which way current is intended to flow through the transistor.
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