Q. In my lessons, the three different transistor configurations are described. I am having trouble telling them apart.
A. A transistor has three elements: collector, base, and emitter. The input signal is applied to one element, and the output is taken off a second element. The one left over is the common element.
For example, say the input signal is applied to the emitter and taken off the collector. That leaves the base, so this describes the common base amplifier. The signal is AC. One side of the input signal source must go to AC ground. The other side must go to the input element of the transistor.
The output signal comes off another element. One side of the load to which the output signal is sent must be at AC ground, too. The remaining element of the transistor must also be connected to the AC ground. This is because current must flow from the input source, through the transistor, and back to the input source.
Output current must also from the transistor, through the load, and back to the transistor through the ground. Both of these currents, the input and output, must flow through the common transistor element. In these circuits, the ground acts as a common conducting path for the AC input current, the transistor, and the AC output current.
This is where the “common” comes in. Since the remaining transistor element is connected to AC common ground, we call the configurations common emitter, common base, and common collector.
Students are often puzzled by the common collector configuration. The collector goes to the power source, Vcc, instead of to ground. While Vcc is the source of the DC voltage for the circuit, Vcc is a short to ground for AC signals. In a power supply operated off the AC power lines, there are filter capacitors with high capacitance values. Their reactances are very low at all the signal frequencies.
Even in battery operated amplifiers there are usually electrolytic capacitors connected across the battery, since as the batteries discharge, their internal resistances rise, and can interfere with the operation of the amplifiers they power. The result is that these capacitors act as short circuits to ground for any AC signal current that flows to Vcc. This is why the common collector has its common element (the collector) at AC ground, even though it is connected to Vcc.