Biasing That Transistor: The Emitter Follower

We were musing upon the relative paucity of education with respect to the fundamentals of electronic circuitry with discrete semiconductors, so we thought we’d do something about it. So far we’ve taken a look at the basics of transistor biasing through the common emitter amplifier, then introduced a less common configuration, the common base amplifier. There is a third transistor amplifier configuration, as you might expect for a device that has three terminals: the so-called Common Collector amplifier. You might also know this configuration as the Emitter Follower. It’s called a “follower” because it tracks the input voltage, offering increased current capability and significantly lower output impedance.

The emitter follower circuit
The emitter follower circuit

Just as the common emitter amplifier and common base amplifier each tied those respective transistor terminals to a fixed potential and used the other two terminals as amplifier input and output, so does the common collector circuit. The base forms the input and its bias circuit is identical to that of the common emitter amplifier, but the rest of the circuit differs in that the collector is tied to the positive rail, the emitter forms the output, and there is a load resistor to ground in the emitter circuit.

As with both of the other configurations, the bias is set such that the transistor is turned on and passing a constant current that keeps it in its region of an almost linear relationship between small base current changes and larger collector current changes. With variation of the incoming signal and thus the  base current there is a corresponding change in the collector current dictated by the transistor’s gain, and thus an output voltage is generated across the emitter resistor. Unlike the common emitter amplifier this voltage increases or decreases in step with the input voltage, so the emitter follower is not an inverting amplifier.

The keen-eyed reader will have noticed at this point that since the base-emitter junction of a transistor is also a diode, it will always maintain approximately the same voltage across itself regardless of the current flowing within it. For a silicon transistor, this is around 0.6 V so the output voltage on the emitter will always be 0.6 V lower than the input voltage on the base. Thus the voltage gain of an emitter follower will always be just a tad less than 1, and you might thus expect that it would therefore be of little use as an amplifier if you neglected that it has significant current gain. The output impedance of an emitter follower is significantly lower than that of a common emitter amplifier, allowing it to drive much more demanding loads. You will often find it used as a buffer stage for this reason, and a handy example can be found on the output of an early op-amp we took a look at earlier in the year.

We’ve now taken a look at the three basic configurations of a transistor amplifier, as well as the fundamentals of biasing a bipolar transistor. It might seem odd to cover this topic on Hackaday when it’s certain that many of you are already familiar with it, but sometimes it’s worth remembering that not everybody is fortunate enough to he well-versed in these fundamentals. The impetus for this series came from a friend lamenting that while his pupils had advanced knowledge of microcontrollers that his generation hadn’t acquired as their age, they had not been given the opportunity to learn these fundamentals.

There is one final piece to come on this topic, these same principles apply to the other three-terminal active components, so we’ll have a quick look at FETs and tubes.

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