5.2: The Need For Biasing

Why bias a transistor in the first place? After all, if the device exhibits current gain (i.e., \(\beta\)), why not just apply an AC signal at the base and obtain an amplified version of it at the collector? The first thing to remember is that current gain is an outgrowth of forward-reverse bias. Given that fact, and without an additional source of energy, amplification cannot be produced. Also, remember the magnitude of the energy hill required for forward-biasing the base-emitter. In order to achieve that, \(V_{BE}\) needs to be around 0.7 volts. If we simply applied an AC signal to the base, we could only hope to forward-bias the base-emitter when that signal exceeded 0.7 volts. The entire negative half of the AC signal would be ignored along with everything positive that's below 0.7 volts. Seeing that the voltage generated from many input devices such as microphones and sensors may only be a few hundred millivolts, the entire signal could be ignored! The solution to these problems is to apply a DC bias to the transistor and then superimpose the AC signal on top of that. In other words, if the AC voltage is riding on a much larger DC voltage, then even the negative peak of the AC signal will be a net positive voltage, and we can maintain proper transistor function.

There are numerous ways to establish a proper polarity DC bias on a transistor. The trick is to find ways to make a stable bias, that is, to establish a Q point that doesn't move in spite of parameter changes such as changes in \(\beta\). As we shall see in following chapters, an unstable Q point can have negative effects on the AC performance of an amplifier. For example, it could make the gain unstable, increase distortion or reduce output power. This lack of stability is a major problem with the base bias configuration examined in the prior chapter. What we would like is a circuit that will establish a collector current that does not shift even when \(\beta\) changes.