In the late 1940s, William Shockley led a team at Bell Laboratories to develop a new kind of amplifier for the US telephone system. He, together with John Bardeen and Walter Brattain announced the invention of the bipolar “junction transistor” at a press conference on July 4, in 1951. Shockley once explained transistor-amplifiers to a student saying: “If you take a bale of hay and tie it to the tail of a mule and then strike a match and set the bale of hay on fire, and if you then compare the energy expended shortly thereafter by the mule with the energy expended by yourself in the striking of the match, you will understand the concept of amplification.”
When a small input voltage results in a large output voltage, the effect is amplification. A transistor can serve as an amplifier by raising the strength of a weak signal. If a DC bias voltage is applied to what’s called the emitter base junction, the transistor transitions into a forward biased condition. Let’s unpack that statement.
A wide mouthed bass, a big funnel, and a large diameter telescope have one thing in common. They each allow easy entry. This can help the fish to scoop up food, the funnel can be used to rapidly fill a narrow jar, and a big lens on the front of the telescope has a lot of light gathering power. In all three examples, this can also be stated as low input resistance. In the case of a transistor, the low resistance input is called the collector. It is, by design, a way to gather as many electrons as possible from a relatively weak source.
Now once the collector has done its collecting, we can apply additional pressure to help push the gathered electrons out of the collector area and towards the output otherwise known as the emitter. Just how this pressure is applied is going to determine just how effective or powerful our push is going to be. We already know that like particles repel one another and such is the case with any gathering of electrons. Sooo, if we have a bunch of contentious electrons already pushing against one another at the collector, and if we inject more electrons at the base, those electrons are going to be motivated to find some means of escape. They’re going to be pushing towards the exit door and thrusting out of the emitter.
When the resistance at an input is low, and when additional pressure is applied from the base, we have a situation that can now be characterized as forward biasing. If you’re driving on the highway at rush hour, and there are a bunch of cars streaming in behind you from an entrance ramp, you are likely to feel the pressure to keep going. If you are tubing on a lazy river, as you pass a powerful tributary or stream, you will be forward biased as you feel the rush.
Within a transistor, the chemical layering, the electron conducting, and the forward biasing all contribute to the process of amplifying. There are many variables. How weak is the incoming signal, how strong is the desired output, and how much pressure can be brought to bear are first among the primary design considerations. Among the others is the ratio of signal to noise. Whenever we are working with conduction, we must also consider the possibility of undesirable induction. Otherwise, when we amplify the signal we want, we may also be amplifying the noise we don’t want.
If, for example, you have a collection of vinyl records, you already know about noise. The dust, the fingerprints, and the peanut butter all conspire to interfere with what might otherwise be an enjoyable listening experience. When the phonograph needle moves through the wavy grooves, it’s not only following the waves, the vinyl itself is porous. Early, in the evolution of audio, recording enthusiasts had to deal with rough records, the rumble of turntables, and the heartbreak of tape hiss.
If you are on a submarine, the levels for the signal of interest relative to the background noise in the ocean can determine whether or not a sonar system will detect an identifiable echo. When the team at Bell Labs was developing their amplifiers, they had to consider the fact that telephone lines share the same utility poles with electrically noisy power lines. Artifacts, within any given recording or transmission medium, each present a unique set of challenges to those engineering our amplification systems. Selective amplification and de-amplification provided a way to boost the signal, where interference or background noise could be anticipated, and then reduce it once the noise was no longer a factor.
Although the right combination of substrate materials within the transistor may affect its response with respect to frequency, the circuitry that complements the amplifying transistor can usually be fine tuned to selectively filter or amplify a signal as needed. It should also be noted that amplifiers can also be used to increase raw power where signaling is not a factor.