Amplifier Classes
Not all amplifiers are the same and there is a clear distinction made between the way their output stages are configured and operate. The main operating characteristics of an ideal amplifier are linearity, signal gain, efficiency and power output but in real world amplifiers there is always a trade off between these different characteristics.
Amplifier Classes is the term used to differentiate between the different amplifier types.
Amplifier classes are mainly lumped into two basic groups. The first are the classically controlled conduction angle amplifiers forming the more common amplifier classes of A, B, AB and C, which are defined by the length of their conduction state over some portion of the output waveform, such that the output stage transistor operation lies somewhere between being âfully-ONâ and âfully-OFFâ. You can see in the diagram above how much a transistor conducts of a sine waveform for each class.
The second set of amplifiers are the newer so-called âswitchingâ amplifier classes of D, E, F, G, S, T etc, which use pulse width modulation (PWM) to constantly switch the signal between âfully-ONâ and âfully-OFFâ driving the output hard into the transistors saturation and cut-off regions.
Transistors
Its difficult to know how much detail to go into, but I think its probably beneficial to first briefly talk about transistors.
Transistors are fundamentally three-terminal devices. On a bi-polar junction transistor (BJT), those pins are labeled
collector (C),
base (B), and
emitter (E). The circuit symbols for both the NPN and PNP BJT are below:
Transistor Construction
Transistors rely on semiconductors to work their magic. A semiconductor is a material that's not quite a pure conductor (like copper wire) but also not an insulator (like air). The conductivity of a semiconductor -- how easily it allows electrons to flow -- depends on variables like temperature or the presence of more or less electrons. Let's look briefly under the hood of a transistor. Don't worry, we won't dig too deeply into quantum physics.
A Transistor as Two Diodes
Transistors are kind of like an extension of another semiconductor component:
diodes. In a way transistors are just two diodes with their cathodes (or anodes) tied together:
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.
The diode representation is a good place to start, but it's far from accurate. 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). There's a whole lot of weird quantum physics level stuff controlling the interactions between the three terminals.
The transistor is kind of like an
electron valve. The base pin is like a handle you might adjust to allow more or less electrons to flow from emitter to collector. Let's investigate this analogy further..
The Water Analogy
We can say that current is analogous to the flow rate of water, voltage is the pressure pushing that water through a pipe, and resistance is the width of the pipe.
Unsurprisingly, the water analogy can be extended to transistors as well: a transistor is like a water
valve -- a mechanism we can use to
control the flow rate.
There are three states we can use a valve in, each of which has a different effect on the flow rate in a system.
1) On -- Short Circuit
A valve can be completely opened, allowing water to
flow freely -- passing through as if the valve wasn't even present.
Likewise, under the right circumstances, a transistor can look like a
short circuit between the collector and emitter pins. Current is free to flow through the collector, and out the emitter.
2) Off -- Open Circuit
When it's closed, a valve can completely
stop the flow of water.
3) Linear Flow Control
With some precise tuning, a valve can be adjusted to finely
control the flow rate to some point between fully open and closed.
A transistor can do the same thing --
linearly controlling the current through a circuit at some point between fully off (an open circuit) and fully on (a short circuit).
From our water analogy, the width of a pipe is similar to the
resistance in a circuit. If a valve can finely adjust the width of a pipe, then a transistor can finely adjust the resistance between collector and emitter. So, in a way, a transistor is like a
variable, adjustable resistor.
A transistor controls a larger current (from the power supply) with a much smaller current (from the audio signal). A small current flowing in the base controls a larger current flowing through the collector emitter. This is how it is used to amplify.
So now you are a transistor expert lets move on to the amplifier classes.
Next post will be about Class A