5 Power amplifiers

5.10 Solid state

Most amplifiers use solid state technology, a.k.a. semiconductors. A transistor is a semiconductor device. Its conduction can be controlled. A small current at the input will result in a higher current at the output, supplied by the power supply. This current gain is the building block for an amplifier. A transistor's current gain is limited. This is especially true for high power transistors. This is why several stages are used to increase the input current about 1 million times and the input voltage about 100 times. Each stage amplifies the signal a bit more, and the final stage drives the loudspeaker. A more recent development is the MOSFET (Metal Oxide Semiconductor Field Effect Transistor). A semiconductor also, but with a different working principle. Its resistance can be controlled by an input voltage. This makes for more efficient (read: cheaper) amplifier design, because high power MOSFETs have a higher gain and can be driven with less stages. Another development is the IGBT (Insulated Gate Bipolar Transistor): a power transistor is driven by a smaller MOSFET, combined in the same housing.

In a transistor amplifier, the mains input is first transformed to a lower AC voltage (and at the same time electrically isolated from the mains, for safety) by a transformer, then rectified by diodes, and then buffered (to get an almost-perfect DC voltage) by one or more pairs of very large capacitors. These are the energy storage units for when the AC voltage is crossing zero and thus not able to supply current. These large capacitors are of huge importance. With a high current, a small capacitor will easily be drawn almost empty every cycle. The voltage drops and the amplifier will clip more easily. Another side-effect of this is hum, because even when running idle, the supply voltage will drop every rectified half-cycle. Capacitors can supply extremely high currents for short periods of time. They play an important part in the peak power specification of an amplifier. Peak power is what matters in bass guitar amplifiers 5.2 Output power.

© Joris van den Heuvel 2001-2009