What Are the Functions of a Single Transistor Amplifier Circuit?

Gain

• An amplifier̵7;s goal is to increase either the current or voltage of a weak incoming signal, such as that coming from a radio antenna. Single-transistor amplifiers come in three configurations: common-emitter, common-collector and common-base, representing the device̵7;s three terminals. The first two configurations amplify current. The common-base arrangement amplifies voltage. A transistor parameter called "beta" indicates how much current amplification a given component can produce. Some types of transistors have higher amplification factors than others.

Distortion

• If you examine the original input signal going into an amplifier closely, as with an oscilloscope, then compare it with the signal coming out of the amplifier, you will notice the two have differences. No amplification process is perfect; they all have degrees of error called distortion. Distortion shows up as changes in a signal̵7;s wave shape. It may have sudden transitions, flattening on the top and bottom, or other imperfections. Naturally, low-distortion amplifiers are better, though an electronic designer always chooses a circuit based on trade-offs of distortion, efficiency and cost.

Noise

• Though a transistor in an amplifier sitting on a workbench appears perfectly still, its atoms are constantly vibrating due to thermal motions at room temperature. The higher the temperature, the greater the vibrations. These motions produce unavoidable electrical noise that shows up in the amplifier̵7;s output signal. Noise has a random characteristic, like the sound of radio static. Though the transistor inherently makes noise, some transistors are noisier than others, so a designer strives to use the best components she can afford.

Power Consumption and Efficiency

• A transistor produces signal gain by consuming power from an independent source, such as a battery or power supply. You might guess that the signal̵7;s output power is the power the transistor uses, but it is somewhat more than that, as the device has less than 100 percent efficiency. As it does the work of amplification, the transistor heats up. It loses energy through this heat. Generally, distortion and efficiency go hand in hand. Higher efficiency means more distortion, so the amplifiers with low distortion have relatively low efficiency.