How Do I Choose the Best Transistor Parts?

When choosing the best transistor parts, you should consider the cost, characteristics of the transistor parts, and your needs. There are many different types of transistors, each with a set of advantages and disadvantages. Your needs should be the top priority when choosing the best transistor parts. You should also compare the shipment costs from vendor to vendor.

Transistor performance includes transistor gain, bandwidth-gain product, maximum currents, and voltages. The transistor gain is the ratio of output to input. For the bipolar transistors, gain is defined as “beta,” which is the ratio of the collector current to the base current. A transferred signal with a high gain or high beta will be of higher amplitude, usually resulting in a lower stage count, or the number of cascaded transistor stages needed to produce the desired effect.

The bandwidth-gain product suggests that the higher the operating frequency, the lower the effective gain of the transistor. This is one reason why building circuits with a higher-frequency band results in lower available output power for transmitters. The price for high-frequency transistors is also higher due to stringent production requirements. Maximum currents and voltages for a transistor also determine the maximum power that the transistor may deliver. The safe power dissipation of a transistor also depends on the thermal resistance, which is a measure of the ability of the transistor package to transfer heat to the cooling device, which is usually an air-cooled heat sink.

Among the different types of transistors, the field-effect transistor is best for low-noise performance. With the improved technology in high-power field-effect transistor parts, the performance of it has risen while its price has dropped. Field-effect transistor circuits have the advantage of being voltage driven, resulting in circuits that are generally more power efficient. When circuits need multiples of the same transistor, a transistor array is very useful because the overall part count drops while helping toward a more compact printed circuit board (PCB) layout. Transistor arrays may have two to as many as eight or more transistors in one package.

Manufacturers release enhanced versions of transistors on transistor schematics. Some transistors are virtually indestructible due to the added protection inside the transistor package, and some packages even have built-in thermal protection that decreases transistor power dissipation to safe limits at the expense of decreasing or even suspending output. The field transistor is specially engineered for fast response, while the planar transistor version is used for transistors on integrated circuits.