About Small-signal Bipolar Transistors (BJT)

Small-signal bipolar transistors (BJT) are semiconductors that amplify small AC or DC signals. They consist of a base n-type or p-type layer sandwiched between emitter and collector layers of the opposite type. With small-signal bipolar transistors, there are two polarities available: PNP and NPN. PNP devices consist of an n-type layer sandwiched between two p-type layers. NPN devices consist of a p-type layer sandwiched between two n-type layers. In both arrangements, the junctions between layers amplify weak incoming signals and the current flow between the emitter and the base controls the current flow between the emitter and the collector. Based on this design, even a small current between the base and emitter connections results in a large current between the emitter and collector connections. In circuits that use small-signal bipolar transistors as switching devices, current in the base-emitter junction creates a low-resistance path between the collector and the emitter.

Performance specifications for small-signal bipolar transistors (BJT) include collector-to-emitter breakdown voltage, collector-to-base breakdown voltage, maximum collector current, current gain bandwidth, and temperature range. Static forward current transfer ratio, which is also known as common-emitter current gain, is the ratio of the input DC current and the output DC current. Power dissipation, the total power consumption of the device, is usually measured in watts (W) or milliwatts (mW). Temperature range for small-signal bipolar transistors is measured in degrees Fahrenheit or degrees Celsius.

Basic IC package types for small-signal bipolar transistors are transistor outline (TO), small outline (SO), and small outline transistor (SOT). For each package type, many variants are available. Transistor outline packages include TO-92, a single in-line package often used for low power devices; TO-220, which is suitable for high power, medium current, and fast-switching power devices; and TO-263, the surface-mount version of the TO-220 package. Small outline transistor packages include SOT23, which is often used in home appliances, office and industrial equipment, personal computers, printers, and communication equipment; SOT89, a plastic, surface mounted package with three leads and a collector pad for good heat transfer; and SOT223, an encapsulated package that provides excellent performance in environments with high temperatures and humidity levels. IC package types for small-signal bipolar transistors include discrete or deca-watt package (DPAK) and flat package (FPAK).

Packing methods for small-signal bipolar transistors (BJT) consist of tape reel, rail, bulk pack, and tube technologies. The tape reel method packs components in a tape system by reeling specified lengths or quantities for shipping, handling, and configuration in industry-standard automated board-assembly equipment. Rail, another standard packing method, is typically used only in production environments. Bulk pack devices are distributed as individual parts, while tray components are shipped in trays. The tube or stick magazine method is used to feed small-signal bipolar transistors into automatic placement machines for through-hole or surface mounting.