One of the most important parameters of the transistor is its forward current transfer ratio, or h fe - this is the current-gain or output/input current ratio of the device (typically 100 to 300 in the two devices listed). I C(max) is the maximum mean current that can be allowed to flow through the collector terminal of the device, and P T(max) is the maximum mean power that the device can dissipate, without the use of an external heatsink, at normal room temperature. Note, when reading the Figure 4 list, that V CEO(max) is the maximum voltage that may be applied between the collector and emitter when the base is open-circuit, and V CBO(max) is the maximum voltage that may be applied between the collector and base when the emitter is open-circuit. General characteristics and outlines of the 2N3904 and 2N3906 low-power silicon transistors. Figure 4 lists the basic characteristics of two typical general-purpose, low-power types - the 2N3904 (npn) and the 2N3906 (pnp) - which are each housed in a TO-92 plastic case and have the under-side pin connections shown in the diagram.įIGURE 4. Today, a very wide variety of excellent silicon bipolar transistor types are readily available. Such devices are robust, have good power-handling capacities, are not excessively temperature sensitive, and generate negligible electronic noise. Virtually all modern bipolar transistors are made from silicon semiconductor materials. Such devices had many practical disadvantages: they were fragile, excessively temperature-sensitive, electronically noisy, and had very poor power-handling capacities. In the early years of bipolar transistor usage, most transistors were made from germanium semiconductor materials. A pnp transistor needs a negative supply - its main terminal current flows from emitter to collector, and is controlled by an emitter-to-base input current that flows to a negative bias voltage. Polarity connections to (a) npn and (b) pnp transistors.Īn npn device needs a supply that makes the collector positive to the emitter - its output or main-terminal signal current (I c) flows from collector to emitter, and its amplitude is controlled by an input “control” current (I b) that flows from base to emitter via an external current-limiting resistor (R b) and a positive bias voltage. In use, npn and pnp transistors each need a power supply of the appropriate polarity, as shown in Figure 3.įIGURE 3. Basic construction (a) and symbol (b) of pnp transistor. If it uses a p-n-p structure, as in Figure 2(a), it is known as a pnp transistor and uses the symbol in Figure 2(b).įIGURE 2. Basic construction (a) and symbol (b) of npn transistor. If it uses an n-p-n construction sandwich, as in Figure 1(a), it is known as an npn transistor and uses the standard symbol in Figure 1(b).įIGURE 1. The term “bipolar” means that the device is made from semiconductor materials in which conduction relies on both positive and negative (majority and minority) charge carriers.Ī normal transistor is made from a three-layer sandwich of n-type and p-type semiconductor material, with the base or “control” terminal connected to the central layer, and the collector and emitter terminals connected to the outer layers. BIPOLAR TRANSISTOR BASICSĪ bipolar transistor (first invented in 1948) is a three-terminal (base, emitter, and collector), current-amplifying device in which a small input current can control the magnitude of a much larger output current. The remaining seven parts of the series will present a wide range of practical bipolar transistor application circuits. This opening episode concentrates on basic transistor theory, characteristics, and circuit configurations. In its discrete form, it can function as either a digital switch or as a linear amplifier, and is available in many low, medium, and high power forms. The bipolar transistor is the most important “active” circuit element used in modern electronics, and it forms the basis of most linear and digital ICs and op-amps, etc.
0 Comments
Leave a Reply. |
Details
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |