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. Paralleling linearly controlled MOSFETs for current sharing means having a feedback loop for each device.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. Here is a recent example of what happens when the MOSFET is not controlled by the feedback loop. IRe1 = \$\frac\$ needs to be actively controlled by a feedback loop. With a first order model of Vbe's change with temperature, a simple equation for current in Re1 is: As the parts heat up, Vbe will reduce allowing more base drive to the transistor from the fixed value of Vc. The problem is that Vbe has a temperature coefficient (\$\gamma\$) of about -1.6mV/C. Re1 and Re2 will help balance current between BJTs. Here is a starting example circuit to show emitter resistor placement. As Olin Lathrop says, the circuit will need to have resistors in series with the BJT emitters to help balance current. For an application where you need to parallel transistors and control current in a linear fashion (not switching the transistors fully on and off), BJTs are your best bet.
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