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B J T Walter Brattain William Shockley John Bardeen

The bipolar transistor

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Page 1: The bipolar transistor


Walter Brattain

William Shockley

John Bardeen

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History• Bell Labs (1948)

• It is to replace Vacuum Tubes

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TJB ipolar


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The Bipolar Junction Transistor


A BIPOLAR transistor has two P-N junctions.

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Review : Diode

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Sample Applications

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If CPU will have to control the motor:

The CPU can’t be connected to the motor directly because it can’ handle the power rating of the

motor (normally high)

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• For this condition, we can use BJT to drive the motor on. This BJT application is called switching (on and off).

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If a microphone is to be connected to the speaker:

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• For this condition, we can use BJT to amplify our voice. As we speak into microphone(input), we can hear our voice in a louder sound in the speaker (output). This is called amplification.

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Trivia about BJT

• The invention of the BJT in 1948 at the Bell Telephone Laboratories ushered in the era of solid-state circuits, which led to electronics changing the way we work, play, and indeed, live.

• The invention of the BJT also eventually led to the dominance of information technology and the emergence of the knowledge-based economy.

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• Solid-state electronics are those circuits or devices built entirely from solid materials and in which the electrons, or other charge carriers, are confined entirely within the solid material.

• Common solid-state devices include transistors, microprocessor chips, and DRAM.

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• A bipolar junction transistor is a three-terminal device that acts like a current-controlled switch (because we use current as input, and amplified current as output).

• It has three terminals, the base, the collector and the emitter.

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• If we put a small current into one of the terminals, called the base, then the switch is “on”—current may flow between the other two terminals, called the emitter and the collector. If no current is put into the base, then the switch is “off”—no current flows between the emitter and the collector

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The circuit symbol for the npn transistor is shown in Figure BJT-1(d). Notice that the symbol contains a subtle arrow in the direction of positive current flow. This also reminds us that the base-to-emitter junction is a pn junction, the same as a diode whose symbol has an arrow pointing in the same direction.

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NPN versus PNP

Generally, PNP and NPN transistors can perform the same functions. The differences are the polarities of the voltages and the directions of the resulting currents. In most applications, an PN device can be replaced with a PNP device or vice versa, the power-supply polarity can be reversed, and the circuit will work in the same way—as long as the new device has the appropriate specifications.

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Die of a KSY34 high-frequency NPN transistor, base and emitter connected via bonded wires

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Doping level and Channel Width

• Base – most lightly doped and its length is 1/150 of the length of the whole transistor. Usually called or labelled as the “control” because the flow of current through the transistor depends critically on what happens at this electrode.

• Emitter – most highly doped because it has the responsibility of emitting the carriers that will circulate during operation.

• Collector – has the widest channel because it has the responsibility of collecting the carriers that the emitter is emitting.

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Biasing (provided with a certain voltage, or made to carry a certain current)

• We usually consider NPN as an example because it is more commonly used.

• The normal method of biasing an NPN transistor is to have the collector voltage positive with respect to the emitter.

• Typical dc voltages for a transistor power supply range between 3 V and about 50 V. A typical voltage is 12 V.

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(1.) Zero Bias

• Condition where the base of a transistor is at the same voltage as the emitter. (no potential difference).

• This prevents current from flowing between the emitter and collector, unless a signal is injected at the base to change the situation.

• Such a signal must, at least momentarily, attain a positive voltage equal to or greater than the forward breakover voltage of the E-B junction.

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(2.)Reverse Bias

• Condition where a battery is connected between the base and the emitter in the circuit with the polarity such that VB becomes negative with respect to the emitter.

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(2.)Reverse Bias

• No current flows through the E-B junction in this situation (as long as the new battery voltage is not so great that avalanche breakdown occurs). A signal might be injected at the base to cause a flow of current, but such a signal must attain, at least momentarily, a positive voltage high enough to overcome both the reverse bias and the forward breakover voltage of the junction.

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(3.) Forward Bias

• A condition where VB is made positive with respect to the emitter, starting at small voltages and gradually increasing.

• If the forward bias is less than the forward breakover voltage, no current will flow.

• But as the base voltage VB reaches the breakover point, the E-B junction will start to conduct.

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(3.) Forward Bias

• The base-collector (B-C) junction of a bipolar transistor is normally reverse-biased. It will remain reverse-biased as long as VB is less than the supply voltage (in this case 12 V). In practical transistor circuits, it is common for VB to be set at a fraction of the supply voltage. Despite the reverse bias of the B-C junction, a significant emitter-collector current, called collector current and denoted Ic, will flow once the E-B junction conducts.

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(3.) Forward Bias

• In a real transistor circuit, the meter reading will jump when the forward breakover voltage of the E-B junction is reached. Then even a small rise in VB, attended by a rise in IB, will cause a large increase in Ic.

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(3.) Forward Bias

• If VB continues to rise, a point will eventually be reached where the Ic versus VB curve levels off. The transistor is then said to be saturated or in saturation. It is wide open, conducting as much as it can.

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BJT’s Modes of Operation

• The two junctions of BJT can be either forward or reverse-biased.

• The BJT can operate in different modes depending on the junction bias.

• The BJT operates in active mode for amplifier circuits.

• Switching applications utilize both the cutoff and saturation modes.

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BJT’s Modes of Operation

• 1. Active Mode• 2. Cut-off Mode• 3. Saturation Mode

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BJT’s Modes of OperationMode Emitter-Base



Active Forward Reverse

Cut-off Reverse Reverse

Saturation Forward Forward

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BJT’s Modes of Operation

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Transistor Circuit Configurations

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Common Base Configuration

• 1. Input signal to the emitter and output is taken from the collector.

• 2. The input circuit is very low impedance, usually 1 to 50 ohms.

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Common Emitter Configuration

• Input – base• Output – collector• Input circuit is low

impedance. (25 to 5k ohms)

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Common Collector Configuration

• Input- base• Output – emitter• Input circuit is very high

impedance and output impedance is LOW.

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Comparison : SummaryCB CE CC