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MEMS1082

Chapter 3-2 Semiconductor devices Transistors and Amplifiers-BJT

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Bipolar Transistor Construction

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npn BJT

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Transistor Structure

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npn BJT

BCE III +=

EBBE VVV −=

ECCE VVV −=

EBC VVV >>

Base-to-emitter pn junction is forward biased

Base-to-collector pn junction is reverse biased

Base is a very thin layer of p type region

Emitter is more heavily n-doped than collector

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Transistor as Current Amplifier The larger collector current IC is proportional to the base current IB according to

the relationship IC =βIB , or more precisely it is proportional to the base-emitter voltage VBE . The smaller base current controls the larger collector current, achieving current amplification.

The analogy to a valve is sometimes helpful. The smaller current in the base acts as a "valve", controlling the larger current from collector to emitter. A "signal" in the form of a variation in the base current is reproduced as a larger variation in the collector-to-emitter current, achieving an amplification of that signal.

β is about 100 Amplification factor

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pnp BJT Transistor

Very thin

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Transistor as Current Amplifier Constraints on Transistor Operation

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BJT packages

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Terminal characteristics of a typical BJT

0.2 V when saturation

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Common emitter transistor circuit Transistor Characteristic Curves Controlled

by collector circuit

ic is proportional to ib

0.2 V when saturation

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Transistors Transistor Operation

Cut off (no collector current), useful for switch operation. In the active region (some collector current, more than a few tenths of a

volt above the emitter), useful for amplifier applications In saturation (collector a few tenths of a volt above emitter), large current

useful for "switch on" applications.

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Transistors Transistor Characteristic Curves

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Transistors Transistor Characteristic Curves

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Bipolar Transistor Switch Vin=0.7V

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Transistor Switch Example The base resistor is chosen small

enough so that the base current drives the transistor into saturation. In this example the mechanical switch is used to produce the base current to close the transistor switch to show the principles. In practice, any voltage on the base sufficient to drive the transistor to saturation will close the switch and light the bulb.

There is no current to the base, so the transistor is in the cut off condition with no collector current. All the voltage drop is across the transistor.

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Transistor Switch Example NPN Common Emitter Switch For switching currents less than an ampere, the

transistor switch can be used. Instead of a mechanical switch in the base circuit, an op-amp could be used. When the switch is open, no current flows in the base so the collector current is cut off. The resistor RB must be small enough to drive the transistor to saturation so that most of the voltage Vcc appears across the load Rc.

In a configuration where the output is taken below the load resistor, this kind of switch can function as an inverting buffer in digital circuits.

A transistor switch with collector resistor can serve as an inverting buffer.

For equal resistors, it has a gain of -1, and is used in digital circuits as an inverting buffer.

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Transistor Switch Example Op-Amp Switching If an op-amp is used instead

of a mechanical switch to operate a transistor switch. The diode is used for protection of the base-emitter junction in case the op-amp swings to its negative supply voltage.

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LED Switch

O V or 5V

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Darlington Pair

The emitter current of TR1 is the base current of TR2. A change in base current of TR1 can give a change 100 times larger in its emitter current. A change in the base current of TR2 has a similar effect on its emitter current. Therefore there is an overall amplification of 100 x 100 = 10000 times. This circuit is sometimes called the Super Alpha Pair. It is often used as a power output stage. The two transistors can come in the same package.

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Phototransistor and Optoisolator

Planar (top) and silicone dome (bottom) layouts - cross-section through a standard dual in-line package

An electronic device designed to transfer electrical signals by utilizing light waves to provide coupling with electrical isolation between its input and output

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Guarantee a transistor in saturation

Q:

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Common emitter npn BJT

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Common emitter npn BJT current amplifier

The Common Emitter Amplifier Circuit This type of configuration is the most commonly used

circuit for transistor based amplifiers and which represents the "normal" method of bipolar transistor connection.

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Common emitter npn BJT current amplifier

The collector current IC is proportional to the base current IB according to the relationship IC =βIB

β has a value between 20 and 200 for most general purpose transistors

BCE III +=

EBC VVV >>

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Common emitter npn BJT current amplifier

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BJT current amplifier

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Common base pnp BJT voltage amplifier

Common base The BASE connection is common to both the input signal AND the output signal with

the input signal being applied between the base and the emitter terminals. The corresponding output signal is taken from between the base and the collector terminals with the base terminal grounded. The input current flowing into the emitter is quite large as its the sum of both the base current and collector current respectively therefore, the collector current output is less than the emitter current input resulting in a current gain for this type of circuit of "1" (unity) or less, in other words the common base configuration "attenuates" the input signal.

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pnp BJT voltage amplifier

This type of amplifier configuration is a non-inverting voltage amplifier circuit, in that the signal voltages Vin and Vout are in-phase. This type of transistor arrangement is not very common due to its unusually high voltage gain characteristics due to its high ratio of output to input resistance or more importantly "load" resistance (RL) to "input" resistance (Rin) giving it a value of "Resistance Gain". Then the voltage gain (Av for a common base configuration

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Emitter Follower Circuit

This type of configuration is commonly known as a Voltage Follower or Emitter Follower circuit.

This type of bipolar transistor configuration is a non-inverting circuit in that the signal voltages of Vin and Vout are in-phase. It has a voltage gain that is always less than "1" (unity).

a large Current gain

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Emitter Follower Switch NPN Emitter Follower Switch The emitter follower can also be

used for switching. The resistor RB must be small enough to drive the transistor to saturation so that most of the voltage Vcc appears across the load.

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Angular Position of a Robotic Scanner Photo-interrupter