Introducing Transistors •
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Transistors are process devices.
This is the symbol for an NPN transistor.
Transistor Terminals
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Transistors have three terminals: Collector
Base
Emitter
Transistor as a Switch •
Transistors can be used as switches.
Transistor •
Transistors can either conduct •
Switch not conduct current.
That is, transistors can either be on
off.
How Transistors Work Collector •
Base
Switching is controlled by the voltage between the Base and the Emitter.
Emitter •
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When VBE < 0.7V the transistor switches off and no current flows between the Collector and the Emitter. When VBE ≥ 0.7V the transistor switches on and current flows between the Collector and the Emitter.
The transistor can be made to operate as a switch by designing the associated circuit so that the transistor is either in the cut-off region or in the saturation region.
The transistor can be operated in three modes:
A. Cutoff Region B. Active Region C. Saturation Region
When IB =0 µA, the transistor is in the cut-off region of its operation. This is drawn with the base lead open, resulting in a base current of zero. Under this condition, there is a very small amount of collector leakage current, ICEO, due mainly to thermally produced carriers.
Because ICEO is extremely small, it will usually be neglected in circuit analysis so that VCE = VCC
In cut-off, both the base-emitter and the base-collector junctions are reverse-biased.
When the base-emitter junction becomes forward-biased and the base current is increased, the collector current also increases and VCE decreases as a result of more drop across the collector resistor.
When VCE reaches its saturation value, VCE(SAT), the base-collector junction becomes forward biased and IC can increase no further even with a continued increase in IB .
At the point of saturation, the relation IC=βIB is no longer valid. VCE(SAT) for a transistor occurs somewhere below the knee of the collector curves, and it is usually only a few tenths of a volt for silicon transistors.
When the transistor is in the cutoff region, the base-emitter junction is reversed-biased.
Neglecting leakage current, all of the currents are zero and V CE is equal to VCC.
When the transistor is in the saturation region, the base-emitter junction is forward-biased. There is enough base current to produce a maximum collector current. The formula for collector saturation current is
Since VCE(SAT) is very small compared to VCC, it can usually be neglected.
The minimum value of base current needed to produce saturation is
Consider the given circuit;
As the input starts positive, current begins to flow in the base and the transistor passes into the active region.
This causes current to flow in the collector circuit and this current flows through resistor RC and so the output begins to fall.
Output
Input
The base-emitter junction is well forward-biased to the current through the resistor RB and the transistor will be saturated. When the voltage at the input goes back to 0 Volt, the process is reversed. The transistor passed from saturation through the active region and is again cutoff.
Output
Input
Therefore,
Turn-On High Collector Current Low Collector Voltage
Turn-Off High Collector Voltage Low Collector Current
Transistor Switching Example X 12V
Variable Voltage Supply •
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When VBE is less than 0.7V the transistor is off and the lamp does not light. When VBE is greater than 0.7V the transistor is on and the lamp lights.
A. Given the circuit, what is VCE when VIN = 0 V? B. What minimum value of IB is required to saturate this transistor if β = 200? Neglect VCE(SAT). C. Calculate the maximum value of RB when VIN = 5 V.
Determine whether or not the transistor is in saturation. Assume β = 25 and VCE(SAT) = 0.2 V.
A. The LED requires 30 mA to emit a sufficient level of light. Therefore, the collector current should be approximately 30 mA. For the given circuit values, determine the amplitude of the square wave input voltage necessary to make sure that the transistor saturates. Use double the minimum value of base current as a safety margin to ensure saturation. Assume that β = 50 and VCE(SAT) = 0.3 V
B. If you change the LED to one that requires 50 mA for a specified light emission and you can not increase the input amplitude above 5 Volts or VCC above 9 Volts, how would you modify the circuit? Specify the component(s) to be changed and the value(s).
