Transistor
A bipolar transistor amplifies fluctuations in current or can be used to switch current on and off. In its amplifying mode, it replaced the vacuum tubes that were formerly used in the amplification of audio signals and many other applications. In its switching mode it resembles a relay, although in its “off” state the transistor still allows a very small amount of current flow, known as leakage.
A bipolar transistor is described as a discrete semiconductor device when it is individually packaged, with three leads or contacts. A package containing multiple transistors is an integrated circuit. A Darlington pair actually contains two transistors, but is included here as a discrete component because it is packaged similarly and functions like a single transistor.
Although the earliest transistors were fabricated from germanium, silicon has become the most commonly used material. Silicon behaves like an insulator, in its pure state at room temperature, but can be “doped” (carefully contaminated) with impurities that introduce a surplus of electrons unbonded from individual atoms. The result is an N-type semiconductor that can be induced to allow the movement of electrons through it, if it is biased with an external voltage. Forward bias means the application of a positive voltage, while reverse bias means reversing that voltage.
Other dopants can create a deficit of electrons, which can be thought of as a surplus of “holes” that can be filled by electrons. The result is a Ptype semiconductor.
A bipolar NPN transistor consists of a thin central P-type layer sandwiched between two thicker Ntype layers. The three layers are referred to as collector, base, and emitter, with a wire or contact attached to each of them. When a negative charge is applied to the emitter, electrons are forced by mutual repulsion toward the central base layer. If a forward bias (positive potential) is applied to the base, electrons will tend to be attracted out through the base. However, because the base layer is so thin, the electrons are now close to the collector. If the base voltage increases, the additional energy encourages the electrons to jump into the collector, from which they will make their way to the positive current source, which can be thought of as having an even greater deficit of electrons.
Thus, the emitter of an NPN bipolar transistor emits electrons into the transistor, while the collector collects them from the base and moves them out of the transistor. It is important to remember that since electrons carry a negative charge, the flow of electrons moves from negative to positive. The concept of positive-tonegative current is a fiction that exists only for historical reasons. Nevertheless, the arrow in a transistor schematic symbol points in the direction of conventional (positive-to-negative) current.
In a PNP transistor, a thin N-type layer is sandwiched between two thicker P-type layers, the base is negatively biased relative to the emitter, and the function of an NPN transistor is reversed, as the terms “emitter” and “collector” now refer to the movement of electron-holes rather than electrons. The collector is negative relative to the base, and the resulting positive-to-negative current flow moves from emitter to base to collector. The arrow in the schematic symbol for a PNP transistor still indicates the direction of positive current flow.
Symbols for NPN and PNP transistors are shown in Figure 28-1. The most common symbol for an NPN transistor is shown at top-left, with letters C, B, and E identifying collector, base, and emitter. In some schematics the circle in the symbols is omitted, as at top-right.
A PNP transistor is shown in the center. This is the most common orientation of the symbol, since its collector must be at a lower potential than its emitter, and ground (negative) is usually at the bottom of a schematic. At bottom, the PNP symbol is inverted, allowing the positions of emitter and collector to remain the same as in the symbol for the NPN transistor at the top. Other orientations of transistor symbols are often found, merely to facilitate simpler schematics with fewer conductor crossovers. The direction of the arrow in the symbol (pointing out or pointing in) always differentiates NPN from PNP transistors, respectively, and indicates current flowing from positive to negative.
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| Figure 28-1. Symbols for an NPN transistor |
To remember the functions of the collector and the emitter in an NPN transistor, you may prefer to think in terms of the collector collecting positive current into the transistor, and the emitter emitting positive current out of the transistor. To remember that the emitter is always the terminal with an arrow attached to it (both in NPN and PNP schematic symbols), consider that “emitter” and “arrow” both begin with a vowel, while “base” and “collector” begin with consonants. To remember that an NPN transistor symbol has its arrow pointing outward, you can use the mnemonic “N/ever P/ointing i/N.”
Current flow for NPN and PNP transistors is illustrated in Figure 28-2. At top-left, an NPN transistor passes no current (other than a small amount of leakage) from its collector to its emitter so long as its base is held at, or near, the potential of its emitter, which in this case is tied to negative or ground. At bottom-left, the purple positive symbol indicates that the base is now being held at a relatively positive voltage, at least 0.6 volts higher than the emitter (for a silicon-based transistor). This enables electrons to move from the emitter to the collector, in the direction of the blue arrows, while the red arrows indicate the conventional concept of current flowing from positive to negative. The smaller arrows indicate a smaller flow of current. A resistor is included to protect the transistor from excessive current, and can be thought of as the load in these circuits.
(other than a small amount of leakage) from its emitter to its collector so long as its base is held at, or near, the potential of the emitter, which in this case is tied to the positive power supply. At bottom-right, the purple negative symbol indicates that the base is now being held at a relatively negative voltage, at least 0.6 volts lower than the emitter. This enables electrons and current to flow as shown. Note that current flows into the base in the NPN transistor, but out from the base in the PNP transistor, to enable conductivity. In both diagrams, the resistor that would normally be included to protect the base has been omitted for the sake of simplicity.
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| Figure 28-2. Current flow through NPN and PNP transistors. |
A voltage divider is often used to control the base potential and ensure that it remains less than the potential on the collector and greater than the potential at the emitter (in an NPN transistor). See Figure 28-3.
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| Figure 28-3. acceptable bias to the base of an NPN transistor |
The amplification of current by a transistor is known as its current gain or beta value, which can be expressed as the ratio of an increase in collector current divided by the increase in base current that enables it. Greek letter β is customarily used to represent this ratio. The formula looks like this:
β = ΔIc / ΔIb
Current gain is also represented by the term hFE, where E is for the common Emitter, F is for Forward current, and lowercase letter h refers to the transistor as a “hybrid” device.
The beta value will always be greater than 1 and is often around 100, although it will vary from one type of transistor to another. It will also be affected by temperature, voltage applied to the transistor, collector current, and manufacturing inaccuracies. When the transistor is used outside of its design parameters, the formula to determine the beta value no longer directly applies.
There are only two connections at which current can enter an NPN transistor and one connection where it can leave. Therefore, if Ie is the current from the emitter, Ic is the current entering the collector, and Ib is the current entering the base:
Ie = Ic + Ib
If the potential applied to the base of an NPN transistor diminishes to the point where it is less than 0.6V above the potential at the emitter, the transistor will not conduct, and is in an “off” state, although a very small amount of leakage from collector to emitter will still occur.
When the current flowing into the base of the transistor rises to the point where the transistor cannot amplify the current any further, it becomes saturated, at which point its internal impedance has fallen to a minimal value. Theoretically this will allow a large flow of current; in practice, the transistor should be protected by
resistors from being damaged by high current resulting from saturation. Any transistor has maximum values for the collector current, base current, and the potential difference between collector and emitter. These
values should be provided in a datasheet. Exceeding them is likely to damage the component.
transistor Connections
Often a transistor package provides no clue as to which lead is the emitter, which lead is the base, and which lead is the collector. Old can-style packaging includes a protruding tab that usually points toward the emitter, but not always. Where power transistors are packaged in a metal enclosure, it is typically connected internally with the collector. In the case of surface-mount transistors, look for a dot or marker that should identify the base of a bipolar transistor or the gate of a field-effect transistor. A through-hole transistor usually has its part number printed or engraved on its package, although a magnifying glass may be necessary to see this. The component’s datasheet may then be checked online. If a datasheet is unavailable, meter-testing will be necessary to confirm the functions of the three transistor leads. Some multimeters include a transistor-test function, which may validate the functionality of a transistor while also displaying its beta value. Otherwise, a meter can be put in diode-testing mode, and an unpowered NPN transistor should behave as if diodes are connected between its leads as shown in Figure 28-5. Where the identities of the transistor’s leads are unknown, this test will be sufficient to identify the base, after which the collector and emitter may be determined empirically by testing the transistor in a simple lowvoltage circuit such as that shown in Figure 28-6.
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| Figure 28-5. NPN transistor diode equivalance |
How to Use transistor
The following abbreviations and acronyms are common in transistor datasheets. Some or all of the letters following the initial letter are usually, but not always, formatted as subscripts:
hFE : Forward current gain
β : Same as hFE
VCEO : Voltage between collector and emitter (no connection at base)
VCBO : Voltage between collector and base (no connection at emitter)
VEBO : Voltage between emitter and base (no connection at collector)
VCEsat : Saturation voltage between collector and emitter
VBEsat : Saturation voltage between base and emitter
Ic : Current measured at collector
ICM : Maximum peak current at collector
IBM : Maximum peak current at base
PTOT : Total maximum power dissipation at room temperature
TJ : Maximum junction temperature to avoid damage
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| Figure 28-6. transistor simple schematic |
are exceeded, damage may occur. A manufacturer’s datasheet may include a graph showing the safe operating area (SOA) for a transistor. This is more common where power transistors are involved, as heat becomes more of an issue. The graph in Figure 28-7 has been adapted from a datasheet for a silicon diffused power transistor manufactured by Philips. The safe operating area is bounded at the top by a horizontal segment representing the maximum safe current, and at the right by a vertical segment representing the maximum safe voltage. However, the rectangular area enclosed by these lines is reduced by two diagonal segments representing the total power limit and the second breakdown limit. The latter refers to the tendency of a transistor to develop internal localized “hot spots” that tend to conduct more current, which makes them hotter, and able to conduct better—ultimately melting the silicon and causing a short circuit. The total power limit and the second breakdown limit reduce the safe operating area, which would otherwise be defined purely by maximum safe current and maximum safe voltage. Uses for discrete transistors began to diminish when integrated circuits became cheaper and started to subsume multi-transistor circuits. For instance, an entire 5-watt audio amplifier, which used to be constructed from multiple components can now be bought on a chip, requiring just a few external capacitors. More powerful audio equipment typically uses integrated circuits to process inputs, but will use individual power transistors to handle high-wattage output.
Darlington Pairs
Discrete transistors are useful in situations where current amplification or switching is required at just one location in a circuit. An example would be where one output pin from a microcontroller must switch a small motor on and off. The motor may run on the same voltage as the microcontroller, but requires considerably more current than the typical 20mA maximum available from a microcontroller output. A Darlington pair of transistors may be used in this application. The overall gain of the pair can be 100,000 or more. See Figure 28-8. If a power source feeding through a potentiometer is substituted for the microcontroller chip, the circuit can function as a motor speed control (assuming that a generic DC motor is being used). In the application shown here, the microcontroller chip must share a common ground (not shown) with the transistors. The optional resistor may be necessary to prevent leakage from the first transistor (when in its “off” state) from triggering the second. The diode protects the transistors from voltage transients that are likely when the motor stops and starts.
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| Figure 28-8. Darlington Pairs circuit |
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| Figure 28-9. Darlington pair embedded in a single transistor |
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| Figure 28-10. Various packaging options for Darlington pairs. |
Seven or eight Darlington pairs can be obtained in a single integrated chip. Each transistor pair in these chips is typically rated at 500mA, but they can be connected in parallel to allow higher currents. The chip usually includes protection diodes to allow it to drive inductive loads directly.
A typical schematic is shown in Figure 28-11. In this figure, the microcontroller connections are hypothetical and do not correspond with any actual chip. The Darlington chip is a ULN2003 or similar, containing seven transistor pairs, each with an “input” pin on the left and an “output” pin opposite it on the right. Any of pins 1 through 7 down the left side of the chip can be used to control a device connected to a pin on the opposite side.
A high input can be thought of as creating a negative output, although in reality the transistors inside the chip are sinking current via an external device—a motor, in this example. The device can have its own positive supply voltage, shown here as 12VDC, but must share a common ground with the microcontroller, or with any other component which is being used on the input side. The lower-right pin of the chip shares the 12VDC supply because this pin is attached internally to clamp diodes (one for each Darlington pair), which protect against surges caused by inductive loads. For this reason, the motor does not have a clamp diode around it in the schematic.
The Darlington chip does not have a separate pin for connection with positive supply voltage, because the transistors inside it are sinking power from the devices attached to it.
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| Figure 28-11. A chip such as the ULN2003 |
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| Figure 28-12. A surface-mount package for a Darlington pair |
transistor as Amplifiers
Two basic types of transistor amplifiers are shown in Figure 28-13 and Figure 28-14. The common collector configuration has current gain but no voltage gain. The capacitor on the input side blocks DC current from entering the amplifier circuit, and the two resistors forming a voltage divider on the base of the transistor establish a voltage midpoint (known as the quiescent point or operating point) from which the signal to be amplified may deviate above and below.
The common-emitter amplifier provides voltage gain instead of current gain, but inverts the phase of the input signal. Additional discussion of amplifier design is outside the scope of this article. In switching applications, modern transistors have been developed to handle a lot of current compared with earlier versions, but still have some limitations. Few power transistors can handle more than 50A flowing from collector to emitter, and 1,000V is typically a maximum value. Electromechanical relays continue to exist because they retain some advantages, as shown in the table in Figure 28-15, which compares switching capabilities of transistors, solid-state relays, and electromechanical relays.
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| Figure 28-13. The basic schematic for a commoncollector amplifier. |
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| Figure 28-14. The basic schematic for a common-emitter amplifier. |
