How To Use The Bipolar Transistors

Thursday, April 12th, 2018 - Active Components

How To Use The Bipolar Transistors

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

Often these terms are used to define “absolute maximum values” for a component. If these maximums
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 the following figure 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.

Safe operating area (SOA) for the componentSafe operating area (SOA) for the component

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 Of Bipolar Transistors

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 the following figure. 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).

Darlington Pairs Of Bipolar TransistorsDarlington pair

Where the emitter of one NPN transistor is coupled to the base of another, they form a Darlington pair (identified by the dashed rectangle in this schematic). Multiplying the gain of the first transistor by the gain of the second gives the total gain of the pair

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.

A Darlington pair can be obtained in a single transistor-like package, and may be represented by the schematic symbol shown in the following figure.

Darlington schematic symbolDarlington schematic symbol

When a Darlington pair is embedded in a single transistor-like package, it may be represented by this schematic symbol. The leads attached to the package can be used as if they are the emitter, base, and collector of a single NPN transistor.

Various through-hole Darlington packages are shown in the following figure.

Various packaging options for DarlingtonVarious packaging options for Darlington pairs.

From left to right: The 2N6426 contains a Darlington pair rated to pass up to 500mA continuous collector current. The 2N6043 is rated for 8A continuous. The ULN2003 and ULN2083 chips contain seven and eight Darlington pairs, respectively.

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 the following figure. 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 chip such as the ULN2003 contains seven Darlington pairsA chip such as the ULN2003 contains seven Darlington pairs.

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.

A surface-mount Darlington pair is shown in the following figure. This measures just slightly more than 0.1” long but is still rated for up to 500mA collector current or 250mW total power dissipation (at a component temperature no higher than 25 degrees Centigrade).

surface-mount package for a Darlington pairA surface-mount package for a Darlington pair. Each square in the background grid measures 0.1”.

Amplifiers With Bipolar Transistors

Two basic types of transistor amplifiers are shown in the following figure. 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 basic schematic for a commoncollector amplifierThe basic schematic for a commoncollector amplifier

The basic schematic for a common-emitter amplifierThe basic schematic for a common-emitter amplifier

The common-emitter amplifier provides voltage gain instead of current gain, but inverts the phase of the input signal.

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 the following figure, which compares switching capabilities of transistors, solid-state relays, and electromechanical relays.

A comparison of characteristics of switching devicesA comparison of characteristics of switching devices

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