Transistor as a switch
The transistors Base biasing voltage is applied when used as an AC signal amplifier in such a way that it always operates within its “active” region, which is the linear part of the curves of the output characteristics. However all bipolar transistors like NPN & PNP can be made to act as a solid state switch type “ON / OFF” by biasing the base terminal transistors differently from a signal amplifier.
One of the major uses for using a transistor to turn a DC output “ON” or “OFF” as solid state switches. Many output devices, such as LEDs, need just a few milliamps at logic level DC voltages and can be powered directly by a logic gate output. Nevertheless, high-power devices, such as motors, solenoids or lamps, frequently need more power than a normal logic gate provides for this we use transistor switches.
If the circuit uses the Bipolar Transistor as a switch, then the transistor’s biasing, whether NPN or PNP, is designed to operate the transistor on both sides of the previously seen “I-V” characteristic curves.
The transistor switch operating areas are referred to as the Saturation Region and the Cut-off Area. This means we can then disregard the operating Q-point biasing and voltage divider circuits needed for amplification and use the transistor as a switch by driving it back and forth between its “full-OFF” (cut-off) and “full-ON” (saturation) regions as shown below.
The pink shaded area at the bottom of the curves is the “cut-off” region, while the blue area at the left is the transistor’s “Saturation” region.
Here the transistor’s operating conditions are zero input base current (IB), zero output collector current (IC) and maximum collector voltage (VCE) resulting in a broad depletion layer and no current flowing through the device.
Then, when using a bipolar transistor as a switch, we can define the “cut-off region” or “OFF mode,” both junctions are reverse biased, VB < 0.7v and IC = 0. The output of the Emitter must be negative with respect to the Base for a PNP transistor.
The transistor will be biased here so that the maximum amount of base current is applied, resulting in maximum collector current resulting in the minimum collector emitter voltage fall resulting in the depletion layer being as small as possible and maximum current flowing through the transistor as possible. The transistor is therefore turned “Fully-ON.”
Instead, when using a bipolar transistor as a switch, we may describe the “saturation area” or “ON mode,” all junctions forward biased, VB > 0.7v and IC= Maximum. The Emitter potential must be positive with respect to the Base for a PNP transistor.
The transistor then acts as a solid state switch “single-pole single-throw” (SPST). It turns “OFF” acting like an open switch and zero collector current flows with a zero signal applied to the transistor base. With a positive signal applied to the transistor base, it switches “ON” acting as a closed switch and flows through the device as full circuit current.
The easiest way to switch moderate to high power is to use the transistor with an open-collector output and the Emitter terminal transistors that are directly connected to the ground The open collector output transistors can thus “sink” an externally supplied voltage to ground by regulating any attached load when used in this way.
Below is an example of an NPN transistor used as a switch to control a relay. A flywheel diode is placed across the load with inductive loads such as relays or solenoids to dissipate the inductive load-generated EMF back when the transistor switches “OFF” and thus protects the transistor from harm. If the load is of a very high current or voltage type, such as motors, heaters, etc., then a suitable relay can be used to control the load current.
Basic NPN Transistor Switching Circuit
The circuit is similar to the one we looked at in the previous tutorials for the Common Emitter circuit. The difference this time is that to use the transistor as a switch the transistor needs to be switched either fully “OFF” (cut-off) or fully “ON” (saturated). When turned “fully-off,” an ideal transistor switch would have infinite circuit resistance between the Collector and Emitter resulting in zero current flowing through it and zero resistance between the Collector and Emitter when turned “full-on,” resulting in maximum current flow.
Low leakage currents pass through the transistor when the transistor is switched “OFF” and when the system has a low resistance value which allows a low saturation voltage (VCE) to flow through it. Although the transistor is not a perfect switch, the energy dissipated by the transistor is at its lowest in both the cut-off and saturation regions.
The base input terminal must be made more positive than the Emitter by increasing it above the 0.7 volts needed for a silicon device in order for the base current to flow Through changing this VBE base-emitter voltage, the base current is also changed which in turn regulates the amount of Collector current flowing through the transistor as discussed previously.
The transistor is said to be saturated when the full collector current flows. The base resistor value specifies how much input voltage is needed and the corresponding base current to completely ON the transistor.
Digital Logic Transistor Switch-
To limit the output current from the logic gate, the base resistor, Rb is required.
PNP Transistor Switch
The PNP transistors can also be used as a switch, this time the difference is that the load is connected to the ground (0v) and the PNP transistor switches the power to it. The base terminal is connected to ground or zero volts (LOW) as shown to turn the PNP transistor operating as an “ON” switch.
PNP Transistor Switching Circuit
The Base Resistance, Collector Current and Voltage measurement equations are exactly the same as for the previous NPN transistor switch. The difference this time is that we use a PNP transistor (sourcing current) to switch power instead of switching ground with an NPN transistor (sink current).
Darlington Transistor Switch
Sometimes the bipolar transistor’s DC current gain is too low for the load current or voltage to be transferred directly, so multiple transistors are used. A small input transistor is used here to turn a much larger current handling output transistor to “ON” or “OFF.” The two transistors are coupled in a “Complementary Gain Compounding Configuration” to optimize the signal gain, or what is more commonly called a “Darlington Setup” where the amplification factor is the product of the two individual transistors.
Darlington Transistors essentially include two independent bipolar NPN or PNP type transistors linked together so that the first transistor’s current gain is multiplied by that of the second transistor’s current gain to produce a system which functions as a single transistor with a very high current gain for a much smaller base current. A Darlington device’s overall current gain Beta (β) or hfe value is the sum of the transistor’s two individual gains and is given as:
And Darlington Transistors are feasible compared to a single transistor switch with very high β values and high collector currents. For example, if the first transistor input has a current gain of 100 and the second transistor input has a current gain of 50 then the total current gain is 100* 50= 5000. For example, if our load current from above is 200mA, then only 200mA/5000= 40uA is the darlington base current. A huge reduction in a single transistor from the previous 1mA.
Below is an example of Darlington’s two basic types of transistor configurations.
Darlington Transistor Configurations
The above NPN Darlington transistor switch configuration shows the Collectors of the two transistors connected together with the Emitter of the first transistor connected to the Base terminal of the second transistor therefore, the Emitter current of the first transistor becomes the Base current of the second transistor switching it “ON”.
The Base receives the output signal from the first or input transistor. In the usual way, this transistor amplifies it and uses it to drive the second larger transistor “output.” The second transistor again amplifies the signal, leading to a very high current gain. One of Darlington Transistors ‘ main characteristics is its high current gains compared to single bipolar transistors.
As well as its high power and voltage switching capabilities, a “Darlington Transistor Switch” has another benefit in its high switching speeds, making it ideal for use in inverter circuits, lighting circuits and DC motor or stepper motor control applications.
One difference to consider when using Darlington transistors over conventional single bipolar types when using the transistor as a switch is that, due to the series connection between the two PN junctions, the Base-Emitter input voltage (VBE) must be higher for silicon devices at approx. 1.4v.
when using a Transistor as a Switch
To switch and power lights, relays or even motors, transistor switches can be used.
If the bipolar transistor is used as a switch it must either be “full-off” or “full-on.”
Darlington Transistors can be used when regulating large currents or voltages.
It is said that transistors that are full “ON” are in their region of saturation.
In their cut-off region, transistors that are fully “OFF”. A “Flywheel Diode” is used when using transistors to switch inductive loads including relays and solenoids.
A low base current regulates a much larger Collector load current when using the transistor as a switch.
Also Read – Difference between BJT and FET