As one of the significant semiconductor devices, transistor has found use in enormous electronic applications such as embedded systems, digital circuits and control systems. In both digital and analog domains transistors are extensively used for different application usage like amplification, logic operations, switching and so on. This article mainly concentrates and gives a brief explanation of transistor application as a switch.
The Bipolar Junction Transistor or simply BJT is a three layer, three terminal and two junction semiconductor device. Almost in many of the applications these transistors are used for two basic functions such as switching and amplification.
The name bipolar indicates that two types of charge carriers are involved in the working of a BJT. These two charge carriers are holes and electrons where holes are positive charge carriers and electrons are negative charge carriers.
The transistor has three regions, namely base, emitter and collector. The emitter is a heavily doped terminal and emits electrons into the base. Base terminal is lightly doped and passes the emitter-injected electrons on to the collector. The collector terminal is intermediately doped and collects electrons from base. This collector is large as compared with other two regions so it dissipates more heat.
BJTs are of two types NPN and PNP, both functioning is same but differ in terms of biasing and power supply polarity. In PNP transistor, between two P- type materials N- type material is sandwiched whereas in case of NPN transistor P- type material sandwiched between two N- type materials. These two transistors can be configured into different types like common emitter, common collector and common base configurations.
Depends on the biasing conditions like forward or reverse, transistors have three major modes of operation namely cutoff, active and saturation regions.
In this mode transistor is generally used as a current amplifier. In active mode, two junctions are differently biased that means emitter-base junction is forward biased whereas collector-base junction is reverse biased. In this mode current flows between emitter and collector and amount of current flow is proportional to the base current.
In this mode, both collector base junction and emitter base junction are reverse biased. This in turn not allows the current to flow from collector to emitter when the base-emitter voltage is low. In this mode device is completely switched off as the result the current flowing through the device is zero.
In this mode of operation, both the emitter base and collector base junctions are forward biased. Current flows freely from collector to emitter when the base-emitter voltage is high. In this mode device is fully switched ON.
The below figure shows the output characteristics of a BJT Transistor. In the below figure cutoff region has the operating conditions as zero collector output current, zero base input current and maximum collector voltage. These parameters causes a large depletion layer which further doesn’t allow current to flow through the transistor. Therefore, the transistor is completely in OFF condition.
Similarly, in the saturation region, a transistor is biased in such a way that maximum base current is applied that results maximum collector current and minimum collector-emitter voltage. This causes the depletion layer to become small and to allow maximum current flow through the transistor. Therefore, the transistor is fully in ON condition.
Hence, from the above discussion, we can say that transistors can be made to work as ON/OFF solid state switch by operating transistor in cutoff and saturation regions. This type of switching application is used for controlling motors, lamp loads, solenoids, etc.
A transistor is used for switching operation for opening or closing of a circuit. This type solid state switching offers significant reliability and lower cost as compared with conventional relays. Both NPN and PNP transistors can be used as switches. Some of the applications use a power transistor as switching device, at that time it may necessary to use another signal level transistor to drive the high power transistor.
NPN Transistor as a Switch
Based on the voltage applied at the base terminal of a transistor switching operation is performed. When a sufficient voltage (Vin > 0.7 V) is applied between the base and emitter, collector to emitter voltage is approximately equal to 0. Therefore, the transistor acts as a short circuit. The collector current Vcc/Rc flows through the transistor.
Similarly, when no voltage or zero voltage is applied at the input, transistor operates in cutoff region and acts as an open circuit. In this type of switching connection, load (here LED lamp) is connected to the switching output with a reference point. Thus, when the transistor is switched ON, current will flow from source to ground through the load.
Consider the below example where base resistance Rb = 50 k ohm, collector resistance Rc = 0.7k ohm, Vcc is 5V and the beta value is 125. At the base input a signal varying between 0 and 5V is given so we are going to see the output at the collector by varying the Vi at two states that is 0 and 5V as shown in figure.
Ic = Vcc/Rc when VCE = 0
Ic = 5V/0.7k ohm
Ic = 7.1 mA
Base Current Ib = Ic / β
Ib = 7.1 mA/125
Ib = 56.8 µA
From the above calculations, the maximum or peak value of the collector current in the circuit is 7.1mA when Vce is equal to zero. And the correspond base current to which collector current flows is 56.8µA. So, it is clear that when the base current is increased beyond the 56.8 micro ampere then the transistor comes into the saturation mode.
Consider the case when zero volt is applied at the input. This causes the base current zero and as the emitter is grounded, emitter base junction is not forward biased. Therefore, the transistor is in OFF condition and the collector output voltage is equal to 5V.
When Vi = 0V, Ib = 0 and Ic =0,
Vc = Vcc – (IcRc)
= 5V – 0
Consider that input voltage applied is 5 volts, then the base current can be determined by applying Kirchhoff’s voltage law.
When Vi = 5V
Ib = (Vi – Vbe) / Rb
For silicon transistor Vbe = 0.7 V
Thus, Ib = (5V – 0.7V)/ 50K ohm
= 86 µA which is greater than 56.8 µA
Therefore the base current is greater than 56.8 micro ampere current, the transistor will be driven to saturation that is fully ON when 5V is applied at the input. Thus the output at the collector becomes approximately zero.
PNP transistor works same as NPN for a switching operation, but the current flows from the base. This type of switching is used for negative ground configurations. For the PNP transistor, the base terminal is always negatively biased with respect to the emitter. In this switching, base current flows when the base voltage is more negative. Simply a low voltage or more negative voltage makes transistor to short circuit otherwise it will be open circuited or high impedance state.
In this connection, load is connected to the transistor switching output with a reference point. When the transistor is turned ON, current flows from the source through transistor to the load and finally to the ground.
Example of PNP Transistor as a Switch
Similar to the NPN transistor switch circuit, PNP circuit input is also base, but the emitter is connected to constant voltage and the collector is connected to ground through the load as shown in figure.
In this configuration base is always biased negatively with respect to the emitter by connecting the base at negative side and the emitter at the positive side of the input supply. So the voltage VBE is negative and the emitter supply voltage with respect to the Collector is positive ( VCE positive).
Therefore, for the conduction of transistor emitter must be more positive with respect to both collector and base. In other words base must be more negative with respect to the emitter.
For calculating the base and collector currents following expressions are used.
Ic = Ie – Ib
Ic = β. Ib
Ib = Ic / β
Consider the above example, that the load requires 100 milli ampere current and the transistor has the beta value of 100. Then the current required for the saturation of the transistor is
Minimum base current = collector current / β
= 100 mA / 100
Therefore, when the base current is 1 mA, the transistor will be fully ON. But practically 30 percent of more current is required for guaranteed saturation of transistor. So, in this example the base current required is 1.3mA.
As discussed earlier that the transistor can be used as a switch. The schematic below shows how a transistor is used to switch the Light Emitting Diode (LED).
- When the switch at the base terminal is open, no current flows through the base so the transistor is in the cutoff state. Therefore, the circuit acts as open-circuit and the LED becomes OFF.
- When the switch is closed, base current starts flowing through the transistor and then drives into saturation results to LED become ON.
- Resistors are placed to limit the currents through the base and LED. It is also possible to vary the intensity of LED by varying the resistance in the base current path.
It is also possible to control the relay operation using a transistor. With a small circuit arrangement of a transistor able to energise the coil of the relay so that the external load connected to it is controlled.
- Consider the below circuit to know the operation of a transistor to energize the relay coil. The input applied at the base causes to drive the transistor into saturation region, which further results the circuit becomes short circuit. So the relay coil gets energized and relay contacts get operated.
- In inductive loads, particularly switching of motors and inductors, sudden removal of power can keep a high potential across the coil. This high voltage can cause considerable damage to the rest circuit. Therefore, we have to use the diode in parallel with inductive load to protect the circuit from induced voltages of the inductive load.
- A transistor can also used to drive and regulate the speed of the DC motor in a unidirectional way by switching the transistor in regular intervals of time as shown in the below figure.
- As mentioned in above, the DC motor is also an inductive load so we have to place a freewheeling diode across it to protect the circuit.
- By switching the transistor in cutoff and saturation regions, we can turn ON and OFF the motor repeatedly.
- It is also possible to regulate the speed of the motor from standstill to full speed by switching the transistor at variable frequencies. We can get the switching frequency from control device or IC like microcontroller.
Have you got the clear idea of how a transistor can be used as a switch? We acknowledge that the furnished information clarifies the entire switching concept with relevant images and examples. Further any doubts, suggestions and comments on this post you can write below.