These two transistors can be configured into different types like common emitter, common collector and common base configurations. Depending on the biasing conditions like forward or reverse, transistors have three major modes of operation namely cutoff, active and saturation regions.
In this mode, the 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 the amount of current flow is proportional to the base current.
In this mode, both collector base junction and emitter base junction are reverse biased. As both the PN Junctions are reverse biased, there is almost no current flow except small leakage currents usually in the order of few nano amps or pico amps. In this mode of operation, both the emitter-base and collector-base junctions are forward biased. Current flows freely from collector to emitter with almost zero resistance. In this mode, the transistor is fully switched ON and is essentially a close circuit.
The below figure shows the output characteristics of a BJT. In the below figure, the cutoff region has the operating conditions when the output collector current is zero, zero base input current and maximum collector voltage.
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 in 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. This type of switching application is used for controlling LEDs, motors, lamps, solenoids, etc. A transistor can be used for switching operation for opening or closing of a circuit. This type solid state switching offers significant reliability and lower cost when compared to conventional relays.
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.
Based on the voltage applied at the base terminal of a transistor switching operation is performed. Therefore, the transistor acts as a short circuit. 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 an LED is used as a load 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. At the base, an input signal varying between 0V and 5V is given. We are going to see the output at the collector by varying the V I at two states that is 0 and 5V as shown in figure. So, the current value at the emitter terminal of the first transistor will form as the input current of the second transistor thus makes it in On condition.
The input transistor which is the first one gets its input signal at the base terminal. The input transistor gets amplified in a general way and this is used to drive the next output transistors. The second device enhances the signal and this results in a maximum value of current gain. One of the crucial features of the Darlington transistor is its maximum current gain when related to the single BJT device.
In addition to the ability of maximum voltage and current switching characteristics, the other added benefit is its maximum switching speeds. This switching operation allows the device to be specifically used for inverter circuits, DC motor, lighting circuits, and stepper motor regulation purposes. The variation to take into account while utilizing Darlington transistors than that of conventional single BJT types when implementing the transistor as a switch is that the input voltage at the base and emitter junction requires to be more which is nearly 1.
In a transistor, unless a current flows in the base circuit, there is no current can flow in the collector circuit. This property will allow a transistor to be used as a switch.
There are a few applications of switching circuits operated by transistors. Here, I considered NPN transistor to explain a few applications which are using transistor switch.
The circuit is designed by using a transistor as a switch, to light the bulb in a bright environment and to turn it off in the dark and a Light-Dependent Resistor LDR in the potential divider. Then the transistor is switched OFF. When the LDR is exposed to the bright light, its resistance falls to less value resulting in more supply voltage and raising the base current of the transistor.
Now the transistor is switched ON, the collector current flows and the bulb lights up. One important component in the circuit of a heat-operated switch is the thermistor. The thermistor is a type of resistor that responds depending upon the surrounding temperature. Its resistance increases when the temperature is low and vice versa. When heat is applied to the thermistor, its resistance drops and the base current increases followed by a greater increase in the collector current and the siren will blow.
This particular circuit is suitable as a fire alarm system. Consider no voltage is applied to the transistor, then the transistor becomes OFF and no current will flow through it. Hence the relay remains in OFF state. The apply of high voltage at the base of transistor BC causes turning ON of the transistor and the relay coil to energize. Here, we are going to know the value of the base current that is required to make a transistor completely into ON condition where the load needs a current of mA when the input value is enhanced to 5v.
Also, know the value of Rb. Or the load like DC Motor might require to have its speed monitored through some continuous pulses. Transistor switches permit this operation to be quicker and more simply than compared with that of traditional mechanical switches.
While implementing a transistor in the place of a switch, even a minimal amount of base current regulates a higher load current in the collector terminal. Using transistors in the place of switch, these devices are supported with relays and solenoids. Whereas in the case when higher levels of currents or voltages are to be regulated, then Darlington transistors are utilized.
On the whole, as a summary, few of the conditions that are applied while operating transistor as a switch are. And, this article has provided comprehensive and clear information of transistor, operating regions, working like a switch, characteristics, practical applications. The other crucial and related topic to be known is what is digital logic transistor switch and its working, circuit diagram?
How to Use Transistor as a Switch. Our control input flows into the base, the output is tied to the collector, and the emitter is kept at a fixed voltage. While a normal switch would require an actuator to be physically flipped, this switch is controlled by the voltage at the base pin. When the voltage at the base is greater than 0.
When the voltage at the base is less than 0. The circuit above is called a low-side switch , because the switch -- our transistor -- is on the low ground side of the circuit. Alternatively, we can use a PNP transistor to create a high-side switch:. Similar to the NPN circuit, the base is our input, and the emitter is tied to a constant voltage. This time however, the emitter is tied high, and the load is connected to the transistor on the ground side. This circuit works just as well as the NPN-based switch, but there's one huge difference: to turn the load "on", the base must be low.
This can cause complications, especially if the load's high voltage V CC being 12V connecting to the emitter V E in this picture is higher than our control input's high voltage. For example, this circuit wouldn't work if you were trying to use a 5V-operating Arduino to switch off a 12V motor. In that case, it'd be impossible to turn the switch off because V B connecting to the control pin would always be less than V E.
You'll notice that each of those circuits uses a series resistor between the control input and the base of the transistor. Don't forget to add this resistor! A transistor without a resistor on the base is like an LED with no current-limiting resistor. Recall that, in a way, a transistor is just a pair of interconnected diodes.
We're forward-biasing the base-emitter diode to turn the load on. The diode only needs 0. Some transistors may only be rated for a maximum of mA of current to flow through them. If you supply a current over the maximum rating, the transistor might blow up. The series resistor between our control source and the base limits current into the base. The base-emitter node can get its happy voltage drop of 0. The value of the resistor, and voltage across it, will set the current. The resistor needs to be large enough to effectively limit the current, but small enough to feed the base enough current.
Here a high voltage into the base will turn the transistor on, which will effectively connect the collector to the emitter.
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