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. And the corresponding base current for this collector current is So, it is clear that when the base current is increased beyond the Consider the case when zero volt is applied at the input.
This causes the base current to be 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.
Thus, the output at the collector becomes approximately zero. 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 the transistor to short circuit otherwise, it will be open circuit.
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.
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.
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. Then the current required for the saturation of the transistor is.
But practically 30 percent of more current is required for guaranteed saturation of transistor. So, in this example the base current required is 1. As discussed earlier, the transistor can be used as a switch. It is also possible to control the relay operation using a transistor. Transistors can operate on a low-voltage supply for greater safety which means they yield higher efficiency and very long life.
The transistors use semiconductor junctions instead of heating electrodes but perform the same function as a vacuum triode. The transistors can control the flow of current through one channel by changing the intensity of a small amount of current flowing via a second channel. Basics of Bipolar Junction Transistors. Since the controlled current must go through two types of semiconductors materials which are P and N.
The current consists of both electron and hole flow, in different parts of the transistor, and these are of two types:. The base is responsible for activating the transistor. The emitter is the negative lead while Collector is the positive lead. Uses of Transistor. Amplifier Circuit. A transistor can be used to amplify current. Vbe biasing voltage produced in the base-emitter junction Due to the forward biasing of the base-emitter junction, the electrons start flowing from emitter to recombine with holes in the base, the base becomes negatively charged.
If the base current Ib is increased by a small amount, hole-electron recombination will get neutralized, the collector current Ic will be increased. Therefore, a small change in current Ib in the base circuit would control a large amount of Ic current. Example of Microphone. The microphone is a transducer that converts our voice or sound wave to an electronic signal. The electrical output of the microphone varies according to the sound waves as the base current Ib is varying because of the small alternating voltage produced by the microphone which means a small change in Ib can cause a large change in Ic.
When this output of the microphone is given to the transistor as an input. The varying collector current Ic flows into the loudspeaker, and we know that if there are changes in the input of the transistor there will be a large change in the output of the transistor. Thus, the transistor amplifies the electronic signal of the microphone.
The frequency remains constant but the amplitude of the sound wave from the loudspeaker is higher than sound waves fed into the microphone. Oscillator Circuit. An electronic oscillator is a device that generates continuous electrical oscillations. In a simple oscillator circuit, a parallel LC circuit is used as a resonant circuit and an amplifier is used to feed energy to the resonant circuit. The frequency gets resonantly amplified, and the output acts as a source of an alternating voltage of that frequency.
The frequency can be varied by varying L or C. Transistor Used as a Switch. The switched controlled current goes between emitter and collector, and the controlling current goes between emitter and base.
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