Transistor as a Switch

Using a transistor as a switch is the simplest application of the device. A transistor can be extensively used for switching operations, either for opening or closing a circuit. On the other hand, the basic concept behind the operation of a transistor as a switch relies on its mode of operation. Generally, the low-voltage DC is turned on or off by transistors in this mode.

Both PNP and NPN transistors can be utilised as switches. A basic terminal transistor can be handled differently from a signal amplifier by biasing both NPN and PNP bipolar transistors with an “ON / OFF” static switch. One of the main uses of the transistor to transform a DC signal “ON” or “OFF” is solid-state switches.

Some devices, including LEDs, only require several milliamperes of DC voltages at the logical level and can be directly controlled via the logical gate output. High-power devices such as generators, solenoids or lamps usually need more power to use transistor switches than the usual logic gate.

Read More: Transistors

Transistor Switch’s Working Regions or Operating Modes

The saturation zone and cut-off area are also known as the transistor switch’s working regions. This implies that, by switching between its “top-off” (saturation) and “absolute OFF,” the transistor is used as a switch to basically overwrite its Q-Point and the voltage divider circuit that is needed for amplification.

Cut-off Region

The “cut-off” area is at the bottom of the curves, the blue, shaded area and the yellow zone on the left is the transistor “saturation” region.

The transistor’s operating specifications include the base current (I_B), the collector current (I_C) and the emitter-collector voltage (V_CE).

Characteristics of Cut-off Region

• The transistor is used as an “open switch”
• The bases and input are grounded (0v)
• The base emission voltage is V_BE > 0.7 V
• The basic emitter is reversed
• The full-OFF (cut-off area) transistor (“Collector Flow = 0”) • V_OUT = V_CC = “1′′”
• No collector current flows (I_C = 0)

Instead, we can describe the “cut-off region” or “OFF mode,” both in reverse bias, with Vb < 0.7 V and I_C = 0, when using a bipolar transistor as a switch.

Also Read: Transistor as Amplifier

Saturation Region

In this mode or region, the highest base current is applied, leading to the overall collector current, causing the average collector-emitter voltage to fall and the leakage surface as small as possible and the maximum current that flows across this transistor. That is why the “Fully ON” transistor is triggered.

Alternatively, we can define the “saturation field” or “ON step,” all junctions forward, V_W > 0.7 V and IK = complete when using a bipolar transistor as a switch.

Let us consider a base-biased transistor in a CE configuration. When we extend the voltage rule of Kirchhoff to the circuit’s input and output side, we can write,

V_BB = I_BR_B + V_BE … (1)

V_CE = V_CC – I_CR_C … (2)

V_BB is the input voltage (Vi), and V_CE  is the output voltage DC (Vo). That’s why we get,

V_i = I_BR_B + V_BE

V_o = V_CC – I_CR_C

First, let’s look at the shift in V_o as V_i rises from zero. A Silicon junction transistor remains in a cutoff state as long as V_i is less than 0.6 V. Also, I_C= 0. V_o= V_CC. Thus, the transistor switches into an active state when V_i goes past 0.6 V. I_C >0, and V_o is also decreasing (because I_CR_C is increasing). Originally, with rising V_i, I_C increases almost linearly.

V_o also decreases linearly until its value drops below 1 V. Post this, the change becomes non-linear, and the transistor moves into the state of saturation. V_o continues to decrease on increasing V_i but never becomes zero. Here’s a V_o vs V_i plot (also referred to as the transition features of a reference transistor).

There are two things to remember here:

When V_i is low and the transistor is unable to forward bias, V_o is high(= V_CC).

If V_i is sufficiently high to saturate the transistor, V_o is very low (~0).

It is also switched off when a transistor is not conducting. On the other side, it is turned on when it is in a state of depletion. Bringing these components together, imagine a resistor that determines the low and high values below and above those points of voltage.

Such levels suit the transistor’s cutoff and saturation. We might say in such a situation that a small input turns off the transistor, and a high input switches on it. These circuits are designed to prevent the transistor from staying in an active state. This is how a transistor can act as a switch.

Also Read: Transformers

Applications Oo Transistor as a Switch

The transistor as a switch has the following uses:

• The LED feature is the most widely employed practical application that is used as a switch for the transistor.
• The relay operation can be managed by making the necessary circuit changes in order to connect and control some external devices with respect to the relay.
• With this idea of transistors, the DC motors can be controlled and monitored. This software is used to turn the engine on and off. The motor speed can be modified by changing the transistor frequency values.
• Light-bulb is one of the example of these switches’. It can switch the light on if the setting is bright and off, depending on the dark surroundings. A light-dependent resistor (LDR) is used to do this.
• An element called a thermistor can be controlled using this switching method, which detects the ambient temperature. The thermistor is called a resistor. The resistance increases when the temperature sensed is low, and the resistance decreases when the sensed temperature is high.

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