Discuss the switching characteristics of transistor. Also explain the transistor as switch with circuit diagram.

Q.) Discuss the switching characteristics of transistor. Also explain the transistor as switch with circuit diagram.

 Subject: electronic devices and circuits

Switching Characteristics of Transistor

Transistors, which can be either Bipolar Junction Transistors (BJTs) or Field-Effect Transistors (FETs), are widely used as switches in electronic circuits. When used as a switch, a transistor changes states between cutoff (off state) and saturation (on state) for a BJT, or between cutoff and triode region for a FET. The switching characteristics of a transistor are crucial for understanding its performance in digital circuits, where it must switch on and off rapidly and efficiently.

Important Switching Parameters

Parameter Description
t_on (Turn-On Time) The time it takes for the transistor to switch from the off state to the on state.
t_off (Turn-Off Time) The time it takes for the transistor to switch from the on state to the off state.
t_r (Rise Time) The time taken for the output to rise from 10% to 90% of its maximum value during turn-on.
t_f (Fall Time) The time taken for the output to fall from 90% to 10% of its maximum value during turn-off.
t_d(on) (Turn-On Delay Time) The time between the application of the input signal and the beginning of the output rise.
t_d(off) (Turn-Off Delay Time) The time between the removal of the input signal and the beginning of the output fall.
I_C(sat) (Saturation Current) The collector current when the transistor is in saturation (fully on).
V_CE(sat) (Saturation Voltage) The collector-emitter voltage when the transistor is in saturation.
V_BE(on) (Base-Emitter On Voltage) The base-emitter voltage required to turn the transistor on.

BJT as a Switch

A Bipolar Junction Transistor (BJT) can be used as a switch by driving it between the saturation region (where it is fully on) and the cutoff region (where it is fully off). The BJT has three regions of operation: cutoff, active, and saturation.

  • Cutoff Region: Both the base-emitter (BE) and base-collector (BC) junctions are reverse-biased. The transistor is off, and the collector current (I_C) is essentially zero.
  • Active Region: The BE junction is forward-biased, and the BC junction is reverse-biased. The transistor operates as an amplifier.
  • Saturation Region: Both the BE and BC junctions are forward-biased. The transistor is fully on, and the collector current is at its maximum.

Circuit Diagram of BJT as a Switch

  V_CC
   |
   R_C
   |
  --- C
 / | \
B   |  E
 \  |
  ---
   |
  GND

In the above diagram:

  • V_CC is the supply voltage.
  • R_C is the collector resistor, which limits the current through the transistor.
  • B, C, and E represent the base, collector, and emitter terminals of the BJT, respectively.

Example of BJT Switching

Let's consider an NPN transistor as a switch in a simple LED circuit.

  +V_CC
   |
   R_C
   |
  --- C
 / | \
B   |  E
 \  |
  ---
   |   /|
   R_B | LED
   |   \|
  GND  ---

Here, R_B is the base resistor that controls the base current (I_B), and the LED is the load.

  • To Turn On the LED: A sufficient base current (I_B) is applied to the base so that the BJT goes into saturation. The voltage across the collector-emitter (V_CE) drops to a low value (V_CE(sat)), and current flows through the LED, lighting it up.
  • To Turn Off the LED: The base current is removed, causing the BJT to enter the cutoff region. The collector-emitter path is now open, and no current flows through the LED, turning it off.

FET as a Switch

Field-Effect Transistors (FETs), such as MOSFETs, can also be used as switches. They operate by controlling the flow of current with an electric field. The FET has three terminals: gate (G), drain (D), and source (S).

  • Cutoff Region: The gate-source voltage (V_GS) is below the threshold voltage (V_th), and the FET is off.
  • Triode Region: V_GS is above V_th, and the drain-source voltage (V_DS) is low. The FET behaves like a resistor, and this is the on state for switching applications.
  • Saturation Region: V_GS is above V_th, and V_DS is high. The FET operates as a constant current source and is not used in this region for switching.

Circuit Diagram of FET as a Switch

  V_DD
   |
   R_D
   |
  --- D
 |   |
 G   S
 |   |
  --- 
   |
  GND

In the above diagram:

  • V_DD is the supply voltage.
  • R_D is the drain resistor, which limits the current through the FET.
  • G, D, and S represent the gate, drain, and source terminals of the FET, respectively.

Example of FET Switching

Let's consider an N-channel MOSFET as a switch in a simple LED circuit.

  +V_DD
   |
   R_D
   |
  --- D
 |   |
 G   S
 |   |
  --- 
   |   /|
   R_G | LED
   |   \|
  GND  ---

Here, R_G is the gate resistor that controls the gate voltage (V_GS), and the LED is the load.

  • To Turn On the LED: A gate voltage (V_GS) higher than the threshold voltage (V_th) is applied, causing the MOSFET to enter the triode region. The drain-source resistance becomes very low, and current flows through the LED, lighting it up.
  • To Turn Off the LED: The gate voltage is removed or reduced below V_th, causing the MOSFET to enter the cutoff region. The drain-source path is now high resistance, and no current flows through the LED, turning it off.

In summary, transistors as switches are fundamental components in digital electronics, and their switching characteristics are essential for designing efficient and fast-switching circuits.