BJT & FET - 1 | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE) PDF Download

The name transistor comes from the phrase “transferring an electrical signal across a resistor.” 

In this EduRev document, we will discuss two types of transistors: 

  1. The Bipolar Junction Transistor (BJT) is an active device. In simple terms, it is a current controlled valve. The base current (IB) controls the collector current (IC).
  2. The Field Effect Transistor (FET) is an active device. In simple terms, it is a voltage-controlled valve. The gate-source voltage (VGS) controls the drain current (ID). 

Regions of BJT Operation

  • Cut-off region: The transistor is off. There is no conduction between the collector and the emitter. (IB = 0 therefore IC = 0) 
  • Active region: The transistor is on. The collector current is proportional to and controlled by the base current (IC = βIC) and relatively insensitive to VCE. In this region the transistor can be an amplifier. 
  • Saturation region: The transistor is on. The collector current varies very little with a change in the base current in the saturation region. The VCE is small, a few tenths of volt. The collector current is strongly dependent on VCE unlike in the active region. It is desirable to operate transistor switches will be in or near the saturation region when in their on state.

Rules for Bipolar Junction Transistors (BJTs)


  • For an npn transistor, the voltage at the collector VC must be greater than the voltage at the emitter VE by at least a few tenths of a volt; otherwise, current will not flow through the collector-emitter junction, no matter what the applied voltage at the base. 
  • For pnp transistors, the emitter voltage must be greater than the collector voltage by a similar amount. 
  • For the npn transistor, there is a voltage drop from the base to the emitter of 0.6 V. For a pnp transistor, there is also a 0.6 V rise from the base to the emitter. In terms of operation, this means that the base voltage VB of an npn transistor must be at least 0.6 V greater that the emitter voltage VE; otherwise, the transistor will pass an emitter-to-collector current. For a pnp transistor, Vmust be at least 0.6 V less than VE; otherwise, it will not pass a collector-to-emitter current. 

Basic Equations for BJT

BJT & FET - 1 | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)

BJT Schematic Symbols (Mnemonics for remembering the direction of the arrows are in 

parenthesis.)

BJT & FET - 1 | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)

Ohmmeters view of the BJT. Clearly a transistor cannot be made on the bench by combining two resistors. (Why is that?) Most ohmmeters can not only measure the resistance, but also measure the forward voltage drop across a diode. From this perspective you can identify the base and the type of transistor based on the following equivalent circuits. 

BJT & FET - 1 | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)


Common Nomenclature (npn example)

BJT & FET - 1 | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)

Types of Amplifiers


  1. The transistor is a three-terminal device; thus, the input and the output must share one terminal in common. 
  2. This is the origin of the nomenclature of the three types of transistor amplifiers: common collector, common emitter, and common base.

Definition of Gain


  • Gain is defined as the ratio of the output signal to the input signal. Because transistor amplifiers often have a quiescent output (a non-zero output when the input is zero) we define gain as the derivative of the output with respect to the input. 
  • For systems where the quiescent output is zero, this reduces to the ratio of the output to the input. Thus, gain is defined as the ratio of the change in output to the change in input. 
  • So far, we have not specified the output quantity, the reason is that we can define the gain with respect to any given output and input quantity.BJT & FET - 1 | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)
  • Note that a negative gain means that the sign of the signal is inverted. Negative gain is not possible for Power Gain. |A| less than unity indicates that the output is smaller than the input. 
  • The quantities need not be the same. If the input and output quantities are different, the gain is no longer unitless. The most common examples are transimpedance gain and transadmittance gain. BJT & FET - 1 | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)

Input Impedance of a Transistor


BJT & FET - 1 | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)

  • Impedance is defined as Z = V/I. In linear circuits (with resistors, capacitors, inductors, batteries, etc.) this ratio is the reciprocal of the slope of the I versus V graph. 
  • In circuits with nonlinear elements such as a transistor, the input impedance of the transistor is defined as the reciprocal of the slope of the I versus V graph. This is simply the derivative of Vin with respect to Iin.
    BJT & FET - 1 | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)
  • We can easily find Zin from what we know already of the behavior of the transistor. We know that the sum of VBE and the IR drop across RE must equal Vin. BJT & FET - 1 | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)
  • Taking the derivative of Vin with respect to Iin, remembering that VBE is a constant, we get the result:
    BJT & FET - 1 | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)
  • Because IE = IB(β + 1) The IR drop across RE is greater than it would be for IB alone. 
  • The amplification of the base current causes Rto appear larger to a source looking into the input by (β + 1). 

Output Impedance of a Transistor for the Emitter Follower (Common Collector)

 

BJT & FET - 1 | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)

The output impedance seen by the load (RE in this example) is defined as: 

BJT & FET - 1 | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)

The minus sign in the derivative comes from the fact the output impedance has the effect of decreasing Vout. The output current Iout is just the emitter current IE which is related to the base current. 

BJT & FET - 1 | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)

Thus we obtain the result that the impedance of the source, as viewed by the load, is reduced by the factor ~1/β. 

BJT & FET - 1 | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)


This topic does not end here. So, to read further the topic of FET, click here

BJT & FET - 1 | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)

The document BJT & FET - 1 | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE) is a part of the Electrical Engineering (EE) Course Electrical Engineering SSC JE (Technical).
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FAQs on BJT & FET - 1 - Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE)

1. What are the different regions of operation for a Bipolar Junction Transistor (BJT)?
Ans. A BJT has three regions of operation: the active region, the cutoff region, and the saturation region. In the active region, both the junctions of the transistor are forward-biased, allowing current flow between the collector and the emitter. In the cutoff region, both junctions are reverse-biased, and no current flows through the transistor. In the saturation region, both junctions are forward-biased, and the transistor allows maximum current flow between the collector and the emitter.
2. What are the rules for Bipolar Junction Transistors (BJTs)?
Ans. The rules for BJTs include the following: 1. The base-emitter junction must be forward-biased for the transistor to operate in the active region. 2. The collector-base junction can either be forward-biased or reverse-biased. 3. The collector current is controlled by the base current through the current gain factor, beta. 4. The base current is very small compared to the collector current in the active region. 5. The transistor operates as a current-controlled device.
3. What are the basic equations for a Bipolar Junction Transistor (BJT)?
Ans. The basic equations for a BJT include: 1. Emitter current equation: Ie = Ib + Ic 2. Base current equation: Ib = Ie / (1 + beta) 3. Collector current equation: Ic = beta * Ib 4. Transistor current gain equation: beta = Ic / Ib 5. Transistor voltage gain equation: Av = Vout / Vin
4. What are the different types of amplifiers that can be built using Bipolar Junction Transistors (BJTs)?
Ans. There are several types of amplifiers that can be built using BJTs, including: 1. Common Emitter Amplifier: Provides high voltage gain and medium input and output impedances. 2. Common Base Amplifier: Provides high current gain and low input and output impedances. 3. Common Collector Amplifier (Emitter Follower): Provides unity voltage gain and high input and output impedances. 4. Differential Amplifier: Used for amplifying the difference between two input signals. 5. Darlington Amplifier: Consists of two BJTs connected in a common-emitter configuration to achieve high current gain.
5. What is the input impedance of a Bipolar Junction Transistor (BJT)?
Ans. The input impedance of a BJT depends on its configuration. In the common emitter configuration, the input impedance is typically medium to high. In the common base configuration, the input impedance is typically low. In the common collector configuration (emitter follower), the input impedance is typically high. The input impedance determines how much the transistor circuit will load the preceding stage or signal source.
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