Question:

A transistor is used as a common emitter amplifier with a load resistance 2 kΩ. The input resistance is 150 Ω. Base current is changed by 20 μA which results in a change in collector current by 1.5 mA. The voltage gain of the amplifier is

Updated On: Jun 12, 2023
  • 1100
  • 1200
  • 900
  • 1000
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The Correct Option is D

Solution and Explanation

The voltage gain of a common emitter amplifier can be calculated using the formula: 
Voltage Gain = - β * \(\frac {Load \ Resistance}{Input \ Resistance}\)
where: 
β is the current gain of the transistor, Load Resistance = 2 kΩ  and Input Resistance = 150Ω.     
To find the voltage gain, we need to determine the value of β, which is the current gain of the transistor. The current gain (β) is defined as the ratio of the change in collector current (ΔIc) to the change in base current (ΔIb). 
β = \(\frac {ΔIc}{ΔIb}\)
Given that the change in collector current (ΔIc) is 1.5 mA and the change in base current (ΔIb) is 20 μA 
β = \(\frac {1.5 \ mA}{20\  μA}\) = 75 
Now 
Voltage Gain = - β * \(\frac {Load \ Resistance}{Input \ Resistance}\)
Voltage Gain = -75 * \(\frac {2000\  Ω}{150\  Ω}\) = -75 * 13.333 = -1000 
Since the voltage gain can be negative for a common emitter amplifier, the correct answer is (D) 1000.

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Concepts Used:

Transistors

Transistor

A transistor is a type of semiconductor device that can be used to both conduct and insulate electric current or voltage. A transistor basically acts as a switch and an amplifier. In simple words, we can say that a transistor is a miniature device that is used to control or regulate the flow of electronic signals.

Parts of a Transistor:

A transistor is a combination of three terminals made of semiconducting materials that help in making a connection to an external circuit and allow current to flow. The three terminals are:

  • Base: The base activates the transistor. It is thin and lightly doped. It is put in the centre of the transistor.
  • Emitter: The emitter is the negative terminal of the transistor. It is heavily doped and is moderately sized.
  • Collector: The collector is the negative terminal of the transistor. It is located on the right side of a transistor and is moderately doped. It is larger than the emitter.

Read More: Difference Between NPN and PNP Transistor

Types of Transistors:

A transistor is a type of electronic device which is formed by p-type and n-type semiconductors.

NPN Transistor

  • NPN transistor is a type of Bipolar Junction Transistor.
  • In this, electrons are major current carriers, and minor ones are holes.
  • Their arrangement is in such a way that N-type doped semiconductors are separated by the layer of P-type doped semiconductors which is a thin layer of material embedded between them.
  • Emitter Current = Collector Current + Base Current

PNP Transistor

  • PNP transistor is also a type of Bipolar Junction Transistor.
  • In these, holes are the major source that carries current, and electrons are minor.
  • Their arrangement is in a way that P-type doped semiconductor is separated by N-type doped semiconductor material which is a thin layer.
  • Emitter Current = Collector Current + Base Current

Read More: Characteristics of a Transistor

Configurations of a Transistor:

Using the three types of configuration can be used to design any transistor circuit. The three types of configuration of a transistor are:

  • Common Emitter Transistor
  • Common Base Transistor
  • Common Collector Transistor

Common Emitter (CE) Configuration of a Transistor

In Common Emitter Configuration, the transistor’s emitter terminal will be connected common between the output terminal and the input terminal.

Input Characteristics

  1. Variation of emitter current (IB) with Base-Emitter voltage (VBE) when Collector-Emitter voltage (VCE) is held constant.
  2. Rin = ΔVBE/ΔIB | VCE = Constant

Output Characteristics

  1. Variation of collector current (IC) with Collector-Emitter voltage (VCE) when the base current (IB) is held constant.
  2. Rout = ΔVCE/ΔIC | IB = Constant

Current Transfer Characteristics

  1. The variation of the collector current (IC) with the base current (IB) when the collector-emitter voltage (VCE) is constant.
  2. α = ΔIC/ΔIB | VCB = Constant

Common Base (CB) Configuration of a Transistor

In Common Base Configuration, the transistor’s base terminal will be connected common between the output terminal and the input terminal.

Input Characteristics

  1. Variation of emitter current (IE) with Base-Emitter voltage (VBE) when the Collector Base voltage (VCB) is held constant.
  2. Rin = ΔVBE/ΔIE | VCB = Constant

Output Characteristics

  1. Variation of collector current (IC) with Collector-Base voltage (VCB) when the emitter current (IE) is held constant.
  2. Rout = ΔVCB/ΔIB | IE = Constant

Current Transfer Characteristics

  1. The variation of the collector current (IC) with the emitter current (IE) when the Collector Base voltage (VCB) is constant.
  2. α = ΔIC/ΔIE | VCB = Constant

Common Collector Configuration of a Transistor

In Common Collector Configuration, the transistor’s collector terminal will be connected common between the output terminal and the input terminal.

Input Characteristics

  1. Variation of emitter current (IB) with Collector-Base voltage (VCB) when the Collector Base voltage (VCB) is held constant.

Output Characteristics

  1. Variation of emitter current (IE) with Collector-Emitter voltage (VCE) when the base current (IB) is held constant.

Current Transfer Characteristics

  1. The variation of the collector current (IE) with the base current (IB) when the Collector-Emitter voltage (VCE) is constant.