Jun 25, 2016

Common Collector Configuration



  • Consider n-p-n transistor in common collector configuration as shown in the figure. The base current IB is in input current and the emitter current IE is output current.
  • The common collector configuration is similar to that of the common emitter configuration, with the expectation that the load resistance is in the emitter circuit rather than in the collector circuit and the output is taken from emitter lead instead of the collector.

Common Collector Configuration

common collector configuration
We know IC = αIE + ICBO    ----->1    and
IE = IC + IB   ------->2
IE = α.IE +ICBO + IB
IE.(1-α) = IB + ICBO

IE = [IB/(1-α)] + [ICBO/(1-α)] ------>3

Since, i/(1-α) = 1+β,    IE=IB.(1+β) + ICBO.(1+β)

IE = (1+β) (IB+ICBO)   ------->4

Neglecting the leakage current ICBO, we have

IE + (1+β) IB
IE/IB = (1+β)  ----------->5

Current Amplification Factor

The ratio of the emitter to the base current is called dc current gain of a transistor in common collector configuration. It is denoted by γdc.
Vdc = IE / IB and    ----------->6

Vdc = IE/IB = (IE/IC) (IC/IB) = (1/α).β  = β/α  ------------->7

Vdc = β / [β.(1+β)] = 1+β   -------------->8

IE = IC+IB = β.IB+IB + (1+β).IB --------->9

In the case of ac operation of a transistor, the current amplification factor is called as the ratio of small change in emitter current (ΔIE) to the corresponding change in the base current (ΔIB).
Vac = ΔIE/ΔIB  ----->10

Relation Between γ and α

We know IC=IC+IB
ΔIE = ΔIC + ΔIB
ΔIB = ΔIE – ΔIC

Current Amplification factor Va = ΔIE / ΔIB   = ΔIE/ (ΔIE – ΔIC)

Dividing numerator and denominator by ΔIE, We get

γ = 1 / [1 – (ΔIC / ΔIE)]  = 1 / (1-α)                            {∵ α = ΔIC / ΔIE}
γ =  1 / (1-α)   ------------>11

Relation between Transistor Currents

We Know
α = IC / IE,    β=IC/IB,   α =β / (1+β) and β= α / (1-α)
  1. IC =β.IB =  α.IB = [β / (1+β)].IE
  2. IB = IC/β = IE/(1+β)   = (1-α).IE
  3. IE = IC/α  = [(1+β) / β].IC     = (1+β).IB   = IB.(1-α)
The three transistor dc currents always bear the following ratio:
                            IE : IB : IC :: 1 : (1-α) : α

Input and Output Characteristics of Common Collector Configuration

input characteristic of common collector configuration

output characteristic of common collector configuration

Transistor Leakage Currents:

Collector to Base Leakage Current (ICBO)

  • When the emitter is open circuited and the collector base junction is reverse biased, a small current called collector to base leakage current flows through the collector base junction.
  • It consists of two components:
  1. The temperature dependent component current due to thermal generation of electron-holes pair and
  2. The voltage dependent component of current due to surface leakage through the collector base junction.
  • It is represented by ICBO. This current is called the reverse saturation current or collector cut-off current and is represented by ICEO. It doubles for every 10oC increase in temperature in silicon transistor.

Collector to Emitter Leakage Current (ICEO)

  • When the base is open circuit and collector-emitter junction is reverse biased, a small current called collector to emitter leakage, ICEO flows from the collector to emitter.
  • This current also depends on the temperature and the voltage of the collector with respect to the emitter, it can be shown that

Emitter to Base Leakage Current ( ICEO)

  • When the collector is open circuited and the emitter-base junction is reverse biased, small current called emitter to base leakage ICEO flows through the junction.

Transport Factor (β):

It is the ratio of a small change in the collector current (ΔIC) to a small change in the whole component of currents (ΔIPE).
β = ΔIC/ ΔIPE






























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