Common Collector CC Transistor Configuration

CB, CE and CC Transistor Configurations

The transistor can be connected in a circuit in the following three configurations.

1. Common base configuration.

2. Common emitter configuration.

3. Common collector configuration.

 

Key Point: Regardless of circuit configuration, the base emitter junction is always forward biased while the collector‒base junction is always reverse biased, to operate transistor in active region. 

3. Common Collector Configuration

• In this configuration, input is applied between base and collector and output is taken from emitter and collector.

• Here, collector of the transistor is common to both input and output circuits and hence the name common collector configuration. It is also known as emitter follower configuration.

• Common collector connections for both npn and pnp transistors are shown in Fig. 4.4.11 (a) and 4.4.11 (b), respectively.

i. Current Relations In CC Configuration

We know that

IE = IB + IC

= IB + αdc1E + ICBO

IE(1‒αdc) = IB +ICBO

 IE = IB/(1‒αdc) + ICBO/(1‒αdc)

We know that

 βdc = αdc  / (1‒αdc)

1+ βdc = 1+ [ αdc  / (1‒αdc) ]

= [ 1‒αdc + αdc ] / (1‒αdc)

 = 1 / (1‒αdc)

IE = IB(1+βdc) + ICBO(1+βdc)

Neglecting ICBO we have

 IE = IB (1+ βdc)

Current gain in CC configuration is given by

 γ = IE/IB = (1+ βdc)

 = 1 +  αdc/(1‒αdc)

= 1 / (1‒αdc)

ii. Common Collector Input Characteristics

• The input characteristics of CC configuration is a graph of input current IB (base current) versus input voltage VCB (collector base voltage) at constant VCE.

• The base current is taken along Y‒axis and collector base voltage VCB is taken along X‒axis.

• Fig. 4.4.12 shows the input characteristics of a typical transistor in common‒collector configuration.

• The common collector input characteristics are quite different from either common base or common emitter input characteristics. This difference is due to the fact that the input voltage VCB is largely determined by the level of collector to emitter voltage VCE.

Fig. 4.4.12 Input characteristics of transistor in CC configuration

• Looking at Fig. 4.4.12 we can write,

VCE = VCB ‒ VBE

Or

VCB = VCE + VBE

• In CC configuration input junction is BC and it is reversed biased so input resistance in CC configuration is very high.

iii. Common Collector Output Characteristics

• It is the curve between emitter current IE and collector to emitter voltage VCE at constant base current IB.

• The emitter current is taken along Y‒axis and collector to emitter voltage along X‒axis.

• Fig. 4.4.13 shows the output characteristics of a typical transistor in common collector configuration.

Fig. 4.4.13 Output characteristics of the transistor in CC configuration

• Since, IC is approximately equal to IE, the common collector output characteristics are practically similar to those of the common emitter output characteristics.