Common Emitter Configuration

>>  Configuration refers to the way of 3 terminals of BJT are used in amplifier.

>>  In Common Emitter Configuration, input is applied at base and output is taken from collector.

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>>  As shown in the figure 6.25, the bias voltage VBB forward biases the base emitter junction and VCC is used to reverse bias the collector base junction.

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>>  The input voltage in the CE configuration is the base emitter voltage, and the output voltage is the collector emitter voltage. The input current is IB and the output current is IC.

INPUT CHARACTERISTICS

>>  It is the curve between input current Iand input voltage VBE at constant collector – emitter voltage VCE.

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>>  When VBE is smaller, carrier injunction into the base will be negligible hence no recombination occurs inside base hence base current IB → 0

>>  When VBE exceeds cut in voltage, carrier injunction into the base becomes significant which causes recombination in the base and IB starts flowing thus increase in VBE, base current continuously increase just like a diode current increases as VBE increases.

>>  When Reverse Bias of Collector junction is increased the width of depletion region of collector junction increases. Since base is lightly doped in comparison to collector, depletion region will penetrate more into base region hence effective base width becomes smaller >> and No. of recombination becomes smaller >> Hence base current decreases, this is known as Early effect or Base width modulation.

>>  So for a constant value of VBE, as Reverse Bias of Collector junction is increased >> Base current decreases.

IB  and VCE has inverse dependency because of Early effect or Base width modulation.

OUTPUT CHARACTERISITCS 

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ACTIVE REGION

>>  The region of the graph where IC is approximately constant but not exactly constant.

VCE = VC – VE

When VCE ↑  ⇒ More reverse bias on Collector Junction ⇒ No. of recombination decreases

⇒ β* ↑  ⇒  α ↑ ⇒  β ↑

IC = β IB   (if ICO ≈ 0)

So IC does not remain constant due to change in β.

SATURATION REGION

>> The region of graph where VCE < VCE(sat).

IB = ( VBB – VBE ) / RB
And VCE = VCC – IC RC
If transistor is in active region means
J→ FB  [ VBE ≈ 0.7V (Constant)]   and   JC → RB
If  VBB ↑ or RB decreases ⇒ IB ↑
As IB ↑ ⇒ IC ↑ ⇒ VCE decreases 
Case 1 :  
Let decreases in VCE is upto 0.2 V

VBE = 0.8 V  and VCE = 0.2 (for Si)

VCB = VCE – VBE = 0.2 – 0.8 = -0.6
⇒ VBC = 0.6 V
⇒ VB > VC means collector junction may be gone in Forward Bias region.

⇒  In saturation region both JE and JC are forward biased at least cut in voltage Vγ.

>>  IC and VCE do not change appreciably even with larger change in IB. And we can say that IC has saturated.

 

Case 2 : 

Let VCE = -4V

VCB = VCE – VBE = -4 – 0.8 = – 4.8V

VBC = 4.8V  (But it can not be possible because when JC becomes forward biased >> (VBC will become constant ≈ 0.6V)
IC = ( VCC – Vsat ) / RC   [Constant in saturation region]
CONCLUSION

IB = ( VBB – VBE ) / RB  and  IC = β IB  [ In active region ]

As IB ↑  ⇒  IC ↑  ⇒  VCE decreases
So as reaches to 0.2 V ( VCE(sat) )  ⇒ IC becomes constant (Saturation Region)
⇒ Hence [ β IB > IC ] Saturation Region
⇒ IB(min) = IC(sat) / β

If  IB ≥ IB(min)  →  Saturation Region

if IB < IB(min)  → Active Region
if IB = -ve  → Cutoff Region 
Or

If  VCE > VCE(sat)  → Active Region

if VCE ≤ VCE(sat)  → Saturation Region
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SOLVED EXAMPLES
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