6.2 Analog electronics lecture 2_partc_i-v output characteristics

Analog Electronics Lecture 2_PartC_I-V output characteristics of BJT

Key words;BJT;

Abstract: This describes the D.C. parameters of BJT.

BJT Common Base Configuration :

For CB Configuration :

I _C = α _F MI _E + I _CBO

Where α _F = D.C. Forward current transfer Ratio of CB BJT = I _C /I _E ;

M = Avalanche Multiplication Factor at Base-Collector Junction given by = 1/[1-(Vcb/BVcbo)^n]

Where n= Miller Indices =2~6

I _CBO = Reverse leakage current at CB Jn with emitter open.

OUTPUT CHARACTERISTICS OF Common Base Bipolar Junction transistor

I _C = I _CBO when I _E = 0 mA. This is the reverse leakage current at CB Junction with Emitter open and is of nA range.

BJT Common Emitter(CE) Configuration:

For CE Configuration :

I _C = α _F MI _E + I _CBO

=>I _C = α _F M(I _B +I _C ) + I _CBO

=>I _C (1-α _F M)= α _F MI _B + I _CBO

If M=1,

Where α _F =0.99.

At low voltages we have M=1.

Then we get:

That is:

OUTPUT CHARACTERISTICS OF Common Emitter Bipolar Junction transistor

NOTE:-The slope in the figure is due to base width modulation which is also known as Early Effect.

I _C = I _CEO when I _B = 0 mA. This is the collector junction leakage current at CB Junction with Base open and is of µA range.

Let us consider :

If α _F M =1, then

At this point , break over occurs. And we have BV _CEO =Break-over Voltage with Base Circuit open.

When α _F M =1

That is

But :

Thus :

Thus:

Putting the required Values i.e. BV _CBO =30V, β _F = 100 , we get V _CB ^* =18 V = BVceo ;

Now we have to know more about BV _CES (Breakover Voltage when the base circuit is shorted)

BV _CBO >BV _CEX >BV _CES >BV _CEO

Where BV _CBO = Breakover Voltage of the collector base junction when the emitter circuit is open.

BV _CEX = Breakover Voltage of CE BJT for a given termination R _X at the base

BV _CEO = Breakover Voltage of CE BJT when the base circuit is open.

BV _CES = Breakover Voltage of CE BJT when the base circuit is shorted to ground.

BV _CEX = Breakover Voltage when the base circuit is connected to ground through a source Resistance (R _S ) .

By proper base termination, the permissible region of operation can be extended upto BV _CBO .

Thus we have seen that breakover occurs at α _F M =1. At low current α _F is very small, almost about 0.1.Therefore voltage has to be taken to a large value to satisfy α _F M =1. But as soon as breakover occurs large current starts flowing. With large current α _F improves from 0.1 to 0.99. Hence α _F M =1 is satisfied at lower voltage Vs. Therefore breakover curves settles down at Vs. This voltage V _S is known as sustaining voltage. Because of the fact that :

V _S <BV _CEO

We get a S Type Negative Impedance Region(NIR). In SCR and UJT also we get S Type NIR but in Tunnel Diode as shown in the Figure below we get N Type NIR.

COMPARISON BETWEEN COMMON BASE AND COMMON EMITTER CONFIGURATIONS

Both CB and CE are Current Controlled Current Source. CB is a near ideal CCCS whereas CE is a non-ideal CCCS.