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Analog Electronics Lecture 2_PartC_I-V output characteristics of BJT

Module by: Bijay_Kumar Sharma. E-mail the author

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

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 :

Figure 1
Figure 1 (Picture 1.png)

IC= αFMIE + ICBO

Where αF= D.C. Forward current transfer Ratio of CB BJT = IC/IE ;

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

Figure 2
Figure 2 (Picture 2.png)

Where n= Miller Indices =2~6

ICBO = Reverse leakage current at CB Jn with emitter open.

Figure 3
Figure 3 (Picture 2.png)
= Avalanche breakdown voltage at CB Jn with emitter open

OUTPUT CHARACTERISTICS OF Common Base Bipolar Junction transistor

Figure 4
Figure 4 (Picture 4.png)

IC = ICBO when IE = 0 mA. This is the reverse leakage current at CB Junction with Emitter open and is of nA range.

BJT Common Emitter(CE) Configuration:

Figure 5
Figure 5 (Picture 5.png)

For CE Configuration :

IC = αFMIE + ICBO

=> IC = αFM(IB+IC)+ ICBO

=> IC(1-αFM)= αFMIB + ICBO

Figure 6
Figure 6 (Picture 6.png)

Figure 7
Figure 7 (Picture 7.png)
Figure 8
Figure 8 (Picture 8.png)

If M=1,

Figure 9
Figure 9 (Picture 9.png)

Where αF =0.99.

At low voltages we have M=1.

Figure 10
Figure 10 (Picture 10.png)

Figure 11
Figure 11 (Picture 11.png)

Then we get:

Figure 12
Figure 12 (Picture 12.png)

That is:

Figure 13
Figure 13 (Picture 13.png)

Figure 14
Figure 14 (Picture 14.png)

OUTPUT CHARACTERISTICS OF Common Emitter Bipolar Junction transistor

Figure 15
Figure 15 (Picture 15.png)

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

IC = ICEO when IB = 0 mA. This is the collector junction leakage current at CB Junction with Base open and is of µA range.

Let us consider :

Figure 16
Figure 16 (Picture 17.png)

If αFM =1, then

Figure 17
Figure 17 (Picture 18.png)

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

When αFM =1

That is

Figure 18
Figure 18 (Picture 19.png)

Figure 19
Figure 19 (Picture 20.png)
Figure 20
Figure 20 (Picture 21.png)

But :

Figure 21
Figure 21 (Picture 22.png)

Thus :

Figure 22
Figure 22 (Picture 23.png)

Thus:

Figure 23
Figure 23 (Picture 24.png)

Putting the required Values i.e. BVCBO=30V, βF = 100 , we get VCB*=18 V = BVceo ;

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

BVCBO >BVCEX >BVCES >BVCEO

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

BVCEX= Breakover Voltage of CE BJT for a given termination RX at the base

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

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

BVCEX= Breakover Voltage when the base circuit is connected to ground through a source Resistance (RS) .

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

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

VS < BVCEO

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.

Figure 24
Figure 24 (Picture 25.png)

COMPARISON BETWEEN COMMON BASE AND COMMON EMITTER CONFIGURATIONS

Table 1
S.No. COMMON BASE COMMON EMITTER
1 hrb(reverse transmission factor)~10-5Thus it behaves as a near unilateral device. hre~10-4Thus it behaves as a non-unilateral device.
2 In RF applications the circuit has a high probability of parasitic oscillations but in CB because of near unilaterality , probability of parasitic oscillation goes down. Hence for RF applications CB is the preferred circuit configuration. At low frequencies there is no danger of parasitic oscillations hence CE can be used even with poor reverse transmission factor.
3 hob = 1/(2M)Thus it behaves as a near ideal current source. Thus it is very suitable for charging a capacitance with a constant current to generate a saw-tooth waveform. hoe=1/(40K)Thus it behaves as a non-ideal current source.

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

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