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AE_Lecture5_PartD_Bootstrapping and Darlington Pair.

Module by: Bijay_Kumar Sharma. E-mail the author

Summary: Ae_Lecture5_PartD describes the circuit technique for realizing a very high input impedance of the order of tens of megaohms for Instrumentation Application.

AE_LECTURE9_Bootstrapping & Darlington Pair Configuration of BJT.

For instrumentation application and particularly for constructing Vacuum Tube VoltMeter(VTVM) typ meters the input stage must offer a very large input impedance of the order of tens of megohms. To achieve this goal we go for emitter follower but emitter follower uses self biasing and the potential divider network made of R1 and R2 considerable load the input impedance of emitter follower thereby making the goal of high input impedance of the order of tens of megaohms unattainable. To overcome this problem we resort to Bootstrapping.

BOOT STRAPPING TECHNIQUE for improving the input impedance of EMITTER FOLLOWER.

Figure 1
Figure 1 (Picture 1.png)
Figure 2
Figure 2 (graphics1.png)

But

Figure 3
Figure 3 (graphics2.png)
is lost .We use bootstrapping to maintain high
Figure 4
Figure 4 (graphics3.png)

Figure 5
Figure 5 (Picture 3.png)

A resistance R3 is connected between I/P and O/P of the amplifier. Here O/P connection is through a coupling capacitance hence it is only AC connection and not DC therefore Q point remains undisturbed. R1 and R2 provide the base biasing as before.

By Miller Transformation:-

Figure 6
Figure 6 (Picture 4.png)

Now

Figure 7
Figure 7 (graphics4.png)

So high input impedence of

Figure 8
Figure 8 (graphics5.png)

is fully realized without being loaded by the biasing network consisting of

Figure 9
Figure 9 (graphics6.png)
,
Figure 10
Figure 10 (graphics7.png)
.

Here the Miller Transformation of R3 to the output node pair is

Figure 11
Figure 11 (graphics8.png)

This is a negative resistance but it does not have a deleterious effect on the performance of emitter follower. It is a vey large negative resistance hence it is very small negative conductance. This in parallel with RE gives a net positive resistance only. Hence it does not make the circuit oscillatory.

By the use of Darlington Pair we can achieve super beta transistor

Figure 12
Figure 12 (Picture 5.png)
Figure 13
Figure 13 (graphics9.png)
Figure 14
Figure 14 (graphics10.png)

Now if

Figure 15
Figure 15 (graphics11.png)

Figure 16
Figure 16 (graphics12.png)
Q1->β1=10 ×Q2->β2=100
Figure 17
Figure 17 (graphics13.png)

Figure 18
Figure 18 (Picture 6.png)

Figure 5. Beta dependence on operating current

Figure 19
Figure 19 (graphics14.png)
.

As seen in the curve, Emitter Injection Efficiency (γ) deteriorates at low operating currents below 0.5mA due to recombination current within EB depletion layer.

Emitter Injection Efficiency (γ) deteriorates at high operating currents above 5mA due to base conductivity modulation.

Hence by this technique, a Darlington Pair Composite Transistor short circuit current gain can never be better than 1000.

If

Figure 20
Figure 20 (graphics15.png)
=
Figure 21
Figure 21 (graphics16.png)
better than 10,000 is to be realized then we will have to go for the fabrication of super beta transistor by realizing ultra thin base layer in vertical NPN transistor. By the use of Ion-Implantation as thin as 10nm base width is being realized to date.

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