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AE_Lecture5_PartB_High Frequency Analysis of CE Amplifier

Module by: Bijay_Kumar Sharma. E-mail the author

Summary: AE_Lecture5_PartB does the analysis for determining upper cut off frequency of CE BJT Amplifier.

MID FREQUENCY ANALYSIS OF CE AMPLIFIER

Figure 1
Figure 1 (Picture 1.png)

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

Figure 3
Figure 3 (graphics2.png)
_______(2)

Figure 4
Figure 4 (graphics3.png)

Figure 5
Figure 5 (graphics4.png)
_____(3)

Figure 6
Figure 6 (graphics5.png)
Figure 7
Figure 7 (graphics6.png)

Dividing Eq(1) by Eq(3) we get:

Figure 8
Figure 8 (graphics7.png)
Figure 9
Figure 9 (graphics8.png)

But

Figure 10
Figure 10 (graphics9.png)

Figure 11
Figure 11 (graphics10.png)

Normally RB>>(rx + rπ) therefore

Figure 12
Figure 12 (graphics11.png)

Figure 13
Figure 13 (graphics12.png)

In the actual gain with respect to the source , source resistance plays a very important role. Smaller is RS larger is the voltage gain.

HIGH FREQUENCY RESPONSE OF CE AMPLIFIER.

Upper -3dB frequency (fH) is determined by Open Circuit Time Constant Method.

The parasitic capacitances Cπ and Cµ are responsible for the fall in the Voltage Gain Response at high frequencies. The time constant associated with Cπ is the Open Circuit Time Constant τ10 and the time constant associated with Cµ is the Open Circuit Time Constant τ20 .

The overall time constant associated with the amplifier is τH = τ10 + τ20 =

Figure 14
Figure 14 (graphics13.png)

Therefore ωH =

Figure 15
Figure 15 (graphics14.png)
;

Figure 16
Figure 16 (Picture 1.png)

GBP=

Figure 17
Figure 17 (graphics15.png)

Figure 18
Figure 18 (graphics16.png)
Figure 19
Figure 19 (graphics17.png)
Figure 20
Figure 20 (graphics18.png)

Figure 21
Figure 21 (graphics19.png)

If

Figure 22
Figure 22 (graphics20.png)

Figure 23
Figure 23 (graphics21.png)

Figure 24
Figure 24 (graphics22.png)
Figure 25
Figure 25 (graphics23.png)
Figure 26
Figure 26 (graphics24.png)

So we see that base spreading resistance is very important from GBP point of view which is the figure of merit of the given transistor. Also the transit frequency solely depends on the base width. Narrower the base width, faster is the response. But narrow base is detrimental to rx which in turn is detrimental for GBP. The only way out is higher doping of Base which is going to reduce the Injection Efficiency. So the new concept of Si-SiGe-Si heterojunction Bipolar Junction Transistor solves all these contradictory requirements. Here we have very narrow base width of the order of 10nm leading to picoseconds transit time through the narrow base, high base doping thereby reducing base spreading resistance and since base is made of Si-Ge alloy which has a narrower band-gap as compared to that of Si, hence inspite of high base doping high injection efficiency is maintained from Emitter to Base thereby obtaining best short circuit forward active current gain. Thus we have achieved the best of all performance parameters. In this process we have been able to achieve Si-Ge HBT with transit frequency of 200GHz.

High Frequency Response Of CB Amplifier.

Figure 27
Figure 27 (Picture 2.png)
Figure 28
Figure 28 (graphics25.png)

Figure 29
Figure 29 (graphics26.png)

Figure 30
Figure 30 (graphics27.png)
Figure 31
Figure 31 (graphics28.png)
Figure 32
Figure 32 (graphics29.png)

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