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AE_Tutorial 4_BJT incremental model.

Module by: Bijay_Kumar Sharma. E-mail the author

Summary: AE_Tutorial 4_This gives a set of problems on the incremental model of BJT.

AE_Tutorial 4_BJT incremental model.

Small signal model of a BJT

(1) BJT Small signal parameters versus current

Given βo=100,

VA [Early Voltage]=75V,

VCE=10V

Figure 1
Figure 1 (graphics1.png)
Figure 2
Figure 2 (graphics2.png)
Figure 3
Figure 3 (graphics3.png)
Figure 4
Figure 4 (graphics4.png)
Figure 5
Figure 5 (graphics5.png)
Table 1
graphics6.png graphics7.png graphics8.png graphics9.png graphics10.png
1µA 4 x 10-5S 2.5MΩ 85MΩ 3400
10µA 4 x 10-4S 250kΩ 8.5MΩ 3400
100µA 0.004S 25kΩ 850kΩ 3400
1mA 0.04S 2.5kΩ 85kΩ 3400
10mA 0.40S 250Ω 8.5kΩ 3400

(2)Calculate the values of gm, rπ, ro, µf for a BJT with βo=75,VA=60V and Q-point(50µA,5V).

(Answer: 2mS, 37.5kΩ, 1.3MΩ, 2600)

(3)Calculate the values of gm, rπ, ro, µf for a BJT with βo=50,VA=75V and Q-point(250µA,15V).

(Answer: 10mS, 5kΩ, 360MΩ, 3600)

(4)What is the small signal limit of BJT?

[Answer: │

Figure 6
Figure 6 (graphics11.png)
│
Figure 7
Figure 7 (graphics12.png)
5mV and │
Figure 8
Figure 8 (graphics13.png)
│
Figure 9
Figure 9 (graphics14.png)
20% of IC]

We will see that small signal limit of FET is 400mV. This is because BJT is an EXPONENTIAL DEVICE whereas FET is a QUADRATIC DEVICE.

5)Find the transit time for a NPN BJT with base width 1µm and µn=500

Figure 10
Figure 10 (graphics15.png)
& T=300K

According to Einstein Eq:

Figure 11
Figure 11 (graphics16.png)

[Answer:

Figure 12
Figure 12 (graphics17.png)
]

(6)In problem (5) for ICQ=1mA determine CD(diffusion capacitance)for the BJT.

[Ans: CDre=τt ; Therefore CD=16pF]

(3)Give the hybrid-π parameters of a BJT operating at ICQ= 2mA,VCEQ= 5V,βF=40 and

hre=3.83x10-4 = η = Base Width Modulation Factor.

[Answer:

Figure 13
Figure 13 (graphics18.png)

Figure 14
Figure 14 (graphics19.png)
Figure 15
Figure 15 (graphics20.png)

(4) Measurement of low frequency h parameters hie, hre, hfe and hoe at ICQ=0.5mA yield

hie=2050 ohm, hre=5x10-4, hfe=40, hoe=2x10-5,CB output capacitance Cob=3pF and |hfe| at 1MHz is 40/√2; At 290K(VT=25mV) calculate gm, rx, rπ, ro, rµ, Cπ, Cµ.

Answer:

Figure 16
Figure 16 (graphics21.png)

Figure 17
Figure 17 (graphics22.png)
Figure 18
Figure 18 (graphics23.png)

Figure 19
Figure 19 (graphics24.png)

Figure 20
Figure 20 (graphics25.png)

(5)If

Figure 21
Figure 21 (graphics26.png)

Answer:-

Figure 22
Figure 22 (graphics27.png)

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