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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

 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).

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

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

5mV and │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

& T=300K

According to Einstein Eq:

]

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

[Ans: CDret ; 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.

(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.

(5)If

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