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# Lecture 8 - PowerPoint PPT Presentation

Lecture 8. Transistor Bias Section 5.1-5.2. K-30/AK-710 FM Wireless Microphone. Radio Frequency Oscillator. Radio Frequency Amplifier. Audio Amplifier. Operating Point Analysis and Design. Simple Biasing. Assumed VBE and β → IB→IC →VC. Comments: IB is sensitive to VBE.

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### Lecture 8

Transistor Bias

Section 5.1-5.2

Oscillator

Amplifier

Audio

Amplifier

Assumed VBE and β → IB→IC →VC

IB is sensitive to VBE.

IC is very sensitive to β.

(Kick off iteration with VBE=0.7)

IC=βIB

Assumed:

RB=1 Mohms

RC=2 Kohm

IS=6.734e-15

Beta=155

IB=11.3 uA

IC=1.76 mA

Beta=155

Variation of gm implies that the gain of the amplifier

will vary significantly as a function of beta.

Assume the base current is negligible,

i.e. negligible compared to the current

in R1.

IC does not depend on Beta!

VBE=R3/(R1+R3)VCC

VBE=5.75/(5.75+17)2.5V=0.631 V

I(R1)/IB=110uA/2.10=52.38

1% error in R2 leads to 14 % error in IC.

5% error in R2 leads to 85 % in IC.

If R2↑→VP↑→IE↑→VRE↑→small ∆VBE

→Small ∆IC

An error in VX due to inaccuracies in R1, R2 and VCC is absorbed by RE, leading to a smaller change in VBE.

VRE should be > 100 mV.

IB=12.4 uA

IC=1.91 mA

Beta=154

1% error in R2 leads to 1.05 % error in IC.

5% error in R2 leads to 5.7 % error in IC.

Initial assumption: VBE=0.7 V