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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

Lecture 8

Transistor Bias

Section 5.1-5.2


K 30 ak 710 fm wireless microphone
K-30/AK-710 FM Wireless Microphone

Radio Frequency

Oscillator

Radio Frequency

Amplifier

Audio

Amplifier



Simple biasing
Simple Biasing

Assumed VBE and β → IB→IC →VC

Comments:

IB is sensitive to VBE.

IC is very sensitive to β.


Iteration solution
Iteration Solution

(Kick off iteration with VBE=0.7)

IC=βIB


Example 1
Example 1

Assumed:

RB=1 Mohms

RC=2 Kohm

IS=6.734e-15

Beta=155


Ads simulation
ADS Simulation

IB=11.3 uA

IC=1.76 mA

Beta=155



Variation
β Variation

Variation of gm implies that the gain of the amplifier

will vary significantly as a function of beta.



Back of the envelope calcuation
Back of the Envelope Calcuation

Assume the base current is negligible,

i.e. negligible compared to the current

in R1.

IC does not depend on Beta!


Negligible base current
Negligible Base Current

VBE=R3/(R1+R3)VCC

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

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


Sensitivity to component variation
Sensitivity to Component Variation

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

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


Biasing with emitter degeneration
Biasing with Emitter Degeneration

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.


Design rules
Design Rules

VRE should be > 100 mV.


Ads simulation1
ADS Simulation

IB=12.4 uA

IC=1.91 mA

Beta=154


Sensitivity to component variation1
Sensitivity to Component Variation

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

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




Iterative method
Iterative Method

Initial assumption: VBE=0.7 V


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