Lecture 12

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## Lecture 12

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**Lecture 12**Transistor Bias Section 5.1-5.2**K-30/AK-710 FM Wireless Microphone**Radio Frequency Oscillator Radio Frequency Amplifier Audio Amplifier**Simple Biasing**Assumed VBE and β → IB→IC →VC Comments: IB is sensitive to VBE. IC is very sensitive to β.**Iteration Solution**(Kick off iteration with VBE=0.7) IC=βIB**Example 1**Assumed: RB=1 Mohms RC=2 Kohm IS=6.734e-15 Beta=155**ADS Simulation**IB=11.3 uA IC=1.76 mA Beta=155**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**Assume the base current is negligible, i.e. negligible compared to the current in R1. IC does not depend on Beta!**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**1% error in R2 leads to 14 % error in IC. 5% error in R2 leads to 85 % in IC.**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**VRE should be > 100 mV.**ADS Simulation**IB=12.4 uA IC=1.91 mA Beta=154**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**Initial assumption: VBE=0.7 V