Lecture 12 Transistor Bias Section

K-30/AK-710 FM Wireless Microphone Audio Amplifier Radio Frequency Oscillator Radio Frequency Amplifier

Operating Point Analysis and Design

Simple Biasing Assumed VBE and β → IB→IC →VC Comments: IB is sensitive to VBE. IC is very sensitive to β.

Iteration Solution IC=βIBIC=βIB (Kick off iteration with VBE=0.7)

Example 1 IterationVBEIB (uA)IC (mA) 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 ParameterADSIteration VBE674 mV683.4 IB11.3 uA uA IC1.76 mA1.8 mA

Execute Iterations in Matlab

β Variation ParameterΒ=100Β=155 VBE672.1 mV mV IB uA uA IC1.1 mA1.8 mA gm43.6 mS69.2 mS Variation of gm implies that the gain of the amplifier will vary significantly as a function of beta.

Resistive Divider Biasing

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 V BE =R 3 /(R 1 +R 3 )V CC V BE =5.75/( )2.5V=0.631 V I(R1)/IB=110uA/2.10=52.38

Sensitivity to Component Variation Nom.1%5% R2 (KOhm) VBE (mV) IB (uA) IC (uA) Gm (mS) % 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 ∆V BE →Small ∆I C 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 V RE should be > 100 mV.

ADS Simulation IB=12.4 uA IC=1.91 mA Beta=154

Sensitivity to Component Variation Nom.1%5% R3 (KOhm) VBE (mV)0.68 IB (uA) IC (mA) Gm (mS) % error in R2 leads to 1.05 % error in IC. 5% error in R2 leads to 5.7 % error in IC.

Misc.

What if base current is not ignored!

Iterative Method Initial assumption: VBE=0.7 V