Lecture 8
Transistor BiasSection 5.1-5.2
K-30/AK-710 FM Wireless Microphone
AudioAmplifier
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=βIB
(Kick off iteration with VBE=0.7)
Example 1Iteration
VBE IB (uA) IC (mA)
1 0.7 11.3 1.8
2 0.6834 11.317 1.8
3 0.6834 11.317 1.8
Assumed: RB=1 MohmsRC=2 KohmIS=6.734e-15Beta=155
ADS Simulation
IB=11.3 uAIC=1.76 mABeta=155
Parameter ADS Iteration
VBE 674 mV 683.4
IB 11.3 uA 11.317 uA
IC 1.76 mA
1.8 mA
Execute Iterations in Matlab
β Variation
Parameter Β=100 Β=155
VBE 672.1 mV
683.4 mV
IB 11.328 uA
11.317 uA
IC 1.1 mA 1.8 mA
gm 43.6 mS 69.2 mS
Variation of gm implies that the gain of the amplifierwill 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 currentin 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
Nom.
1% 5%
R2 (KOhm)
5.75 5.8075 6.037
VBE (mV) 623 626 639
IB (uA) 2.10 2.35 3.59
IC (uA) 229 262 424
Gm (mS) 8.8 10.1 16.3
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 uAIC=1.91 mABeta=154
Sensitivity to Component Variation
Nom.
1% 5%
R3 (KOhm)
6.8 6.868 7.140
VBE (mV) 0.68 0.68 0.68
IB (uA) 12.4 12.5 13.1
IC (mA) 1.91 1.93 2.02
Gm (mS) 73.5 74.2 77.7
1% 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