1. Transistor Bias Section 5.1-5.2
Lecture 8
Transistor Bias
Section

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

3. Operating Point Analysis and Design

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

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

6. Example 1 Iteration VBE IB (uA) IC (mA) 1 0.7 11.3 1.8 2 0.6834 11.317
Assumed:
RB=1 Mohms
RC=2 Kohm
IS=6.734e-15
Beta=155

7. ADS Simulation Parameter ADS Iteration VBE 674 mV 683.4 IB 11.3 uAIC
1.76 mA
1.8 mA
IB=11.3 uA
IC=1.76 mA
Beta=155

8. Execute Iterations in Matlab

9. β Variation Parameter Β=100 Β=155 VBE 672.1 mV 683.4 mV IB 11.328 uAIC
1.1 mA
1.8 mA gm
43.6 mS
69.2 mS
Variation of gm implies that the gain of the amplifier
will vary significantly as a function of beta.

10. Resistive Divider Biasing

11. 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!

12. Negligible Base Current
VBE=R3/(R1+R3)VCC
VBE=5.75/( )2.5V=0.631 V
I(R1)/IB=110uA/2.10=52.38

13. 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.

14. 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.

15. Design Rules
VRE should be > 100 mV.

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

17. Sensitivity to Component Variation
Nom. 1% 5%
R3 (KOhm)
6.8
6.868
7.140
VBE (mV)
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.

18. Misc.

19. What if base current is not ignored!

20. Iterative Method
Initial assumption: VBE=0.7 V