1. Lecture 11
Bipolar Amplifiers (1)

2. Output/Input Impedance
It is desirable to maximize the input impedance
and minimize the output impedance of the amplifier.

3. Measurement of Input/Output Imepdance
Not connected to
any external source
Disable the effect
of any input voltage
Source.
Apply a test voltage (Vx)
Measure the resulting current (IX)
Calculate Vx/IX

4. Summary of Impedances Seen at Terminals of a Transistor
(Into the base)
(Into the collector)
(Into the emitter)

5. DC and Small-Signal Analysis
2-step analysis:
DC analysis
Small signal analysis
(Premise: the change in IC due to the signal must remain small)

6. Operating Point Analysis and Design

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

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

9. 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

10. 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

11. 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.

12. Resistive Divider Biasing

13. 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!

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

15. 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.

16. 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.

17. Design Rules
VRE should be > 100 mV.

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

19. 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.

20. Misc.

21. What if base current is not ignored!

22. Iterative Method
Initial assumption: VBE=0.7 V