Lecture 11

Bipolar Amplifiers (1)

Output/Input Impedance

It is desirable to maximize the input impedanceand minimize the output impedance of the amplifier.

Measurement of Input/Output Imepdance

Not connected to any external source

Disable the effectof any input voltageSource.

1. Apply a test voltage (Vx)2. Measure the resulting current (IX)3. Calculate Vx/IX

Summary of Impedances Seen at Terminals of a Transistor

(Into the base) (Into the collector) (Into the emitter)

DC and Small-Signal Analysis

2-step analysis:1.DC analysis2.Small signal analysis(Premise: the change in IC due to the signal must remain small)

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

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