                            Amplifier Teaching Aid (GTZ, DED; 86 pages)  Preface  Introduction  Lesson 1 - Semiconductor Review  Lesson 2 - Bipolar Transistor  Bipolar Transistor II  First Evaluation  Lesson 4 - Transistor Fundamentals  Lesson 5 - Transistor Biasing  Lesson 6 - Transistor Biasing II   Lesson Plan    Transistor biasing II    VDB analysis   Worksheet No. 6  Second Evaluation  Lesson 7 - Small Signal Amplifier  Lesson 8 - Small Signal Amplifier II  Lesson 9 - Small Signal Amplifier III  Lesson 10 - Large Signal Amplifier  Third Evaluation

#### VDB analysis

Design errors of 5% or less are acceptable, because of resistor tolerances. Fig. 6-3: VDB example circuit

Find the base voltage:

Assumption: Base current is so small that it has no effect on the voltage divider.

5% error - > base current is 20 times smaller than the divider current. VB = I * R2 = 0.82 mA * 2.2KΩ = 1.8V

VE = VB - VBE = 1.8V - 0.7V = 1.1V VC = VCC -(RC * IC) = 10V - (3.6KΩ * 1.1 mA) = 6.04V

VCE = VC - VE = 6.04V - 1.1V = 4.94V

Checking the assumption:

5% error --> The current gain can vary from 30 to 300. Even under the worst case condition the calculation is within the 5% limit, hence the assumption can be done.

Summary of Process and Formulas

 Divider current Base voltage VB = I * R2 Emitter voltage VE = VB - VBE Emitter current Collector voltage VC = VCC - (IC * RC) Coll.- emitter voltage VCE = VC - VE

HO: What will change if the emitter resistor increases to 2KΩ? (unchanged voltage divider) Fig. 6-4: VDB circuit

Solution:

I = 0.82 mA

VB = 1.8V

VE = 1.1V VC = VCC - (RC * IC) = 8.02V

VCE = VC - VE = 6.92V Fig. 6-5: VDB circuit

Saturation point:

Visualize short between collector and emitter

VRC = VCC - VE = 10V - 1.1V = 8.9V

- - > Cutoff point:

Visualize open between collector and emitter

- - > VCE (cut) = VCC - VE = 8.9V

Q-point: VC = VCC - (IC * RC) = 10V - (1.1 mA * 1KΩ) = 6.04V

VCE = VC - VE = 6.04V - 1.1V = 4.94V

Now we plot these values and get the load line and the Q-point: Fig. 6-6: Output curve with load line and Q-point

The values VCC, RC, R1, and R2 are controlling saturation current and cutoff voltage. To move the Q-point is possible by varying the emitter resistance (RC).

Get the Q-point in the Middle of the Load Line

To set the Q-point is a important preparation as you will see later on.

Effect of RE:

RE too large -- > Q-point moves into cutoff
RE too small --> Q-point moves into saturation

Q - point in the middle of the load line:

Half the value of IC (sat) and redesign RE

IC (sat) = 2.47 mA ==> 1.23 mA Look for the nearest standard value:

===> 910 Ω Fig. 6-7: Output curve, Q-point in the middle   