                            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  Second Evaluation  Lesson 7 - Small Signal Amplifier   Lesson Plan    Small signal amplifier    Coupling capacitor    Bypass capacitor    Amplifier analyzing method   Worksheet No. 7   Experiment No. 4  Lesson 8 - Small Signal Amplifier II  Lesson 9 - Small Signal Amplifier III  Lesson 10 - Large Signal Amplifier  Third Evaluation

#### Amplifier analyzing method

From a given amplifier circuit first do -the dc analysis (recall lesson 6) and than do the ac analysis. Fig. 7-2: CE amplifier circuit

DC equivalent circuit

For dc, all capacitors are acting like open switches; therefore we can draw the following dc equivalent circuit: Fig. 7-3: DC equivalent circuit

Now the dc analysis can easily be done: (see Lesson 6)

VB = 1.8V
VE = 1.1V
IE = 1.1 mA
VC = 6.04V
VCE = 4.94V

AC equivalent circuit

For the ac all capacitors are shorted and the dc sources are reduced to zero: Fig. 7-4: Ac equivalent, circuit

The top of the 10K and 3.6K resistors are grounded. The resistors 10K/2.2K and 3.6K/10K are in parallel so we can combine them: Fig. 7-4: Simplified ac equivalent circuit

Now we got a really simple circuit for the ac analysis.

Voltage Gain

One of the most important characteristics for small signal amplifiers is the voltage gain (AV). The lowercase letters are used to indicate ac values. The output voltage is given by:

Vout = ic * rc

The input voltage is given by:

Vin = ie * re

Substitute of these two expressions: Because ic approximately equals ie: AC emitter resistance (re)

The first step in calculating the voltage gain is to estimate the ac emitter resistance (re). (formula derived by using calculus)

This relation applies to all transistors that means it is a universal formula.

Let's remember our example circuit (Fig. 7-4): AC collector resistance

Due to the ac analyzing method we easily get the ac collector resistance (re). See Fig. 7-4:

rc = 2.65KΩ

So now we are ready to calculate the voltage gain: HO: What will be the voltage gain for the following circuit? Fig. 7-5: CE amplifier circuit

Solution:

DC analysis

VB = 3.6KΩ, VE = 2.9 KΩ, IE = 2.9 mA

VC = 9.5V, VCE = 6.6V

AC analysis

rc = 2.65KΩ     