Amplifier Teaching Aid (DED Philippinen, 86 p.) Lesson 8 - Small Signal Amplifier II Lesson Plan (introduction...) Signals in a CE amplifier Input impedance Impedance matching Handout No. 1 Worksheet No. 8

Amplifier Teaching Aid (DED Philippinen, 86 p.)

Lesson 8 - Small Signal Amplifier II

Lesson Plan

Title: Small Signal Amplifier II

Objectives:

- Able to calculate the input impedance
- Understand the purpose of impedance matching

Figure

Signals in a CE amplifier

One characteristic of the CE amplifier is the phase inversion.

See Handout No. 1 (let the students complete)

For a better understanding the different voltage signals are plotted in four graphs:

Graph 1: A small ac signal is applied at the amplifier input.
Graph 2: The small ac signal is topping the biasing dc.
Graph 3: The amplified and inverted ac signal is topping the biasing dc.
Graph 4: The output capacitor C₂ is blocking the dc, the amplified inverted ac signal is applied at the load.

Input impedance

Another important value to analyze is the input impedance of an amplifier circuit.

Recall the ac equivalent circuit in Lesson 7:

(see Fig. 8-1 on the next page)

Fig. 8-1: AC equivalent circuit

R_G is equal to the internal resistance of the signal source. Input impedance looking into the base:

i_e/i_b is approximately equal to b:

z_b = b * r_e

The input impedance of a amplifier stage is the combination of base impedance and biasing resistance:

z_in = R₁//R₂//b * r_e

Ex: What is the input impedance in Fig. 8-1?

re = 22.7 W, b = 100

z_in = 1.8 KW//22.7 W * 100

= 202 W

Impedance matching

When you couple some amplifier stages or connect electronic appliances, the input/output impedance of every stage will effect the efficiency.

Fig. 8-2: Impedance matching

The internal resistance/impedance of the signal source (R₁) and the loudspeaker (R₄) is fixed. Only R₂ and R₃ can be designed to match the impedance.

Voltage optimum

Between signal source and amplifier it is important to transfer the highest possible voltage.

Fig. 8-3: Voltage optimum

In order to get the highest, possible voltage (V₂) let's try several values for R₂.

Ex: V = 10V, R₁ = 100 W

The input impedance of an amplifier should be much bigger than the internal resistance of the signal source.

Power Optimum

Between amplifier and loudspeaker it is most important to transfer the highest possible power.

Fig. 8-4: Power optimum

Ex: Power in the load under different values of R4.

V = 10V, R3 = 8W

P_L = R₄ * I²

Input impedance of the load and the output impedance of the amplifier should have the same value.

R₃ = R₄

R_i = R_L

Handout No. 1

Figure

Worksheet No. 8

Figure

No. 1 a) The generator voltage doubles. What is the input impedance?

b) The generator resistance R_G doubles. What is the input impedance?

No. 2 You like to connect a microphone (R = 100KW) to an amplifier. What should be the input impedance of the amplifier to get a voltage optimum?

Microphone output voltage V_out = 10 mV

No. 3 The output impedance of your amplifier is 6W. What kind of loudspeaker (4W or 8W) do you choose to get the best power transfer?