231 COMMON EMITTER TRANSISTOR AMPLIFIER

OBJECTIVES:

1. To design a common emitter transistor amplifier.
2. To assemble the circuit and measure the gain-frequency characteristics.

RELEVANT DATA:

Data of the amplifier:

V_CC = 12V, R_L = 4.7kW, Voltage gain (A_v) = 20,

Output voltage = 2V_p-p, Input voltage = 100mV_p-p,

Small signal frequency = 5kHz.

Data of the components:

1. Transistor:

BC109 silicon NPN general purpose

Maximum Ratings:

Collector – emitter voltage (V_CEO max) 30V

Collector current (dc) 100mA

Power dissipation at 25°C 300mW

h_FE/h_fe

Min 200

Typ 450

Max 800

h_{ie =} 1kW (Typ)

2. Resistors:

Resistor values available are from E12 series (ie 12 values per decade as 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82).

Eg: 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82 Ω

100, 120, 150, 180, 220, 330, 390, 470, 560, 680, 820 Ω

1.0, 1.2, 1.5, 1.8, 2.2, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2 kΩ etc

3. Capacitors:

4.7μF and 10μF capacitors are available for the design.

PROCEDURE:

Part A: Design of the Amplifier

· Use the available resistors. For example if you get 0.9kΩ value for a resistor after calculations, you will have to use the nearest available resistor of value 1kΩ or 820Ω.

· Use the typical value of h_FE and h_fe for calculations.

Step 1: Let the quiescent collector current be 5mA. Hence let the dc load line to cut the y-axis (collector current) at 10mA. This enables us to operate the transistor nearly in the middle part of the dc load line. Taking this value (10mA) and the voltage gain (A_v = R΄_L/R_E where R΄_{L =} R_C//R_L) into account, find suitable values for R_C and R_E. Check whether (1 + h_fe)R_E >> h_ie and confirm the validity of A_v = R΄_L/R_E.

Step 2: Draw the Thevenin Equivalent circuit of the biasing arrangement which contains R_1, R₂ and Vcc. Let R_b/R_E = 5 where R_b = R₁//R₂ and select suitable resistors for R₁ and R₂. Note that the collector current is 5mA and you may assume that V_BE = 0.7V.

Step 3: Find the values of capacitors such that their impedances are negligible compared with the series resistances with them. In the case of output, R_L is the resistance in series with the output capacitor and in the case of input you will have to find the input impedance of the amplifier by using the ac model.

Step 4: With the selected resistors, determine the new operating point. Draw the ac load line and obtain the maximum undistorted peak-to-peak swing of the output voltage and check whether it is larger than your requirement (2V_p-p).

Part B: Measurement of Amplifier Gain-Frequency Characteristics

Assemble the circuit. Before connecting the dc power supply (12V), check whether the connections are correct.

Connect the signal generator through a 2.2kΩ resistor to the input of the amplifier. This will prevent you from applying a large base voltage that will damage the transistor. Using the CRO adjust the signal generator output until the input becomes 100mV_p-p at 5kHz (observe the waveform before the input capacitance and after the 2.2kΩ resistor).

Observe the amplifier output (across R_L) on the CRO using the second channel. Measure the amplitude of the output signal. Vary the frequency while keeping input amplitude constant at 100mV_p-p and measure the output amplitude for different values of frequency from 10Hz to 1MHz (10Hz, 30Hz, 70Hz, 100Hz, 300Hz, 700Hz, 1kHz, 3kHz, etc.). Plot gain Vs frequency using the log scale for both gain and frequency [log₁₀(gain) Vs log₁₀(f)] and hence determine the bandwidth of the amplifier.

Figure 01: Common Emitter Transistor Amplifier