Title:

Bipolar Junction Transistor: A Single-Stage Amplifier

Objective:

To demonstrate the concept of large- and small-signal models, to determine the DC biasing, and to determine the small-signal parameters at low frequency, such as the voltage gain, current gain, input resistance, and output resistance of common-collector and common-emitter transistor circuit configurations. Various biasing regions will be observed, with major emphasis on the active region.

Equipment and components:

1 BJT (2N3904); 1 Potentiometer (5 kW); 5 Resistors (100 W, 510 W, 1 kW, 10 kW, 20 kW); capacitor (100 m F), oscilloscope; function generator; DC power supply.

Note:

Analyze (calculate) the circuits first. Compare your experiment measurements with your analysis predictions. Remember to include the resistance of the potentiometer in your analysis. Keep the transistor that you use properly after use, you will need to use it on later labs.

1. Set up the circuit shown below on a solderless breadboard.

2. Set up the common-emitter circuit shown below.

1. Start with only the dc input. Is Q₂ in active mode? Explain.
2. Find the operating point of Q₂ (I_E, I_B, I_C, V_E, V_B, V_C ). Adjust V_I so that V_O = 5 V. Find the operating point again. Also find the b. Is the measured b in the expected range?
3. Add the ac input voltage. Measure the magnitude of the small signal voltage gain (v_o/v_i) and current gain (i_o/i_i) of the circuit.
4. Change V_I to -3 V. What happens to V_O? Explain.
5. Pick one of the active-region operating points, and adjust V_I and V_CC to bias your circuit at this point. Then measure the magnitude of the voltage gain, current gain, input resistance, and output resistance.

[r_out is obtained by applying an ac test voltage on the output, with a capacitor (100 mC) placed between v_o and v_test. v_i is turned off. r_out = v_test / i_test]

6. Let v_i be a triangular wave with an initial amplitude of 0.2 V_p-p. While observing the output on an oscilloscope as the amplitude of v_i is increased, determine the maximum value v_i that results in no observable distortion. Indicate whether the distortion is caused by the saturation region or the cut-off region. To illustrate the distortion, plot v_i and v_o vs. time on the same graph.

3. Set up the common-collector circuit shown below.

1. Start with only the dc input voltage (i.e., v_i = 0). Is Q₃ in active mode? Explain.
2. With V_I = 3 V, and then adjust it so V_O = 2 V, measure the operating point of Q₃ (I_E, I_B, I_C, V_E, V_B, V_C ). Also find the b . Is the measured b in the expected range?
3. Include the ac input voltage. Measure the ac voltage gain (v_o/v_i) and current gain (i_o/i_i) of the circuit.
4. Measure r_in for the circuit by measuring the ac voltage and current at node X.
5. Measure the output resistance looking into node v_o. (Refer to 2(e)).
6. Observing the output voltage on an oscilloscope as the amplitude of v_i is increased from 0.2 V_p-p to 2 V_p-p. Sketch the output for both extremes of the input. What happens at the output for the 2 V_p-p amplitude sinusoidal input? Remember to keep V_I at the value set in (b).