EET2222 – Electronic Devices and Circuits

Lab 6 – Common Emitter Amplifier

1. Calculate the operating (Q) point and voltage gain of the circuit below (see reverse). Measure β for your transistor using the curve tracer. Graph the load line and operating point of the circuit.

2. Set up the circuit above. Note the polarity mark on the 100 and 10 µF capacitors; make sure that you install them properly. Check the operating point to make sure that it is close to your calculated value (include your measured operating point parameters in your report). Now measure the voltage gain of your amplifier at a frequency of 1 kHz using the scope. Make sure that the output signal is not distorted (reduce the amplitude of the input signal to eliminate distortion). Sketch v_in and v_out showing the phase relationship between them (label each axis with suitable units and calibration points).

3. Compare your calculated and measured values of A_v.

4. Vary the frequency of v_in over a range of 10 Hz to 100 kHz, measure the input and output voltages, and plot the voltage gain of your circuit vs frequency. Use the semilog paper provided with frequency as the independent variable (on the horizontal axis). Label each axis with units and suitable calibration points.

Your report is due at the end of the lab period. Please use the green engineering paper.

Calculating the Q-Point and Voltage Gain:

The type of circuit used in this week’s lab is called a voltage-divider biased common-emitter amplifier. As we’ll see in class, this is a very stable biasing method that does not vary appreciably with changes in β.

Analysis of the circuit requires a two part process: First we solve for the DC voltages and currents, including the Q-Point. Secondly, we‘ll solve for AC parameters, such as voltage gain.

DC Analysis	AC Analysis
	, ac resistance of emitter
	, ac resistance of emitter & unbypassed emitter resistor.
	, ac collector resistance
	, ac voltage gain

Example: