Lab 2 - Common-Emitter Amplifier

The purpose of Lab 2 was to familiarize us with the common-emitter amplifier and the basics of gain and bandwidth. I became familiar with LTSpice and used it to analyze CE amplifier circuits.

Reviewing BJT circuits was the primary goal of the lab lecture. DC biasing, Q-points, and hybrid-pi models were discussed. In the lab, we built and tested a common emitter amplifier. I began by building the DC portion of the CE amplifier and used the DMM to measure various voltages across the base, emitter, and collector terminals. Obtaining these voltages gave me a good indication that my circuit was assembled properly and that my transistor was working in the forward-active mode.

Figure 1: DC voltages for CE amplifier

Figure 2: 2 breadboarded CE amplifiers. configuration on right at bottom used for a special project

I then added the AC circuit and applied a 1kHz, 20mV AC input signal, carefully observing the polarity of my capacitors. I measured and plotted the gain of the CE amplifier vs load resistance as shown in figure 4.

Figure 3: 1kHz input signal supplied by the BK Precision 4040 function generator

Figure 4: Gain vs resistance

In our lab, we were given special instructions to redesign the given CE amp. The default circuit is adjusted to provide for the cleanest and largest amplitude sinusoidal output possible for a 100 mV input amplitude using parts from the RF kit. By adjusting the circuit, I produced a clean sinusoidal output with amplitude of 4 V in LTSpice, but I was only able to get up to about 2.7V in lab. I added a 10 ohm resistor at the emitter (AC feedback resistor) which causes the Q-point to be stabilized and prevents clipping. I also added a -9V Vee voltage source and increased the load resistance to 100k to balance the output and increase the overall performance of the circuit.

Figure 5: Input and output waveforms for redesigned amplifier

After studying my circuit and trying a few different things, I figured out that I could have maximized the gain in my circuit by completely removing the emitter resistor and bypass capacitor. Also, my circuit would need further adjustments if it were to drive an 8 ohm speaker, which is much lower impedance than the 100k resistor I used as a load. Overall, I am satisfied with my design and will use it to help improve upon my knowledge of amplifier circuit design techniques for use in my radio.