ELEC 3030 Lab 2: The Common-Emitter Amplifier

In this week's lab activity, I dove into the technicalities of the common-emitter amplifier (CE amp). Studying the circuit through simulations in LTspice (Figure 1) before physically constructing the amp in lab, I studied the results and became familiar with the important features of the amp such as the gain, bandwidth, and power dissipation. After completing this very in-depth study, I physically assembled the circuit in lab (Figure 2) and made measurements against which I compared my simulation results.

Figure 1: Common-Emitter Amplifier Circuit

Figure 2: Common-Emitter Amplifier Circuit

Setting the supply voltage Vcc (white wire in Figure 2) to 9 V and the input signal, the AC signal Vin (red wire in Figure 2), to 0 V, I measured the voltage values at the transistor and recorded these values in Table 1. Then, I connected the input signal, the 1 kHz sine wave at amplitude 10 mV, to my amp and analyzed the output on the oscilloscope, as shown in Figure 3.

Table 1: Measured voltage values at transistor

Next, I considered the response of the gain to varying values of resistance for the load resistor (shown in Figure 1). Analyzing the input vs output response of the amp with varying levels of the load resistance, I recorded the values measured by the oscilloscope into Table 2. Then, using the data from Table 2, I created the gain vs resistance plot shown in Figure 4.

Figure 3: Output vs input of CE amp

Table 2: Gain compared with load resistance

Figure 4: Graph of Gain vs Resistance at Load

As evident from the approximate logarithmic trendline, this CE amp will not provide a suitable gain for the 8 Ohm speaker that I plan to use. Next, I replaced the load resistance with the default value of 1 kilo-Ohm to measure the gain of the amp at varying frequencies. Using the formula 20*log(Vo/Vin) to calculate the gain in dB, I measured the voltage gain at the frequencies shown in Table 3.

Table 3: Measured gain at varying frequencies

Using this table, I plotted these points on a Bode plot to view the bandwidth (half-power bandwidth) of the amp (shown in Figure 5) which I calculated as approximately 10 MHz (a much smaller bandwidth than the simulated amp circuit with a bandwidth of 66 MHz).

Figure 5: Gain vs Frequency of CE Amp Circuit

Studying the function of a CE amp in the lab, I thoroughly enjoyed visually observing the amplification via the oscilloscope. I found the circuit's response to changes in resistance values very interesting as well, and it prompted me to think of other beneficial uses of a CE amp (other than amplification through a speaker). Also, the bode plot revealed that this circuit supports amplitude modulation much more than FM because of the wide range of frequencies that the amp will permit. After examining this CE amplifier circuit in simulation and in lab, I am more intrigued in the uses of such devices and how I can use them in my own electronics projects.