Lab 2

Common Emitter Amplifier

DC Bias Circuit

In the first step of the lab, the DC bias circuit was created and the Q-point was measured. Figure 1 shows the DC circuit schematic.

Figure 1: DC Circuit Schematic

The following values were measured from the circuit using a DMM.

Vb = 1.84 V

Vc = 5.59 V

Ve = 1.11 V

Vbe = Vb-Ve = 0.73 V

Vce = Vc - Ve = 4.48V

Now, the AC components were added. Figure 2 shows the complete CE schematic that was built for this section.

Figure 2: CE Amplifier Circuit Schematic

Using the function generator and oscilloscope, the gain was measured for different values of load resistance. This data is shown in Table 1 and Figure 3.

Figure 3: Gain Compared with Load Resistance Plot

The plot shown in Figure 3 reveals that as the load resistance drops below around 1k Ohms, the gain drops by 3 dB. When the load resistance is 10 Ohms, which is around the value of the 8 Ohm speaker, the gain drops to an almost unusable 1.3849 v/v. This means that the common emitter amp alone cannot work as an audio amp.

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Bode Plot

Next, a Bode Plot was created for the CE amp. The load resistor was replaced with the default 1k Ohm resistor. The data in Table 2 was then retrieved.

Figure 4: Bode of CE Amp

From inspection, the 3 dB bandwidth is found to be 1.998 MHz.

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Conclusion

In this lab, a CE amp was simulated, built, and tested. It was observed that when the load resistance is low relative to the collector resistor, the gain suffers in magnitude. This is particularly inconvenient because the speaker we are trying to drive is an 8 Ohm speaker. One method to fix this issue would be to create a two stage amp with the second stage being a CC amp. The input resistance to the CC is high and is able to drive a low impedance load.

A bode plot was also generated from the CE amp that was built. From the bode plot, the 3 dB bandwidth of this particular amplifier was found to be 1.998 MHz.

The goals of this chapter were to become familier with gain/bandwidth, become familiar with a CE amp, simulate a CE amp with LTspice, and breadboard and test a CE amp. All of these goals were successfully met.