Common-Emitter Amplifier
A Common-Emitter (CE) Amplifier is a common and easy way of building amplifiers. There is plethora of ways to make an amplifier, which is a device or circuit that scales up, or down, an input voltage. The ratio of the input and output is referred to as the gain, it has no unit, but is usually represented to have the units (V/V) or dB after some conversion. In this lab, a Bipolar Junction Transistor (BJT) is used to build the amplifier, in particular, a 2N3904 npn BJT.
For the purposes of this lab, the input voltage going into the amplifier is an AC voltage. However, a DC operating point or a quiescent point (Q-point) needs to be established in order to stabilized the biasing circuit. Some of these parts include R1, R2 and Re which are added for voltage biasing and circuit stabilization, Ce (commonly known as the bypass capacitor) which reduces the voltage drop across Re, thus lowering the gain drop caused by Re.
Part 1: Basic CE Amp
Building the DC CE amp, shown in figure 1 and measuring the values of the circuit, the resulting values are presented in table 1. While the values seem like they do not serve much purpose, as more components are added later, this particular circuit represents the DC portion of the circuit and is crucial in finding the Q-point. The Q-point is important for the biasing of the circuit, as well as, the optimum forward voltage.
Table 1: list of simulated and real values of the circuit shown in figure 1
Figure 1: DC part of the CE amp
After measuring the needed values from the previous circuit, the AC components were added to the circuit to get the circuit shown in figure 2. The input signal into the amp has a 10 mV amplitude (20 mVpp) and a frequency of 1 KHz, the input signal was generated using the function generator discussed in previously. Then, the oscilloscope was used to read the input signal and the output signal indicated in figure 2. Figure 3 shows the input and output signals, input in yellow, output in blue.
To see how the load resistor affects the gain, table 2 shows several values for the load resistance and how the gain is affected by it. Additionally, figure 4 provides a visual representation of how the load resistance affects the gain. Based on the results, it is expected that when the speaker (8 Ω) is connected to the circuit in place of the resistor, the gain will drop even lower than this.
Table 2: List of different resistances used and their gain
Part 2: Bode plot
Going back to the circuit presented in figure 2, with a load resistance of 1 kΩ the bode plot is found by changing the frequency at which the input signal operates. Table 3 shows the frequencies and their respective gain, while figure 5 shows the bode plot found from such values.
Table 3: List of different frequencies tested along with their respective gain
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Figure 5: Bode plot of the CE amp, gain (dB) vs. frequency (Hz)