​

​

​

​

​

 

2.1 Gain and Bandwidth 

​

In this section the student is introduced to the concept of gain and bandwidth. The gain is characterized as the ratio of output voltage divided by input voltage, A(f) = Vout/Vin. This is plotted on a bode plot in units of dB, A(dB) = 20log(A(f)). From this you can determine the mid-band gain. When the gain drops to -3 dB of the max gain that defines the high and low frequencies of the mid-band gain. The bandwidth is the defined as the difference between the high and low frequencies. 

​

2.2 Common Emitter Amplifier

​

The premise of this circuit is to bias the NPN BJT in the forward active region, resulting in a nice current gain. To accomplish this a 4-Resistor biasing network to stabilize the Q-point in the center of the forward active region. Then the AC signal in series with a coupling capacitor is added as the input to the circuit. Another capacitor is added in parallel with the emitter resistor to allow the AC signal to pass through. This is referred to as the bypass capacitor. 

​

2.3 LTspice Simulation of the CE Amp

​

Next the student is to use the LTspice simulation software to construct the CE Amp. The manual walks them through the basics of this software. Then the student is asked to build and simulate their design.

​

2.4 Build and Test a Common Emitter Amplifier

​

Now the student is to begin constructing their circuit in the lab. They begin by bread boarding the DC portion of the amp and take various voltage readings, which are shown in table 1. This is to make sure the transistor is operating in the forward region. After confirming this, the AC components are added, including a 1 kHz AC sine wave input signal. They are then to display both the input and output signal on the oscilloscope, shown in figure 2. 

​

The gain I got from my circuit was around 43, or 32.69 dB. This was with a load resistance of 1 K Ohms. Then we were asked to test out the circuit with various load resistance, and see how this effected the gain. From this I observed that at lower resistances the gain was low (RL = 10 ohms, Gain of 6) and as the resistance increased, so did the gain (RL = 100 k Ohms, Gain of 60). Considering that we will be using an 8 ohm speaker, this means that this circuit has some room for improvement. 

​

Lastly, the student is asked to replace the load resistor with the original 1 k Ohm resistor, and then test the gain out at different input frequencies. From this readings, we are to plot the result on a bode plot and determine the half power bandwidth. This results are shown in figure 3. Sorry you have to look at my horrible handwriting. 

​

In conclusion, this lab did a good job introducing students to the CE Amplifier, as well as develop so other skills to further their understanding. Upon completion of this lab, students should be familiar with the functions of a CE amplifier, how to build and simulate a simple circuit in LTspice, and become more comfortable operating the lab equipment. 

​