RF Systems Lab

Lab 2: Common Emitter Amplifier

2.1 Gain and Bandwidth

Amplifiers are characterized by gain and bandwidth, where gain is the ration of the output voltage to the input voltage, and bandwidth is the voltage gain in decibles. The 3-dB Bandwidth occurs when the voltage gain drops to 0.707%, or 3dB, of it's maximum value. The signal generator is used to control the amplitude and frequency of the signal, and an o-scope be be used to observe input and output signals simultaneously.

2.2 Common Emitter Amplifier

This configuration harnesses the capabilities of an NPN transistor and a four-resistor biasing network to uphold the transistor at a designated operating point (Q). Here, it assumes the role of an amplifier, significantly exceeding unity gain. The Pre-Lab report for sections 2.2 and 2.3 can be found below.

2.4 Build and Test Common Emitter Amplifier

The DC portion of the common emitter amplifier is constructed and probed with the DMM to ensure functionality. Vb is found to be 1.79V; Vc is 5.69V; Ve is 1.07V; Vbe is 0.72V, and Vce is 4.61V. These measurements indicate proper operation of the circuit. The AC portion of the circuit is then added in with a 1kHz AC input signal. Figure 1 shows the input and output waveforms for the circuit. Then the load resistance is varried from 10 to 100k ohms to produce the gains found in Table 1. Figure 2 shows the graph of gain vs resistance for the circuit. Since the CE amp will be driving a speaker with 8Ω resistance, it can be expected that performance will be around 2.4V/V, similar to the 10Ω resistance load.

By keeping the load resistance constant at 1kHz and varying the input frequency from 100Hz to 10MHz, the values from Table 2 and the Bode Plot from Figure 7 are generated. Using these values, the 3dB bandwidth is found to be