RF Systems Lab

Lab 3: Audio Amplifier

3.1 Buffer Amplifier

A buffer amplifier can be realized with a BJT through a common-collector configuration. This type of amplifier is also referred to as an emitter-follower, since the output on the emitter ideally follows the input signal on the= base (that is, the gain is about 1). Notice the input has a 1 V amplitude; about a volt or two is the typical value we’d like to have at our speaker for a loud output. However, these signals can become very distorted. A class AB amplifier solves the distortion problem, but doesn’t have as high efficiency. Adding a pair of diodes in the base bias string ensures that there is just sufficient voltage to turn on the pair of transistor pn junctions, and the output signal is far less distorted. The pre-lab report for section 3.2 and 3.4 can be found below.

3.3 Build and Test the Two-Stage Amplifier

The speaker is prepared by soldering leads onto the positive and negative connection points. The speaker was verified to function, albeit quietly, by connecting it directly to the function generator.  Then, the speaker was attached to the CE amplifier, producing a clean signal. The addition of the CC amplifier distorted the sound a little bit, but not to an inaudible quality. However, when  the oscilloscope was used to view the output signal, the signal became incredibly distorted. This is believed to be due to an issues with the oscilloscope, as the circuit functioned properly when it wasn't being probed. Figure 1 below shows the output signal, while Figure 2 shows the signals from the two-stage amplifier.  Table 1 shows the amplitudes and gain of the circuit, and the quiescent power is found to be 429.5mW.

3.4b Build and Test the AB Push-Pull Amplifier

The addition of two diodes turns the two stage amplifier into an AB push-pull configuration. The output of the circuit becomes shakier, but cleaner, and the result is a clear, but quite sound. The amplitude and gain for the new circuit is denoted in Table 2, with the amplification output shown in Figure 3.

3.5 Chip Level Design

Sections 3.5 and 3.6 of the lab call for experimentation with the LM386 Op-Amp. However, since this chip was not included in the ordered lab materials, this portion of the lab was bypassed.