The goal of this lab was to become familiar with, analyze, and construct several audio amplifiers. These include the Common-collector (CC) amp, the CE-CC two stage amp, the Class AB push-pull amp, the CE-class AB two stage amp, Op-amp audio amplifier, Op-amp-class AB two stage amp, and LM386 audio amp. All of these are meant to boost an audio signal to drive a speaker. During this lab, I learned that some things to take into consideration when choosing an audio amplifier are gain, distortion, and power consumption. Gain controls loudness of the speaker, distortion is important in sound quality, and an excessive power requirement will shorten battery life.
The first amplifier I became about in this lab is the buffer amp. They are often placed between the amplifier and a low resistance load, ideally have unity gain, and have high input impedance. In lab 2 the CE amplifier gain dropped considerably as the resistance decreased revealing that it would perform poorly with the 8 ohm speaker as a load which is why the high input impedance of the buffer amp is important.
As expected, the 8 ohm speaker made a very low sound when connected to the CE amp alone in this lab.
The CC amp is a unity gain amp that can be used as a buffer amp. The design used in this lab had a gain of about 0.9 V/V in simulation. The current through the transistor, and therefore the power requirement, can be reduced by increasing the resistance at transistor's emitter at the cost of max gain.
By connecting the output of a CE amp to the input of the CC, you get a CE-CC two stage amp. With the CC acting as a buffer, the simulated gain is about 54 V/V and distortion occurs around 20 mV for the design of this amp I used. In lab testing of this amp only provided about 25% (13.2 V/V) of the gain in simulation, but the speaker was still significantly louder than when powered by the lone CC amp. The simulated quiescent power for this amp was 1.23 W.
The Class AB push-pull amp is another unity gain amp that can be used as a buffer. In simulation, it dissipates 41.6 mW and gain of about 0.85 V/V. Lab tests gave similar results of 45 mW and .71 V/V. When acting as a buffer for the CE amp, the combination has a gain of about 30 V/V in simulation and a quiescent power of 116.6 mW. The gain was measured to be about 25 V/V in lab. The speaker gave off a clean, loud sound when driven by this CE-Class AB amp.
The Op-amp is like the CE amp because the gain decreases when driving a low impedance load, but its gain is proportional to -R2/R1 in the inverting Op-amp schematic. Using the Class AB amp as a buffer, the two stage Op-amp circuit achieved a gain of about -320 V/V with an R2 of about 500K and R1 of 1K. The speaker output a clear sound that was nice and loud. However, the Op-amp requires two sources rather than just one as the others do.
The LM386 amp is able to work as a stand-alone amp. It needs no buffer, but it produces a noisy signal. Its gain is theoretically 20 V/V, but only measured to be about 16 V/V in lab. Still the sound comes through the speakers loud and clear, but it does have sound distortion if the input is too high or too low.
