Download Project Part 2 Report on Electronic Circuit Analysis II | ELEG 312 and more Study Guides, Projects, Research Electrical and Electronics Engineering in PDF only on Docsity! CPEG322 Project – Part 2 (Due in class on Friday 5/2/08) The audio power amplifier that you have built in part 1 has a relatively low volume compared to the two previous designs you have built before. Why is that? OK, I will answer this question – the circuit in part 1 used an emitter follower configuration that has a gain of 1X. So the peak-to-peak voltage swing from the MP3 player is not being amplified by the power amplifier. Worse than that, the voltage divider formed by the source output resistance of the MP3 player and the input resistance of the power amplifier can have an attenuating effect on the audio signal. On the other hand, the other two audio amplifiers we built used a common-source and differential amplifier configurations –these circuits had a voltage gain of 4X to 8X. So why in the world did we bother building the circuit in part 1? OK, I will answer this question – the circuit in part 1 has a property that its power consumption scales with music volume. When the MP3 player is off, the power consumption is few milli-watts. As the music volume increases, so does the power consumption. In contrast, the other two amplifiers we have built burn the same power regardless of music volume. So these other amplifiers are highly power inefficient at low music volume. How can we fix the problem of low volume? The answer to that is that we can add a separate stage that will do the amplification of the voltage signal. In fact, this is what is typically done in modern audio amplifier systems – they call this additional stage the pre-amplifier. The preamplifier concept is shown below: One simple pre-amplifier topology is a common-emitter amplifier with degeneration as shown below. Because this amplifier does not need to drive a low-impedance (e.g. 8 ohm) speaker, we can design is for much lower power consumption using larger load resistors. The limiting factor becomes speed because if RC and RE become too big they will create poles that will attenuate audio frequencies (e.g. we do not wish this amplifier to attenuate parts of signal spectrum below 10KHz). We would probably want this stage to have a gain of 4X to 8X just as before but with lower power consumption because otherwise it would defeat the purpose of going to a special power-saving power amplifier. How does connect pre-amplifier and power amplifier? The answer is simple – the magic of AC coupling capacitors and resistor dividers (to set bias at input) will do the trick of interfacing the two stages. But that is something for you to think about…
