Week 3/4 - Audio Amplifier
This page will cover the lab meeting from week 3 and 4. During these two meetings, we learned about, built and test a few audio amplifiers. After completing this lab, students should have a good understanding of how these circuits work, and maybe even know which amp they want to use in the final radio build. Now let's look at the amplifiers we learned about.
Two-Stage Amplifier
The first audio amplifier we looked at was the two-stage amplifier, also called a CE-CC amplifier. As the name suggests, it consists of a common emitter amp connected to a common collector amp. As we learned last week, as the load impedance on the common emitter circuit increases, so does the load. While we could go and drop $100 dollars on a 75 ohm speaker, the common collector circuit serves the purpose of increasing our load impedance to increase our gain.
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2 Stage Amp Schematic from Lab Manual (1)
The first thing the lab manual instructs us to do is setup the speaker by soldering wires to the positive and negative eyelets, and then test the speaker by applying an audio wave directly to it. One thing to note, when the speaker is connected directly to the function generator, the output of the generator slightly drops dues to the added impedance. For this lab we will be using an input sine wave with amplitude of 20mV and 1kHz frequency.
After running this signal into the speak sound is produced, but it is not very loud, so let's work on amplifying this input signal!
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First the student is asked to first build the CE portion of the circuit, and see how it sounds. In my experiment, the sound was slightly louder than before, but not by much. This is expected since the load impedance is so low.
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Next, they are to build the CC amplifier separate from the CE and test how that sounds. This one sounded weaker than the CE amp, which is to be expected. The purpose of the CC amp is not to amplify the signal, but to increase the load impedance, so these results are to be expected.
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Now we put these two circuits together to create the two-stage amplifier, and see how that changes the output. One thing to note, we had some difficulties in lab getting the input to properly show on the oscilloscope, so the screenshots are a little off due to the noise and interference from all the equipment running in the lab.
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2 Stage Amp Oscilloscope output
While the input signal isn't appearing properly, the output actually looks pretty good, but since we already know what our input should look like this isn't a big deal. We can see our output is around 1V, which puts out gain from this circuit at 50! You could definitely hear the difference in this too.
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Lastly, the quiescent power was to be measure. This was accomplished by adding a 100 ohm resistor to the Vcc input, and adjusting the power supply voltage until the voltage seen by the circuit was 9V. This voltage drop across the 100 ohm resistor gives us the current drawn in by the circuit which was then used to calculate a quiescent power of 0.99 W.
Class A-B Push Pull Amp
Push Pull Amp Schematic from Lab Manual (1)
The next amplifier circuit we will look at is the Class AB push pull amplifier. One thing to note, Class AB is the schematic shown above with the two diodes. Class B is the same circuit, but with the diodes replaced by shorts.
The benefit of this circuit is that the npn transistor is only on during the positive half cycle, and the pnp is only on during the negative portion of the half cycle. This results in a lower quiescent power. However, in the class B circuit, neither transistor is on when their pn junctions have less the 0.7 V resulting in distortion in the lower voltage parts of the waveform. This issues is fixed in the class AB circuit by adding two diodes, forcing the transistors into an on state, but you are using more power this way.
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Now that we understand this basics of these circuits, let's build them! First the student is to build a class B push-pull amplifier. They are to run the supply voltage through a 100 ohm resistor and adjust the voltage until the circuit side is seeing 9V. This allows us to easily determine the Pq because we can easily calculate the current the circuit is drawing. My circuit measured a Pq of 1.055 W. As expected, there was also some distortion in the output during the lower voltage parts of the input wave.
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Now the student is to repeat this process with the class AB amp. I measured a Pq of 1.08 W, and still got some distortion in my output, but not as much as with the class B. Next we were asked to replace the dummy load with our speaker and do a sound test. The sound was not bad, but it seemed to distort more at higher frequencies.
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Lastly, the student is asked to replace the 2nd stage CC amp with the class AB amp. So know you have a CE connected to the AB Push-Pull. They are then to determine the gain, and do a sound test.
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AB Push Pull output
CE-AB Push Pull amp output
Once again I'd like to reiterate that the oscilloscope was not being very cooperative during this lab, so the screen shot does not perfectly reflect my results, but it shows generally what was going on. I measured an output voltage peak of around 1.6 V with an input voltage of 60 mV resulting in a gain of 26.66. I then did a sound test with an input sine wave of 20 mV and frequency of 1kHz, and it sounding just fine.
LM386 Amplifier
20 Gain LM386 schematic from lab manual (1)
Now we move on to the LM386 amplifier, which is an IC amplifier designed for low voltage consumer applications, and an adjustable gain.
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We were asked to construct an audio amplifier with a gain set to 20 using the above schematic from the lab manual. We were then asked to conduct a sound test, and measure the quiescent power and gain from the amplifier.
20 Gain LM386 output
The Pq was once again measure using the 100 ohm resistor to power supply method, and I calculated a quiescent power of 0.216 W. The gain measured was 24.5, which is slightly higher then the design calls for, but not off by an egregious amount. Overall this amplifier worked quit easily for me despite my TA saying sometimes they just don't work.
Op Amp Audio Amplifier
Op Amp schematic from lab manual (1)
The last amplifier we looked at is the Op Amp, which is another type of IC amplifier. The Op Amp, or Operation Amplifier, amplifies the difference in voltage at a pair of input terminals. It has a very high gain and input impedance, as well as a low output resistance. On thing to note, is that the signal output is an inverted from of the input.
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Students were asked to build an op amp circuit from the schematic shown above, and add the class AB push-pull circuit to it. So the input is taken at the op amp, and the output is measure at the push-pull. We were then ask tp determine the gain a quiescent power of this circuit.
Op Amp - AB Push Pull output vs input
As you can see, this circuit resulted in a really strong gain, about 75, but there is some slight distortion. I measured a Pq of 0.504 W. The sound from this circuit was pretty loud, as you'd expect with that kind of gain.
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In conclusion, this lab was a great exercise in learning about may different audio amplifiers. For me, I feel like the Ce-CC and the Op Amp are the two most likely candidates for being my radio's amplifier. The biggest take away i got from this lab is that with the speaker were using you have got to increase the load impedance to get a nice gain. Overall this was a fun and informative lab that I really enjoyed.
Citations
1. Lab Manual written by Stuart M. Wentworth, 2018