Lab Three: Audio Amplifier
During this lab the audio amplifier portion of the radio is constructed. This portion is important because the radio speaker requires a strong signal in order for it to be able to be heard clearly. The weak signal will enter the two-stage amplifier and exit the amplifier with a notable gain as discussed below.
Building and Testing The CE Amplifier
To begin, two wires are soldered onto the probes of the 8\(\Omega\) speaker. This now enables the speaker to be connected as the load at the end of the amplifiers to hear the amplified signal.
The speaker is then connectd as the load of the CE amplifier from Lab 2. When the amplifier is fed an input, the output to the speaker is relatively the same, as the speaker is only 8\(\Omega\). When the speaker is connected there is about 600\(mV_{PP}\) verus a 2\(V_{PP}\) when the speaker is not connected. The sound is not extremely loud, but it is certainly audible at this point.
Next the CE audio amplifier is connected to the speaker. When used alone with the speaker, there is almost no gain to be detected as shown in Figure 1. This is due to the small load resistance of the speaker.
Next the CC amplifier shown in Figure 2 is created.
Building and Testing The CC Amplifier
Once the CC ampliefer is breadboarded and debugged accordingy, it is ready for testing. The speaker is now connected to the CC amplifier. Once fed the same waveform from the CE amplifier, the output sound seems to be relativelt the same with no audible gain detectable. The output waveform is nearly identical to that of Figure 2. It was also found that too much power was being dissipated in the 47\(\Omega\) resistor, and three 100\(\Omega\) resistors are used in replacement to dissipate the power more evenly.
Building and Testing The Two Stage Amplifier
Now the two stage amplifier shown in Figure 3 is constructed. Once created, the testing of the circuit ensure. The fist stage is fed a very small(approx. 40\(mV\)) signal. And the speaker is connected to the output of the second stage. The audio from the speaker is now significantly louder. There is a slight bit of noise detected in the signal but overall there is a clear signal coming through. Next the oscillosocope is connected to a 10\(\Omega\) load instead of the speaker. The measurements shown in Table 1 are recorded. The quiescent power dissipation of the amplifier is found to be 1.29 Watts.
\begin{array} {|c|c|c|} \hline Output Amplitude & Input Amplitude & Gain (\frac{V}{V}) \\ \hline 1.24 V_{PP} & 40.59 mV_{PP} & 30.5 \\ \hline \end{array}
The Class AB Push-Pull Amplifier
The class B amplifier (Figure 3) is constructed because it will have much less quiescent power dissipation. This is due to the NPN PNP combination of BJT transistors that narrows the current flow without greatly effecting the opperation of the amplifier. THe power dissipation of this class B amplifier is now found to be 36 mWatts. However, there is significant clipping in this particular configuration due to the lack of biasing on the base of the transistors. This distortion is shown in Figure 4.
This distortion problem can be solved by using diodes to help in biasing the bases. This is called a class AB Push-Pull Amplifier as shown in Figure 5. The output of this amplifier is shown in Figure 6 with significantly less distortion, though a higher power dissipation of 52 mWatts.
CE and AB Push-Pull Generator
Finally, the function generator is connected to the CE Amplifier output. The overall voltage gain of the multistage amplifier is shown in Table 2.
\begin{array} {|c|c|c|} \hline Output Amplitude & Input Amplitude & Gain (\frac{V}{V}) \\ \hline 847.1 mV_{PP} & 20 mV_{PP} & 42.35 \\ \hline \end{array}
This lab has helped greatly in my understanding of the implementation and design of amplifiers. There are many different kinds of amplifiers to choose from, each with their own advantages and disadvantages