Audio Amplifiers
Audio amplifiers, or amplifiers in general, take a small input and scale it. The ratio of the amplitude of the output and input signal is called the gain, and is unitless. Some amplifiers are inverting, meaning that a positive input will have an output of negative amplitude and vice versa, however, in audio amplifiers the inversion makes no difference, as the voice would be the same regardless.
Additionally, amplifiers can be used in series in order to further amplify the signal, making the circuit a multi-stage amplifier. Commonly, only one amplifier with a gain higher than one is used, the second amplifier used is a unity amplifier. A unity amplifier is an amplifier that has a gain equal to or around one. The unity amplifier could be used to invert the signal after it was inverted by the first amplifier or to reduce the noise in the output signal. Demonstrations are shown below.
The amplifiers discussed below are:
Single-stage: Common Collector (CC), B push-pull, AB push-pull, op amp
Two-stage: CE-CC, op amp-AB push-pull
Part 1: Two-stage CE-CC
The Common Collector amplifier is a unity amplifier, it has a gain of a little less than one, usually around 0.9 (V/V). When building the CC amplifier, shown in figure 1, it can be seen in the graph, in figure 2, that the gain is slightly under one.
When replacing the load resistor with a speaker, the sound coming out of the speaker is very low, compared to the CE amp which produced a much louder sound, although with some noise.
As demonstrated previously, the CE amp has a high gain that can be boosted by increasing the load resistance, this works well with CC amp, which has a high input resistance. Which leads to the CE-CC amp being a simple, yet effective two-stage amplifier. After testing the CC amp, it was connected to the CE amp to make the two-stage CE-CC amp. The resulting circuit shown in figure 3.
The input voltage (peak-to-peak) chosen to test the CE-CC circuit was 20 mV and a speaker was connected in place of the load resistor. The sound From the speaker is nearly as loud as that from the single-stage CE amp. Going back to the resistor and using an oscilloscope to see the output signal, the graph in figure 4 was observed.
One important aspect of the two stage amplifiers is that they dissipate more energy than the single stage, in the case of the CE-CC circuit, the power dissipation is 1.8 W.
Part 2: AB push-pull
A CC amp is also called class A amplifier, it dissipates power regardless of the input signal, meaning, if there is no input signal, it would still dissipate power. However, a class B amplifier consists of two transistors, a pnp and npn, they are connected such that only one of them is active at one time. In the class B amp, when the input signal is positive only the npn is active, while the pnp is not, by the same token, the pnp is only active during the negative half cycle of the input, while the npn is not. Which leads to lower power dissipation compared to the CC or CE amps. One drawback of a class B amp is that the output signal is distorted, as shown below.
The distortion in the previous circuit, class B, can be addressed by adding a diode between the input and base of each transistor, as demonstrated below. The distortion in the previous circuit happens due to the transistor requiring the input to reach a certain voltage to reach forward operation mode. Adding the diodes ensures that the voltage going into the base is sufficient to reach forward operation. Such a circuit is called class AB push-pull amplifier.
