Radio Principles

No4. Modulator

The modulator is something of a misnomer here as it is in fact a power amplifier and a component of the modulated amplifier wherein the actual modulating takes place. It is worth noting the difference between a voltage and power amplifier at this point.
In the previous amplification stages we have been concerned with passing the maximum voltage from stage to stage. In an audio system we would have to match the last stage to the impedance of the speaker; this could be 4, 8, 16, 32, 64 ohm or in the case of a high impedance one hundreds of ohms. There are also non-standards. The maximum power transfer theorem states that a source delivers maximum power to the load when the output impedance of the source equals the input impedance of the load. Consider the diagram below:

The power passed from the source to load consists of the voltage and the current. If you reduce Zout and Zin (the output impedance of the source and the input impedance of the load) to a simple series impedance circuit, then the current must be the same value at any point. The two impedances divide the voltage, and can be treated in the same way as any other voltage divider. Therefore, if the two impedances are equal, the power delivered to the load must equal the power in the source.

If Zout is greater than Zin, then more power will be in the source than load, if Zin is greater than Zout, more in the load. In either case a very poor audio signal will be received and if too much power is in the amplifier it may overload and burn out. In our case we wish to match the power amplifier with a modulator and are further burdened with a highly varying impedance dependent on the frequency of audio. If we measure impedances at a standard, say 1 kHz, we can use that as our benchmark and match the two to that. Usually the output impedance of the source will be higher than that of the load. The simplest way is to match through a transformer, in which the impedance of the primary coil is approximately equal to that of Zout, and the secondary that of Zin. This way is usually preferred because a transformer is extremely efficient, can handle high power and seldom if ever breaks down. In this way we could now attach our modulator to the modulated amplifier, and marry the carrier generated and audio signal. We discuss this on the next page.