6.6. AMPLIFIER

The inputsignal connected to the so called amplifier has a very tiny power compared with the output power. The amplifier has two terminals for the powersupply where the additional energy is delivered into the circuit and an output where the signal is produced which has the same shape but a bigger “size” (energy) than the input signal.

fig. 40

Using all the blocks which we came up to here we can now achieve a radio which would give us a reasonable sound also for stations which are not very near.

The result of the construction of a radio in that way would be a very simple radio receiver like it was built soon after the technology for amplifiers had been invented (the basics for this technology were the VALVES).

But very soon it was found that this simple construction had always a very high distortion, which was due to two reasons: So there was only one amplifier it was necessary in order to achieve a signal which was strong enough to have a very high AMPLIFICATION (the signal had to be enlarged very much in a simple amplifier circuit - called STAGE). Therefore it was necessary to have a signal as big s possible coming from the tuned circuits with a bad SELECTIVITY, which means they could not filter very exactly.

CONSEQUENCE: there are always radio stations transmitting on a carrierwave whose frequency is near to the frequency of the desired station, which reached the level of AUDIBILITY.

If a single amplifier stage has to amplify which very high amplification, it tends to produce oscillations itself, therefore it produces distortion itself. So the next step was to built in a second amplifier, but now one which was amplifying the still modulated signal appearing just out of the tuned circuit. This amplifier was called the RF-amplifier. The result was the a so-called TUNED FREQUENCY RADIO RECEIVER or TRF-receiver which was used from about 1930 till 1950.

fig. 41