28/10/2010 and 02/11/2010

The last stage of our radio receiver.
We had stopped the last time writing about the positive regeneration obtained inserting an another solenoid in the same ferrite kernel of the primary, we obtain an higher amplify and a better selectivity.

I think that to understand better the last necessary parts of our radio receiver i have to make a summary of it:
- first of all we have to build a separating stage;
- second the envelope detector;
- then an audio amplifier;
- finally the transformer fnecessary if we want to connect some speakers.

The need for the separating stage.
We saw in the last lecture how to polarized the transistor to obtain an output voltage in DC of 4,5 V. It's clear that at the output of the transistor we have to insert other circuit for our radio development, but what happen if we directly connect the rest of the circuit in the output terminal of the transisor, obviously we change the value of the resistence and capacitance used for the polaritation.
Therefore we have to build up "something" that separate the rest of the circuit that we will connect to the transistor, in technical language we said that we need to transform our low impedence terminal in an high impedence one.
To make this separating stage we used a common Op-Amp. Correctly Josè Maria spend some time during the lesson to explain to us the carachteristics of these Op-Amp, the most interesting thing is that we have to pay a very high attention reading the specific and understand that we are working in high frequency and therefore the characterist are very very different from what we remember in our last electonic circuit theory.
Finally we understand that we used this Op-Amp in our separating stege for his carachteristic of high input impedance and not for amplify our signal.

Obtaining the original information: the envelope detector.
We have to remember that the original information is in the envelope of the signal that we receive (AM modulation) therefore now that our signal is amplified we need to restore this information. To do it there is a very simple way, we only need a diode a resistence an a capacitance.
First of all we revised the way how to obtained a continuos corrent from a sinusoid, therefore we understand better how this work also in the laboratory (trying different diode -Si or Ge-and different value of resitance).
We have to note that in a sense we need a circuit that follow the trend of the voltage (the envelope of our signal) and therefore we understand that there are some upper (to avoid diagonal distorsion) and lower limits (for the portant) in the value of the resitance.
We went to the laboratory another time and we remain surprise how good is the envelope detector used to recover a voice signal.
(Note that this receiver and this envelope detector works fine with no fast transient music).

The last step: the audio amplifier and the transformer.
In the last part of the today lesson we saw the audio amplifier need to connect the speaker. We have to understand that now we are at audio frequency (20hz - 4khz) therefore we use an active amplify TL081.
We saw the limitation of this amplify in term of current in output and after a few analysis we arrive at the conclusion that we can't connect directly at the output terminal of the amplify the commerical speakers that have a typical value of impedence of 8 Ohm.
Josè Maria showed us a very clever (but commercially expensive) solution to connect the speaker, this idea is based on the transformer, but we have to observed that we need a very high inductance value therefore it can't disturb our circuit, to obtained this high impedance we have to make a lot of coils (with the same number ratio).