HF filter and trasformer.
After seeing that our MW receiver correctly works (was very surprising and exciting!) we start to re-study the circuit that we used in the MW receiver to obtain a more general knowledge about HF filters.
In class we study the basic circuit: inductance, capacitance, low pass, high pass. After that we revise the concept of Q (quality factor) and we try to extend the knoweldge about the antenna-tuner-band pass filter to other circuit: we used the Thevenin equivalent.
Then we study the "dipole RLC" and a very simple but very useful ciruit called "tank", that is a resonant circuit with a resistance Rs in series with the L.
The osservation that we made after a few step of easy math is that for a particular frequency (the resonance one) and for a quality factor Q>5 we can replace the Rs with a resistance in parallel called Rp: this technics is very very useful when we want to understand the trend of different circuits without making a lot of math.
Jose Maria showed us two possible application of the tank, the first is converting a simply inverting amplifier in a band pass filter, and the second is to obtain a tuned amplifier with a transistor.
Changing topic, we started talking about how to make inductances, the first way is around a toroidal ferrite (we understand also why we use a ferromagnetic material and not simply iron!), the second is around a kernel of ferrite (like our antenna in the MW receeiver) and the third is without ferrite component by only rolled in free air (the value of L is lower but we have less parasite resistance and capacitance). N.B: Nagaoka obtained a sperimental equation to describe this kind of coils.
The natural flow of thought moves from inductors to the magnetic coupling and transformer. The professor advised us not to run in error and to understand very well the difference between the ideal-theory concept of transformer and a realistic way of made them: the different is very deep and we have to take it into account.
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