Fluffyboii
Active Member
Hi, I kind of don't understand circuits that operate at frequencies like FM range or more. I look at radio circuits, transmitters and see LC tank circuits instead of resistors to make an class A amplifier for example. When I try to replicate on LTSpice I get inconsistent results with these types of circuits. It is probably because I never really understood how to calculate AC currents and voltages and memorized my way through it. Also stuff like regenerative receivers with feedback and such seem to be impossible to understand with rigorous way.
For example Class C amplifiers doesn't make sense to me since it will short to ground at DC (I get that this one at least has an resistor to prevent it). And there is no bias for the transistor itself. Maybe it is assumed that it is supposed to be used with regular AM signal so that it will have some DC bias but that would make it very unreliable. Testing it in LTSpice confirms my suspicion since it doesn't seem to work. When I add feedback from collector to base with a resistor to possibly turn it into Class A amplifier it outputs an distorted amplified signal. But it is impossible to set the Q point so that it is not distorted even with adding an emitter resistor it is always very bad at the output. Maybe the filtering at output supposed to fix it.
There are very weird circuits like this FM transmitter for example:
C9 and C10 should act like an voltage divider. Why is it decreasing the input signal before amplifying it. How bias voltages are determined here. How does the oscillator change its frequency exactly according to input signal.
This one looks similar as well. I understand that first stage is just an amplifier and second is an oscillator but how exactly that oscillator changes its frequency according to the voltage at base. I would expect it to amplitude modulate it instead of changing its frequency.
I read that it is possible to demodulate FM by taking input signal's derivative first than using an envelope detector to approximate message signal. I know the basic op amp circuit to take derivation but idk how to do something similar with an BJT. I saw a tuned RC filter can approximate derivation. I couldn't find a circuit that uses this unreliable way of demodulating FM but I am sure it can be done with 2 transistors or something like that since people seem to be able to use a single transistor for like 4 things at the same time. How can that be possible anyway? Like how can it both amplify audio frequencies and demodulate AM in a Reflex receiver. The simple analogy of thinking it as an valve that is controlled withvoltage current doesn't apply there.
It is just so bizarre. Just when I think I understand something another black box of an circuit appears.
I learned that RF course in my collage just teaches drawing RF integrated circuits in Cadence. I know I can't get anything more than C of a class of that caliber but should I give it a try. It is probably only using mosfets in ways it is not applicable in discerete designs though.
For example Class C amplifiers doesn't make sense to me since it will short to ground at DC (I get that this one at least has an resistor to prevent it). And there is no bias for the transistor itself. Maybe it is assumed that it is supposed to be used with regular AM signal so that it will have some DC bias but that would make it very unreliable. Testing it in LTSpice confirms my suspicion since it doesn't seem to work. When I add feedback from collector to base with a resistor to possibly turn it into Class A amplifier it outputs an distorted amplified signal. But it is impossible to set the Q point so that it is not distorted even with adding an emitter resistor it is always very bad at the output. Maybe the filtering at output supposed to fix it.
There are very weird circuits like this FM transmitter for example:
C9 and C10 should act like an voltage divider. Why is it decreasing the input signal before amplifying it. How bias voltages are determined here. How does the oscillator change its frequency exactly according to input signal.
This one looks similar as well. I understand that first stage is just an amplifier and second is an oscillator but how exactly that oscillator changes its frequency according to the voltage at base. I would expect it to amplitude modulate it instead of changing its frequency.
I read that it is possible to demodulate FM by taking input signal's derivative first than using an envelope detector to approximate message signal. I know the basic op amp circuit to take derivation but idk how to do something similar with an BJT. I saw a tuned RC filter can approximate derivation. I couldn't find a circuit that uses this unreliable way of demodulating FM but I am sure it can be done with 2 transistors or something like that since people seem to be able to use a single transistor for like 4 things at the same time. How can that be possible anyway? Like how can it both amplify audio frequencies and demodulate AM in a Reflex receiver. The simple analogy of thinking it as an valve that is controlled with
It is just so bizarre. Just when I think I understand something another black box of an circuit appears.
I learned that RF course in my collage just teaches drawing RF integrated circuits in Cadence. I know I can't get anything more than C of a class of that caliber but should I give it a try. It is probably only using mosfets in ways it is not applicable in discerete designs though.