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How to understand RF circuits.

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.
1672147831586.png

There are very weird circuits like this FM transmitter for example:
1672148328609.png

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.
1672148397233.png

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 voltage 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.
 
Found one from 1971 for fraction of the cost. Is that modern enough. There is one from 74 as well. Seems like it sold well for an english electronics book here. Can get one for like 3USD

Ok those were all sold listings :/ Will find one for sure at some point.
Old ones are fine, the principles haven't changed.
 
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.
View attachment 139738
There are very weird circuits like this FM transmitter for example:
View attachment 139739
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.
View attachment 139740
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 voltage 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.

Base V changes the collector reverse voltage which in turn changes the Ccb,
hence FM modulates a tank circuit.


Hybrid Pi model, fairly good model for use medium RF frequencies....

1672236108221.png

Well CπCπ is mainly determined by the Base-Emitter Capacitance. This is not only the static capacitance between wires etc (which does not change much over biasing) but also the Base-Emitter PN Junction. That PN junction is a diode and its capacitance depends on the voltage across the junction and therefore the biasing conditions.

Same for CμCμ as it sits between Collector and Base and also is mostly determined by the capacitance of the Collector to Base junction. Note that this junction is reverse biased in active mode so the capacitance of that junction is highly dependent on the Base-Collector voltage.


Regards, Dana.
 
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There are downloadable ones on archive.org, I though I posted as link earlier in one of your c=threads?

eg.
I also saw its pdfs while looking one for sale. I bought one nontheless. Got two other random electronics books from the same store. I don't know the year of the one I bought but it was too cheap so it will be nice even if it only covers tubes. Having pysical copy is always the best.
 
I also saw its pdfs while looking one for sale. I bought one nontheless. Got two other random electronics books from the same store. I don't know the year of the one I bought but it was too cheap so it will be nice even if it only covers tubes. Having pysical copy is always the best.

I completely agree.
 
Base V changes the collector reverse voltage which in turn changes the Ccb,
hence FM modulates a tank circuit.


Hybrid Pi model, fairly good model for use medium RF frequencies....

View attachment 139756



Regards, Dana.
It is the varactor diode business all over again. Well it is just two pn junctions instead of a single one anyway. So if you add these inner capacitances with the capacitors of the oscillator, you can write an equation to determine what is the normal frequency and maximum frequency derivation. Only if I had the mental capacity to put all the information together and process it I could have designed an oscillator from scratch. Why everything in Analog realm has to be so difficult to understand. Digital is just some set states and there are no in between. I wish everything were like that.

Looks like the books I bought weren't actually in stock so I had to cancel my order. Payed little more and got the 1967 version of the radio handbook instead since it was the only one other than 57 ones.
Will I ever truly understand all the basics from the movement of electrons to wireless communication. I know I don't need to but the inability of being unable to do so is consuming me.
 
Must be something wrong at your end, the 1980, '81 & '83 ones download fine for me.
Downloads fine for me as well.

The OP might find these books useful from the same site:


The Babani books are usually pretty straightforward to understand.
 
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There it is. It is mostly about tubes and most of the materials in the circuit diagrams are probably unobtainable for me but looks like principles indeed did not change. Wonder if John is ok.
 

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In a nut shell:

Recall that with Class C it only conducts on the positive half cycle of the input signal. And depending on the bias level for a desired conduction angle, the period of duty cycle will vary. Usually, a point contact resistor connected from the ground to the base is used for self-bias, where the charge on the input capacitor due to the input signal creates the bias for the resistor. This idea comes up from the vacuum tube days of using self-bias.

Here is the best video on self-biased Class C amps. It explains the relationship of the input capacitor with the bias resistor. It would be good to watch all of this video and build the circuit in Spice (LTSpice).


For precise bias settings, for particular conduction angles on the positive pulse, the bias needs to be set using a conventional bias voltage divider. Which is more stable when you think about it.

Now to restore the negative half cycle, the signal on the transistor collector needs to go to a resonant tank circuit, where via the principle of the fly wheel action of resonance, restores the negative half cycle.

Your idea of feedback from the collector to base is not a good idea since this can lead to a parasitic oscillation.
 

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