# Need help for calculation of the AM circuit.

#### guptamridul477

Joined Apr 28, 2024
13
Hey community,
Today my comms professor just randomly gave an AM circuit(simulation file and image attached) and asked me to derive from where did those values of resistor and capacitors came?? He just told me to the derive what did each resistor and capacitor contribute (with calculations). Can anyone help please.

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#### ericgibbs

Joined Jan 29, 2010
19,118
hi gup,
How can your professor have given you this circuit which was posted on AAC a couple of days ago.?

E

#### guptamridul477

Joined Apr 28, 2024
13
hi gup,
How can your professor have given you this circuit which was posted on AAC a couple of days ago.?

E
View attachment 321527
When I discussed about this circuit with my professor and told him that I was'nt able to make a good simulation, he then just told me derive how these values of resistances and capacitances arrived rather than finetuning it in the simulation. I wasn't able to find about it on the net then he told me take help of my friends on AAC. Yes it is the same query but posting in a diff thread coz I thought new thread will give me more responses. Sorry, if I am wrong.

#### sparky 1

Joined Nov 3, 2018
785
Assembling a radio is fun but designing a circuit for a radio takes much more understanding concepts and calculations.
Looking at a miscellaneous schematic, it can be referred to for comparison. See R4 and R7

Starting with BC547 data sheet the IV graph or curves that shows 20mA and 40 mA and so on. But what else is needed on top of that.
Logically you could just choose an operating point in the center allowing adjustment. But saturation or cut off is not good for Oscillation.
If you focus on just the ac and dc load lines, you could choose the collector and emitter resistors. To understand where clipping occurs gives
the designer the known constraints unlike choosing half the supply voltage, there is a smarter method that leads to the teacher's question.
Using an AC diagram instead of the schematic you can better understand the AC Calculations since AC flows differently than DC in capacitors and inductors. The AC diagram takes getting used to, but it is part of the process that the teacher would like you to be aware of.

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#### ericgibbs

Joined Jan 29, 2010
19,118
Hi gup,
I would advise against analysing and submitting your posted circuit simulation circuit results.

As I have already pointed out, it does not work correctly, the Modulated signal is inverted.
It maybe OK for a continuous sine wave, but not for audio.

E

#### MrAl

Joined Jun 17, 2014
11,725
Hey community,
Today my comms professor just randomly gave an AM circuit(simulation file and image attached) and asked me to derive from where did those values of resistor and capacitors came?? He just told me to the derive what did each resistor and capacitor contribute (with calculations). Can anyone help please.
View attachment 321524
Hello there,

Are you do to this all with a circuit simulator or do you have to do some hand analysis of the circuit?
Also, what models are you allowed to use for the transistor?

#### guptamridul477

Joined Apr 28, 2024
13
Hello there,

Are you do to this all with a circuit simulator or do you have to do some hand analysis of the circuit?
Also, what models are you allowed to use for the transistor?
With a circuit simulator. I am allowed to use BC547B

#### MrAl

Joined Jun 17, 2014
11,725
With a circuit simulator. I am allowed to use BC547B
Hi again,

Oh you got it made then. You just have to run the simulator and, I would think, measure the current and voltages in the circuit and try to make sense out of what each part is doing. That's kind of a guess though because I don't entirely understand what this means:
"...derive what did each resistor and capacitor contribute (with calculations)."

If you have to do hand calculations then you will have to dig into the analysis a little deeper. You'll also have to choose a simpler model for the transistor I would think because using a full spice model for hand calculations is not very practical, although you could do it. I assume the simulation uses a full spice model, but if you are allowed to tone that down a little you can use a simpler model and then match your calculations to the simulation. You may have to ask for more information though unless you know what you have to do exactly.

BTW, when I talk about the transistor model, I am referring to the 'level' of the model, not the transistor part number. That's the level of similarity to a real-world transistor where the spice model would be fairly close while a current controlled current source would be a much lower-level model (although still very useful).

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#### sparky 1

Joined Nov 3, 2018
785
It is simple right.
Swagatam showed some of his history as he progressed his MW variation on a reciever, it used the BC547
He also provided a circuit board and parts list. Interesting project, a lot of work,

I started mixing audio and carrier with different small projects. I got interested in the SA602 or NE602 mixer chip after reading
about it in popular electronics. On the bench I was nervous connecting it up so I re-read the datasheet to make sure I did not exceed 200 mV AC maximum. It was popular and it was already designed to work correctly and it did. No impedance mismatch or inverted signals. Later I replaced it with my own home-made mixer. So the learning curve was exploring an already engineered mixer by testing it with my scope according to specs given.
Having built a few kits I had some experience and studied them carefully. I recall messing up a few things changing out the NE602.
Needless to say I could'nt improve the performance. I did not want to modify the resistor bias network for the transistor.
After that adventure I realized getting the impedance match in spec and the operating point optimal would give the best chance of modulating the carrier for peak performance.

Having a working simulation gives test point measurement data allows you see how your circuit is behaving.
You can work your way through the circuit in a logical path one step at a time. The measurement data should be nearly the same on the breadboard.
Ltspice is good for setting up and displaying the specific values of interest at a glance.

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