r/ECE • u/iseeyouuu__ • 21d ago
HOMEWORK (GOOD) Can someone help me with a simulation on Ltspice pls🥹
I tried simulating a circuit however I'm not getting the expected output. I would be really grateful if someone helps me simulate that plss. This is the circuit and 2nd pic shows the connection i tried to make. I also tried adding a buffer circuit in between but that didn't work as well.
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u/HumbleHovercraft6090 21d ago
Break the loop. Feed a sine wave at frequency it is supposed to oscillate, check for gain and phase shift at the output. Check whether your bias point is close to what you had in simulation.
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u/PM_ME_UR_PET_POTATO 21d ago
It feels dumb to ask this but how would this oscillate? Isn't your feedback on the first stage phase shifted by only 270 degrees?
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u/iseeyouuu__ 21d ago
Uh from what I know each capacitor resistor pair causes a phase shift of 60 degree so total 180 degree the amplified output already has a phase diff of 180 so 180+180= 360
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u/Automatic_Somewhere2 21d ago
Hmm, I thought there was no phase shift from resistors? A 60 degree phase shift for each pair would imply a -30 degree shift from the resistor and a 90 degree shift from capacitor?
Edit: Nevermind, I just hadn't learned that yet. Pretty neat.
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u/S4vDs 21d ago
Look I’m not that great at electronic analysis yet but stage 2 is a simple amplifier looks about right (didn’t check values). Stage 1 looks like some sort of amplifier whose output is passed through 3hpf and then refed. My question is, everything here is DC, so how will it actually oscillate?
I mean the HPFs do add phase shift but phase shift on dc doesn’t do anything. This might be some different thing I don’t know about and I may be wayyy off but that was just my initial analysis. I believe you’re seeing noise. Could you tell me Vpp of your output?
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u/iseeyouuu__ 21d ago edited 20d ago
I kinda made a change in the entire circuit. Now it is like oscillator,buffer circuit and class AB power amplifier. Oscillator gives 3.4 Vpp, Buffer gives 3.3 Vpp and power amplifier gives 3.2 Vpp. ( Oscillator components values changed as well)A continuous sine wave is obtained from power amplifier. Ig the problem there was due to load on oscillator. Also isn't the voltage from buffer high enough to be better passed to power amplifier rather than rc coupled amplifier which is meant for small signals? Wouldn't I be able to hear the sound through the speaker?
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u/Kush3225m 21d ago
See, what you have is an oscillator and and an amplifier in CE configuration, to drive the speaker you need two things voltage and current, but since the resistance of speaker is usally very less like 8ohms, it doesnt load the CE amplifier so you need a speaker driver circuit which is a cc configuration, i actually made similar project and have the circuit in ltspice, so if you want you can dm and i'll share the pic.
The power from in the speaker should be around 200 mW for audible sound so you need to make the circuit that way
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u/iseeyouuu__ 20d ago
My circuit simulation worked better when I added Class AB power amplifier. But I'm looking forward to improving it a bit more can't call it satisfactory yet
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u/VC_rowdy_farmer 21d ago
I think you should separate the 2 stages to debug this, idk what is oscillating since I see that you are feeding DC source into the output, but try to add a sine wave signal into the input of state 1, see whats at the the output, I've got a feeling the problem lies at the 1st stage since the 2nd stage looks simple enough
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u/iseeyouuu__ 21d ago
Uh the first stage is an oscillator and is meant to give sinusoidal waveform as output by making use of the noise within the circuit
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u/VC_rowdy_farmer 21d ago
huh so thats what those 3 HPFs are for, ur trying to compensate for the 180deg phase shift of the 1st stage itself
So what do you want at the output of the 2nd stage?
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u/iseeyouuu__ 21d ago
Yes
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u/VC_rowdy_farmer 21d ago
So I just went to watch the video you showed in the 1st picture on youtube, have you checked if you got a nice sine wave at the output of stage 1?
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u/iseeyouuu__ 21d ago
Yes oscillator part was working very well
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u/VC_rowdy_farmer 21d ago
well if you get a nice periodic sine wave at the output of stage 1
then the problem must be stage 2, maybe check your connections?those breadboards are notorious for contact issues, maybe some of your components are connected wrong or even not making contact with the metal rails underneath the breadboard
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u/iseeyouuu__ 21d ago
Didn't try it out on breadboard. We have to submit the simulation first and only then we are permitted to implement it. My assumption is that the rc amplifier part causes load on oscillator and oscillation eventually dies
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u/iseeyouuu__ 21d ago
So I tried another circuit on Tinati. Oscillator - buffer circuit - class AB power amplifier. The buffer circuit gives a wave of around 3.6 Vpp. I don't think that signal is suitable for a rc coupled amplifier. Since it is kinda large ,gave it directly to power amplifier. Would that be fine?
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u/VC_rowdy_farmer 21d ago
Its fine, 3.6 Vpp is kinda small anyway, try to simulate that thing first. See how it turn out
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u/iseeyouuu__ 21d ago
I get a sinusoidal output when I enable initial conditions in transient analysis
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u/HumbleHovercraft6090 21d ago
Test the oscillator alone with an 8k load. The base resistors of second stage are probably loading the oscillator IMO. If this is the case, have an emitter follower between the two stages.
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u/iseeyouuu__ 21d ago
Oh ok I'll try that. But for now, I changed the circuit as oscillator - buffer circuit - AB power amplifier as I will have to show the simulation results today. May I dm you to ask a few doubts if you don't mind pls
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u/HumbleHovercraft6090 21d ago
Post it here. Multiple eyes always better.
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u/osama_been_lagging12 19d ago
add a emitter follower ( CC ) configuration amplifier to last stage beofre connecting to load (speaker) . speaker requires good amount of both current and voltage as well
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u/sackbomb 21d ago
You should post the output you're seeing from the simulation as well, and what you expected to see instead.