- #1
Click For Summary
Discussion Overview
The discussion revolves around the presence of voltage and current spikes in an LTspice simulation of an audio amplifier circuit. Participants explore potential causes, including component values and circuit topology, while also discussing related concepts such as crossover distortion and quiescent current.
Discussion Character
- Exploratory
- Technical explanation
- Debate/contested
- Mathematical reasoning
Main Points Raised
- One participant suggests that the spikes may be due to a dead-band and step during each crossover of the output stage, indicating a potential issue with component values rather than simulation parameters.
- Another participant identifies crossover distortion as a cause of the spikes, attributing it to too low output stage quiescent current.
- Concerns are raised about the appearance of base, collector, and emitter currents in the simulation, questioning whether the observed shapes are normal.
- Participants discuss the differences in collector currents between a current mirror circuit and the advanced amplifier circuit, noting that the currents should be out of phase in one case and in phase in the other.
- Suggestions are made to verify the source of oscillation in the operational model and to consider the feedback configuration in the circuit.
- Alternative circuit designs are proposed to address the issues raised, including a schematic that operates in class-A up to a certain output before switching to class-B operation.
Areas of Agreement / Disagreement
Participants express differing views on the causes of the voltage and current spikes, with some attributing them to component values and others to circuit design issues. The discussion remains unresolved regarding the optimal configuration and the specific reasons for the observed behaviors in the simulation.
Contextual Notes
Participants note limitations in the provided schematic clarity and the need for careful measurement and specification of currents, as well as the potential for thermal runaway due to high quiescent current settings.
Who May Find This Useful
Individuals interested in audio amplifier design, LTspice simulations, and the analysis of circuit behavior may find this discussion relevant.
- #2
Baluncore
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Can you please rename a copy of your .asc file to .asc.txt and attach it to your next post.
Then I can run your original file with all the same parameters.
Then I can run your original file with all the same parameters.
- #3
ntetlow
- 21
- 2
ok. thanks
- #4
Baluncore
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@ntetlow
The component values are too blurry to read, but when I follow the schematic in my head with typical values, the topology seems OK. My guess would be that you are seeing a dead-band and step during each crossover of the output stage, that is being fed back and around the loop. I suspect your selection of component values is the problem, rather than the simulation parameters.
The component values are too blurry to read, but when I follow the schematic in my head with typical values, the topology seems OK. My guess would be that you are seeing a dead-band and step during each crossover of the output stage, that is being fed back and around the loop. I suspect your selection of component values is the problem, rather than the simulation parameters.
- #5
ntetlow
- 21
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Thank you for looking and for your info though I'm going to have to look up 'dead band' and 'step' to try and understand what you mean.
I got the schematic from http://www.ecircuitcenter.com/Circuits_Audio_Amp/Advanced_Amplifier/Advanced_Audio_Amplifier.htm which I would expect would work without error. I have run both the schematic and the accompanying netlist with the same result (i'm fed up with finding errors in published documents!)
I got the schematic from http://www.ecircuitcenter.com/Circuits_Audio_Amp/Advanced_Amplifier/Advanced_Audio_Amplifier.htm which I would expect would work without error. I have run both the schematic and the accompanying netlist with the same result (i'm fed up with finding errors in published documents!)
- #6
Jony130
- 626
- 137
Crossover distortion are the cause of this spikes. Too low output stage quiescent current.
Try this circuit:
Try this circuit:
- #7
Baluncore
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Attached is a copy of the original, but for LTspice.
I have not labelled the nodes.
Let me know what problems you find.
I have not labelled the nodes.
Let me know what problems you find.
- #8
Baluncore
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Errata:Jony130 said:
Q10, a PNP type QMPSA56 is cross labelled with itself.
Replace PNP text with QMPSA56
Replace text QMPSA56 with Q10
- #9
Jony130
- 626
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Yes, I notice this. And I reupload the corrected file.Baluncore said:
- #10
ntetlow
- 21
- 2
Audi_amp_bjt_1 seems to have got rid of the spikes (thank you) but why are the base, collector and emitter currents of Q3 so ugly, is this normal? Also, the current shapes of Q4 and Q5 don't look the best either but are beautifully righted in Q6 and Q7, is this too normal?
May I also draw your attention to http://www.ecircuitcenter.com/Circuits_Audio_Amp/Diff_Amp_w_Mirror/Diff_Amp_w_Mirror.htm. this is a preamble to the advanced amp circuit and is used to double the current gain from Q1, Q2 using a current mirror, this seems to work OK. However, if the same is supposed to happen in the Advanced amp, it doesn't. In the mirror circuit the collector currents of Q1 and Q2 are 180 degrees out of phase whereas in the advanced amp circuit they are not. Any ideas why?
May I also draw your attention to http://www.ecircuitcenter.com/Circuits_Audio_Amp/Diff_Amp_w_Mirror/Diff_Amp_w_Mirror.htm. this is a preamble to the advanced amp circuit and is used to double the current gain from Q1, Q2 using a current mirror, this seems to work OK. However, if the same is supposed to happen in the Advanced amp, it doesn't. In the mirror circuit the collector currents of Q1 and Q2 are 180 degrees out of phase whereas in the advanced amp circuit they are not. Any ideas why?
- #11
Baluncore
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@ntetlow
I might be an idea to find the source of oscillation in your OP model. Is the same oscillation present in the Advanced_Amplifier_0.asc that I attached to post 7 ?
The Advanced Amplifier is a learning exercise, not a practical optimised audio circuit.
One problem with Audi_amp_bjt-1.asc is that the feedback is taken from the second last stage, not from the output. Another is that, to reduce crossover distortion, the output stage has a quiescent current of 800 mA, without any current limit to prevent the catastrophic thermal runaway. You need a checklist of parameters to verify, so when you optimise a design, you do not end up with something like 800 mA quiescent in the output stage.
Here is an alternative schematic that operates in class-A up to 5 mA output, then switches on one half of the class-B circuit for higher loads. The base current is balanced to reduce offset voltage. It is not optimised, and was designed to test alternative subcircuits to the Advanced Amplifier.
I might be an idea to find the source of oscillation in your OP model. Is the same oscillation present in the Advanced_Amplifier_0.asc that I attached to post 7 ?
The Advanced Amplifier is a learning exercise, not a practical optimised audio circuit.
One problem with Audi_amp_bjt-1.asc is that the feedback is taken from the second last stage, not from the output. Another is that, to reduce crossover distortion, the output stage has a quiescent current of 800 mA, without any current limit to prevent the catastrophic thermal runaway. You need a checklist of parameters to verify, so when you optimise a design, you do not end up with something like 800 mA quiescent in the output stage.
The collector currents in the mirror should be in phase. The collector currents in the differential pair should be out of phase. You must be careful how you specify and measure currents because currents can be reversed by reversing a component.ntetlow said:
Here is an alternative schematic that operates in class-A up to 5 mA output, then switches on one half of the class-B circuit for higher loads. The base current is balanced to reduce offset voltage. It is not optimised, and was designed to test alternative subcircuits to the Advanced Amplifier.
- #12
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