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Op Amp Circuit Gain Does not Match LTSpice Simulation

epsolutions

Sep 7, 2019
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I created an amplifier in LTSpice as shown in the diagram and asc. file below. It is intended to accept a line level audio signal from a USB audio interface, and increase the voltage and current to drive a coil. The simulation indicates an output swing of about 0-9Vpp (blue trace). This is consistent with the parts and circuit topology. However, when constructed (twice from new parts), the circuit's gain only achieves half of that. IOW the output swings from 0V to only 4.5V, at which point it begins to clip if the gain is increased. I am aware that the offset and gain adjustments are interact, but I have tried all possible settings.

I am not sure what is happening. Judging by the sim plot, LTSpice wants to double whatever voltage I enter into V3. Maybe I am not using the correct analysis term? In the plot the 2.3V bipolar signal from the interface (green trace) is shown as reduced to 1.15V (purple) at pin 3 of the LM358.

1. Can anyone please help me understand why the built circuit's output voltage is low?

2. Why is this lack of gain not reflected in the simulation?

I posted on another forum but did not receive a detailed answer. I have tried to make my question here as clear as I can, in the hope someone can point me to the mistake.
 

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  • Op Amp Emitter Follower.png
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  • Op Amp Emitter Follower CURRENT.asc
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Bluejets

Oct 5, 2014
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Perhaps your "9v power suppply" ( not shown) is not up to the job.
 

epsolutions

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Sorry, I don't understand. Please explain your reply. V1 is shown as 12VDC.
 

Bluejets

Oct 5, 2014
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Whatever...same same.
Perhaps power supply you are using is not up to the job.
You do not say what it is and crystal ball is upta this end.
 

Harald Kapp

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Perhaps your "9v power suppply" ( not shown) is not up to the job.
A reasonable assumption. The load current (through L1) reaches 1.8 A peak:
1709707396537.png
Your power supply needs to be able to supply this current without dipping in voltage.

@epsolutions : btw it is much easier to follow such a simulation when you label the nodes like so:
1709707502796.png
Nobody can know what V(n005) or V(noo6) is, but it is easy to recognize V(out) as such.
 

Delta Prime

Jul 29, 2020
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. Can anyone please help me understand why the built circuit's output voltage is low?

Show me your real world circuit of the LT spice circuit simulation.
Any simulation tool is a user defined device i. e. component models.
Random tangent Parameters, simulation of transients, that sort of thing.
I make no (ass)(u)(me) trying to cover up for those not in the know! Rubbish...
 

epsolutions

Sep 7, 2019
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Well then, ignore the simulation and cast your expert eyes upon the circuit design. Why is it outputting only 0-5Vpp when the supply is 12VDC?
 

Harald Kapp

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Usually a comparatively simple circuit like this one should not deviate in practice so far from the simulation.
Double check your components and your connections. There's at least one mismatch in your schematic:
1709712362091.png
4k4 + 21k don't add up to a 50 k potentiometer - although this should be irrelevant for the function in this case.

What did you do with the 2nd opamp in the LM358's case? Have you tied the inputs to a fixed potential (e.g. both unused inputs connected to GND)? Leaving unused inputs open (floating) is never a good idea. It may lead to any of a number of unforeseeable behaviors.
 

epsolutions

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While appreciated, neither of those two observations explain the unexpectedly low voltage output. This would undoubtedly relate to the overall design, and possibly the effect of the offset on the LM358 gain headroom. I am not sure, which is why I am asking. If I use a 24VDC supply, the swing doubles to over 9Vpp. There is no mistake in the constructed circuit. It is as shown.
 

Delta Prime

Jul 29, 2020
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Okay a different approach.
While appreciated, neither of those two observations explain the unexpectedly low voltage output. This would undoubtedly relate to the overall design, and possibly the effect of the offset on the LM358 gain headroom
Appreciate this!
The simulation indicates an output swing of about 0-9Vpp (blue trace). This is consistent with the parts and circuit topology. However, when constructed (twice from new parts),
You're not listening... You're going to make an excellent electronic engineer show me your construction!! Now is not the time to be shy.
 

Alec_t

Jul 7, 2015
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However, when constructed (twice from new parts), the circuit's gain only achieves half of that. IOW the output swings from 0V to only 4.5V
How were you measuring that? At only 8Hz, any AC-coupled instrument may be having difficulty in giving an accurate reading.
 

danadak

Feb 19, 2021
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1709755785777.png

Your input to OpAmp is ~1.56V. Your fdbk R6, R3 sets G thru NonInv OpAmp
circuit of ~ 1.5. The output stage is a G = 1 (its an emitter follower),
so output would be 1.5 x 1.56 ~= 2.34V.

1709756944852.png

And your OpAmp is running into the rail (clipping) with the 8V offset at Vin.
The OpAmp output swing is limited to ~ 2-3 V off its supply rail per spec.



Regards, Dana.
 
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danadak

Feb 19, 2021
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When input is DC swept from 0 to 12 V you can see effective range
is ~ 9V. Also note at 0 V in a problem getting emitter follower shut off.

1709757321266.png

Regards, Dana.
 

epsolutions

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Thanks danadak, but for some reason your calculations do not match what LTSpice shows here, i.e. that the output of the LM358 (base of Q1) swings between about 1 and 10Vpp. LTSpice shows V3 to be +/- 1.2Vpp. The offset at Vin is about 1.2V (not "8V"), leaving a 1.5Vpp signal in the positive region into pin 3 of the op amp.

My question is, why does the sim (.asc file attached) show an output of twice of what I am measuring in the constructed circuit? 8Hz is no problem for my scope. Yes, the LM358 is clipping. But how is this related to the offset, or some other factor?
 

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danadak

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Here is no offset, 1.15 sim, correct OpAmp

1709762302488.png
 

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danadak

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Here is stepping R3 from 2K to 8K, effective increase offset.

1709762516737.png


Regards, Dana.
 

epsolutions

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Yes, that is what I see here as well. The signal amplitude at pin 1 is good, but the wave form is not. I do not understand how that answers my question.
 

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  • output-with-no-offset.png
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epsolutions

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Irrespective of the sims, the question is how can I get full expected 0-9V swing from the LM358, or at the emitter of the 2N3055, and still provide a DC offset for the bipolar input?
 

danadak

Feb 19, 2021
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1) Does input have to be DC coupled ?
2) What is input range pk - pk and is it always ground referenced ?

Sorry if you have already made that clear, I keep re-reading and not sure....

Regards, Dana.
 
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