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Using info from simulation software such as LTspice to improve your design

PurpaSmart

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This is more of question. But Ive heard using info from a square wave can tell many flaws in an amplifier , I so I tried it it out and lo and behold!
Here is the input signal
square_in.PNG


Now here is output signal
out2.PNG

Hmm lets take a closer look...
out.PNG

From what I heard this is "Severe Low Frequency Attenuation" this should be not be rolling off like this, now the question is? How can one overcome this problem? here is the schematic. This is from Bob Cordells Burning Amp 2016 slide using a diff pair with current mirrors, I'm curious on how one should fix this problem?
schematic.PNG
 

restorer-john

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Schematic is wrong.

Your -OUT is actually the + rail.
Your +OUT is actually the -rail.

Why have you got Q14 and Q15 collectors shorted together?

Edit: A bias spreader is in his talk (a Vbe multiplier) 1.2v or so.

The front end is +/-15v with constant current feeding each pair.

His main rails are +/-55v
 
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PurpaSmart

PurpaSmart

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Here is the the project, Also the shorted transistors is what bob had in his slide
Skip to 32 minutes in
EDIT, I removed the C1 and and that did the trick, I guess I had to much filtering going on I thought originally it wasnt problem since I had already lowered the value some and it did not change anything
fixed.PNG
 

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PurpaSmart

PurpaSmart

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Location
Washington State
Schematic is wrong.

Your -OUT is actually the + rail.
Your +OUT is actually the -rail.

Why have you got Q14 and Q15 collectors shorted together?

The simulation just shows a spike going through each 10pF cap when each of Q5 and Q7 get pulled above or below the input. The decay is discharge.

The only power supply you have is for the front end
Power supply is V1
 

restorer-john

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can you link the LTspice file, I want to see what you are seeing.
 
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