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Old 9th April 2005, 10:54 PM   #1
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Default Amplifier Attempt 2

I have adjusted some things that people pointed out to me.
(And did the schematic on the computer!)

Am I getting closer?
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Old 9th April 2005, 11:27 PM   #2
ilimzn is online now ilimzn  Croatia
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Ummm... no.
I really hate to be so terse, but you have one truly trivial but dangerous mistake there and one not so trivial. Are you sure you understand how it all works?
I am sure it would help if you could discover what they are for yourself, as you are likely to learn a lot that way.
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Old 9th April 2005, 11:40 PM   #3
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I realize I don't fully understand what is going on. I am trying to learn.
While I appreciate your interest in my learning, it might be helpful to at least point me in the right direction.

-Thanks,
Scott
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Old 10th April 2005, 12:37 AM   #4
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why did you draw only npn's ??

one problem is you have no voltage divider of output voltage to feedback.
You need to compare input signal with only small fraction of output, otherwise you wouldn't get voltage gain.

second problem is (for me) bad use of current mirror. It mirrors one leg current PLUS current of the emitter of Q8. I wouldn't use mirror in VAS like this.

I've made some corrections and 'guessed what you've meant' (see attachment).
It should work if done like this:
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Old 10th April 2005, 01:09 AM   #5
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thank you for putting in so much effort to help a newbie, it is much appreciated!
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Old 10th April 2005, 01:40 AM   #6
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bias adjustion circuit (Vbe multiplier)
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Old 10th April 2005, 02:36 AM   #7
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hey scott,

you seems to have peculiar but dangerous connections.
don't worry keep learning.

soon you'll find your sellf rewarding.

all of us have been with this stage.
regards,

hienrich
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Old 10th April 2005, 02:06 PM   #8
MikeB is offline MikeB  Germany
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Hi Scott !

I think you should get some simulationsoftware, that's the easiest
way to learn electronics. You have at least 6 fatal errors in your circuit,
I am not sure if you understood the difference between npn and pnp ?
Your 2 diodes will give a nice firework ! Schematic not completed ?

darkfenriz, is this is a typo ? in the diffamp-cfp you should use 2.2k, not 22k ?

Mike
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Old 10th April 2005, 09:34 PM   #9
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yes, you're basically right, 0.3mA biasing of Q3/Q4 seems reasonable. I was playing with simulator and tried to minimise offset, which seemed high ('bout 60mV) and I didn't know what is to blame.
2.2k is correct.
regards
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Old 10th April 2005, 09:46 PM   #10
MikeB is offline MikeB  Germany
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Hi darkfenriz !

You have DC-offset because of the asymetric resistors to gnd for
both bjts of diffamp. You have 47k on the left and 82k on the right.
This gives different inputoffsets on both sides, this difference multiplied
by the gain of the amp gives your DC-offset.
If you replace the 2 resistors (82k+1.5k) through 47k and 860ohms,
you get identical inputoffsets on both sides.

Mike
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