Amplifier Attempt 2

scott_jcp · 9th April 2005, 09:54 PM

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

Am I getting closer?

ilimzn · 9th April 2005, 10:27 PM

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.

scott_jcp · 9th April 2005, 10:40 PM

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

darkfenriz · 9th April 2005, 11:37 PM

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:

scott_jcp · 10th April 2005, 12:09 AM

thank you for putting in so much effort to help a newbie, it is much appreciated!

darkfenriz · 10th April 2005, 12:40 AM

bias adjustion circuit (Vbe multiplier)

hienrich · 10th April 2005, 01:36 AM

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

MikeB · 10th April 2005, 01:06 PM

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

darkfenriz · 10th April 2005, 08:34 PM

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

MikeB · 10th April 2005, 08:46 PM

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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9th April 2005, 10:27 PM	#2
ilimzn diyAudio Member Join Date: Feb 2005 Location: Zagreb	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.

9th April 2005, 10:40 PM	#3
scott_jcp diyAudio Member Join Date: Feb 2005 Location: Richardson, TX	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

10th April 2005, 12:09 AM	#5
scott_jcp diyAudio Member Join Date: Feb 2005 Location: Richardson, TX	thank you for putting in so much effort to help a newbie, it is much appreciated!

10th April 2005, 01:36 AM	#7
hienrich diyAudio Member Join Date: Dec 2004 Location: City of Gentle People	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

10th April 2005, 01:06 PM	#8
MikeB diyAudio Member Join Date: Apr 2004 Location: Gütersloh	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

10th April 2005, 08:34 PM	#9
darkfenriz diyAudio Member Join Date: Jun 2004 Location: Warsaw	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

10th April 2005, 08:46 PM	#10
MikeB diyAudio Member Join Date: Apr 2004 Location: Gütersloh	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