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Old 29th June 2007, 02:45 PM   #21
x-pro is offline x-pro  United Kingdom
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Quote:
Originally posted by happyboy
What i understand from the example is that i must use a feedback resistor that when matched with the input capacitance of the Opamp must be out of the unity gain bandwidth of the opamp? If what i understand is right, then how do i calculate what value of a resistor to use with different input capacitance of different opamp?
This is just an example of a possible problem - for many opamps there is no data available for the input capacitance. It is sensible in an audio opamp application to keep the value of a feedback resistor between 1K and 10K - most of the time you shouldn't have a problem. You need to be aware about potential stability issues if you try to use opamps designed for high frequency applications and not for audio.

Cheers

Alex
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Old 30th June 2007, 04:17 AM   #22
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Quote:
Originally posted by x-pro


This is just an example of a possible problem - for many opamps there is no data available for the input capacitance. It is sensible in an audio opamp application to keep the value of a feedback resistor between 1K and 10K - most of the time you shouldn't have a problem. You need to be aware about potential stability issues if you try to use opamps designed for high frequency applications and not for audio.

Cheers

Alex
Point noted there and i think i will use OPA2134 instead of AD826.

Here is the redrawn schematic. Hope its stable now.
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Old 30th June 2007, 09:21 AM   #23
x-pro is offline x-pro  United Kingdom
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Quote:
Originally posted by happyboy
Here is the redrawn schematic.
Q2 is upside down. It may even work but rather badly

Cheers

Alex
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Old 1st July 2007, 02:09 AM   #24
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Quote:
Originally posted by x-pro


Q2 is upside down. It may even work but rather badly

Cheers

Alex
Yikes! No wonder there was alot of distortion if i biased that thing below 1.4A. Now i can bias it at 1.2A and still make it work properly.

Just a question. Is an input impedance of 150K normal and acceptable?
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Old 1st July 2007, 09:32 AM   #25
AndrewT is offline AndrewT  Scotland
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Hi,
the input impedance of the circuit you last posted is 2k5.
But it is not an e24 resistor value.
2k5//10k=2k.
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Old 1st July 2007, 10:36 AM   #26
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Quote:
Originally posted by AndrewT
Hi,
the input impedance of the circuit you last posted is 2k5.
But it is not an e24 resistor value.
2k5//10k=2k.
Hi, sorry for the misconception there, i changed that resistor to 150K but did not update it here. I was quite worried cause its quite high i suppose.

Erm sorry but may i know whats a e24 resistor value?

Thanks!
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Old 1st July 2007, 12:50 PM   #27
AndrewT is offline AndrewT  Scotland
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Hi,
1% resistors usually come in 24values per decade. so increments after 2k0 are 2k2, 2k4, 2k7,3k0.
If you can find someone offering e96 values then you can fill in the gaps, but they often cost much more.
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Old 1st July 2007, 01:34 PM   #28
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Quote:
Originally posted by AndrewT
Hi,
1% resistors usually come in 24values per decade. so increments after 2k0 are 2k2, 2k4, 2k7,3k0.
If you can find someone offering e96 values then you can fill in the gaps, but they often cost much more.
Oh now i get it. Thanks!
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Old 1st July 2007, 05:59 PM   #29
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This will give you better way to adjust your current so you can remove the resistor out of the output.
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Old 2nd July 2007, 02:18 AM   #30
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Originally posted by jerluwoo
This will give you better way to adjust your current so you can remove the resistor out of the output.
Hi Jerluwoo,
Thanks for your help. Here is the schematic that i redrawn. Is using 2N3906 for the PNP transistors alright?

Thanks!
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