Re: LM38xx typical application circuit and the Ci feedback to ground capacitor.
As I understand it, Ci is used with Ri as a highpass LC filter to remove any possible DC in the feedback loop.
Usually specified as a non-polar 22uF, there are various designs where Ci is polarised or only 10uF. Ri is nearly always 1K.
Also, the datasheet application notes say:
At this point, it is a good idea to ensure that the Gain-
Bandwidth Product for the part will provide the designed gain
out to the upper 3 dB point of 100 kHz. This is why the
minimum GBWP of the LM3886 is important.
GBWP ≥ AV x f3 dB = 13 x 100 kHz = 1.3 MHz
GBWP = 2.0 MHz (min) for the LM3886
Solving for the low frequency roll-off capacitor, Ci, we have:
Ci ≥ 1/(2π Ri fL) = 4.85 μF; use 4.7 μF.
...which is a bit beyond my level of understanding *ook ook - wanders off to look for a banana* : I'm assuming giving Ci a value between 10 and 20-or-so uF will suffice for a general purpose audio amp into an 8ohm nominal load.
So... I've the following options for Ci using components I have or can get easily:
1) Mitsumi 10uF MPT - good quality metallized polypropylene but bigger than the rest of the gainclone circuit put together (~ 5cm x 5xm x 3cm). Will long leads lead to undesirable effects here or doesn't it matter? An increasingly strange prospect is stacking two of these monsters in parallel for 20uF.
2) Vishay Sprague 22uF tantalum.
3) Electrolytic 10 or 22uF. Low ESR or low leakage (similar to tantalum) kinds available.
Comments as to which option sounds good (as it were) are very welcome.
As I understand it, Ci is used with Ri as a highpass LC filter to remove any possible DC in the feedback loop.
Usually specified as a non-polar 22uF, there are various designs where Ci is polarised or only 10uF. Ri is nearly always 1K.
Also, the datasheet application notes say:
At this point, it is a good idea to ensure that the Gain-
Bandwidth Product for the part will provide the designed gain
out to the upper 3 dB point of 100 kHz. This is why the
minimum GBWP of the LM3886 is important.
GBWP ≥ AV x f3 dB = 13 x 100 kHz = 1.3 MHz
GBWP = 2.0 MHz (min) for the LM3886
Solving for the low frequency roll-off capacitor, Ci, we have:
Ci ≥ 1/(2π Ri fL) = 4.85 μF; use 4.7 μF.
...which is a bit beyond my level of understanding *ook ook - wanders off to look for a banana* : I'm assuming giving Ci a value between 10 and 20-or-so uF will suffice for a general purpose audio amp into an 8ohm nominal load.
So... I've the following options for Ci using components I have or can get easily:
1) Mitsumi 10uF MPT - good quality metallized polypropylene but bigger than the rest of the gainclone circuit put together (~ 5cm x 5xm x 3cm). Will long leads lead to undesirable effects here or doesn't it matter? An increasingly strange prospect is stacking two of these monsters in parallel for 20uF.
2) Vishay Sprague 22uF tantalum.
3) Electrolytic 10 or 22uF. Low ESR or low leakage (similar to tantalum) kinds available.
Comments as to which option sounds good (as it were) are very welcome.