LM4780 Datasheet Confusion

midway2k · 8th April 2006, 08:30 AM

Hi everyone, I am trying to design an inverting 4780 amp. So i took a look at the datasheet,
http://www.national.com/ds.cgi/LM/LM4780.pdf
and there is a little section about inverting configuration in page 19 and 20. In page 19 it refers to equation 8 to calculate the lower -3dB point if a coupling capacitor is used. However, it seems to me that the equation is not right for the schematic in page 20, since the input resistance is suppose to be RIN//Ri, not RIN alone. Thus the shematic in page 20 should only give me -3dB frequency of about 162.541 Hz, while equation 8 gives a -3dB point of about 3.386 Hz. Can anyone confirm with me whether ppl who wrote the data sheet didn't know what they were doing or I wasted money on my education? Thanks for any help.

Midway

peranders · 8th April 2006, 01:37 PM

You are quite right. 1 uF is a bit little together with 1 kohms. The input impedance is 47//1 = 979 ohms

1 uF and 979 ohms = 162 Hz

I think they meant to have 10 kohms instead!

midway2k · 8th April 2006, 07:19 PM

If they had Ri = 10K ohm instead, then the overall gain will be 2 instead of 20, will the 4780 be stable in this case? I have been wondering about this for quite a while too, does the minimum gain of 10 rule apply to the inverting configuration? or just the non-invering configuration? thanks.

peranders · 8th April 2006, 08:45 PM

The feedback resistor must be 220 kohms or something.

8th April 2006, 08:30 AM	#1
midway2k diyAudio Member Join Date: Dec 2004 Location: us	LM4780 Datasheet Confusion Hi everyone, I am trying to design an inverting 4780 amp. So i took a look at the datasheet, http://www.national.com/ds.cgi/LM/LM4780.pdf and there is a little section about inverting configuration in page 19 and 20. In page 19 it refers to equation 8 to calculate the lower -3dB point if a coupling capacitor is used. However, it seems to me that the equation is not right for the schematic in page 20, since the input resistance is suppose to be RIN//Ri, not RIN alone. Thus the shematic in page 20 should only give me -3dB frequency of about 162.541 Hz, while equation 8 gives a -3dB point of about 3.386 Hz. Can anyone confirm with me whether ppl who wrote the data sheet didn't know what they were doing or I wasted money on my education? Thanks for any help. Midway

8th April 2006, 01:37 PM	#2
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	You are quite right. 1 uF is a bit little together with 1 kohms. The input impedance is 47//1 = 979 ohms 1 uF and 979 ohms = 162 Hz I think they meant to have 10 kohms instead! __________________ /Per-Anders (my first name) or P-A as my friends call me

8th April 2006, 08:45 PM	#4
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	The feedback resistor must be 220 kohms or something. __________________ /Per-Anders (my first name) or P-A as my friends call me

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8th April 2006, 07:19 PM	#3
midway2k diyAudio Member Join Date: Dec 2004 Location: us	If they had Ri = 10K ohm instead, then the overall gain will be 2 instead of 20, will the 4780 be stable in this case? I have been wondering about this for quite a while too, does the minimum gain of 10 rule apply to the inverting configuration? or just the non-invering configuration? thanks.

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