Question re LM4780 inverting configuration

sam9 · 8th February 2006, 02:59 AM

The inverting circuit decribed in the LM4780 says that the value of Rb, the non-inverting ground resistor, should equal the parrallel value of Ri and Rf, the gain setting resistors. No problem. However, I'm curious - in the experience of those who have tried it, how critical is this match and what is the effect of a a mismatch.

The obvious compulsive-obsesive solution is to use two parrallel resistors for Rb having the same values as Ri and Rf. But how critical is this in real ife?

sangram · 8th February 2006, 08:41 AM

Why not try it out?

Actually I have, but not with LM4780.

My first 'chip'amp was a 4-way TDA2030 circuit, 2 chips per channel in bridge configuration. I used the single resistor bridge method for simplicity, one of the main goals of the circuit at the expense of sound quality. The chips were all in inverting configuration with Rb=Ri.

I don't remember any problems with DC-offset.

The second time I have tried inverting configuration with LM4766 P2P wiring, I paralleled the resistors because I could.

I think you may not face any problems apart from a slightly higher than usual DC-offset. If that.

sam9 · 8th February 2006, 01:58 PM

After posting, it also ocurred to me that using a small trim pot for Rb might be a possability if DC offset is the main concern.

Bazukaz · 8th February 2006, 02:03 PM

Resistor mismatch would set slightly different gaint for each IC. I used 5% metal film resistors without any problems.You can match resistors with ohmetter.

sam9 · 8th February 2006, 02:36 PM

Bazukaz,

I was concerned not with the gain setting resistors but with the resistor between non-inverting input and ground. The AppNotes say this should be the same value as the value you would get if the gain setting resistors were parralleled.

If Rf//Ri = 20k//1k= 953 I.e., A=20), then Rb should be 953. This is all very neat. But if you are using different Rf and Ri an exact Rb is hard to find. Exact alternatives are

Rf=200k
Ri=10k
Rb=9.53k

or 2k, 1k, 93.5 etc. If you want a gain other than 20 is also gets tricky. While you can generally find a Rb within 1% of the desired value exact, I was curious about how sensative the device was to mis matches. Sometime a mis match of 0.5% can be important sometime 5+% is a matter of indifference.

I can try it myself of, course and it looks likley I will have to but I thoght to save some time.

8th February 2006, 02:59 AM	#1
sam9 diyAudio Member Join Date: Jun 2002 Location: Left Coast	Question re LM4780 inverting configuration The inverting circuit decribed in the LM4780 says that the value of Rb, the non-inverting ground resistor, should equal the parrallel value of Ri and Rf, the gain setting resistors. No problem. However, I'm curious - in the experience of those who have tried it, how critical is this match and what is the effect of a a mismatch. The obvious compulsive-obsesive solution is to use two parrallel resistors for Rb having the same values as Ri and Rf. But how critical is this in real ife?

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8th February 2006, 08:41 AM	#2
sangram diyAudio Member Join Date: Sep 2002 Location: India	Why not try it out? Actually I have, but not with LM4780. My first 'chip'amp was a 4-way TDA2030 circuit, 2 chips per channel in bridge configuration. I used the single resistor bridge method for simplicity, one of the main goals of the circuit at the expense of sound quality. The chips were all in inverting configuration with Rb=Ri. I don't remember any problems with DC-offset. The second time I have tried inverting configuration with LM4766 P2P wiring, I paralleled the resistors because I could. I think you may not face any problems apart from a slightly higher than usual DC-offset. If that.

8th February 2006, 01:58 PM	#3
sam9 diyAudio Member Join Date: Jun 2002 Location: Left Coast	After posting, it also ocurred to me that using a small trim pot for Rb might be a possability if DC offset is the main concern.

8th February 2006, 02:03 PM	#4
Bazukaz diyAudio Member Join Date: Nov 2005	Resistor mismatch would set slightly different gaint for each IC. I used 5% metal film resistors without any problems.You can match resistors with ohmetter.

8th February 2006, 02:36 PM	#5
sam9 diyAudio Member Join Date: Jun 2002 Location: Left Coast	Bazukaz, I was concerned not with the gain setting resistors but with the resistor between non-inverting input and ground. The AppNotes say this should be the same value as the value you would get if the gain setting resistors were parralleled. If Rf//Ri = 20k//1k= 953 I.e., A=20), then Rb should be 953. This is all very neat. But if you are using different Rf and Ri an exact Rb is hard to find. Exact alternatives are Rf=200k Ri=10k Rb=9.53k or 2k, 1k, 93.5 etc. If you want a gain other than 20 is also gets tricky. While you can generally find a Rb within 1% of the desired value exact, I was curious about how sensative the device was to mis matches. Sometime a mis match of 0.5% can be important sometime 5+% is a matter of indifference. I can try it myself of, course and it looks likley I will have to but I thoght to save some time.

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