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Old 23rd December 2012, 01:48 PM   #1
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Default Single supply inverting chip amp.

Had a bit of a search, but haven't found anything on this topology. Is it doable? Am using an lm3875 and trying to get gain as low as possible, (hence inverting).

It's for a guitar amp, so Hi-Fi is not especially needed.
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Old 23rd December 2012, 02:34 PM   #2
DUG is offline DUG  Canada
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Page 7 (Fig 3.) of datasheet ( http://www.ti.com/lit/ds/symlink/lm3875.pdf )shows single supply cct.
Add inverting opamp in front of it and reduce signal for overall gain of -1.

Done.
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Old 23rd December 2012, 02:53 PM   #3
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Quote:
Originally Posted by DUG View Post
Page 7 (Fig 3.) of datasheet ( http://www.ti.com/lit/ds/symlink/lm3875.pdf )shows single supply cct.
Add inverting opamp in front of it and reduce signal for overall gain of -1.

Done.
Thanks, but attenuate to then amplify is what I'm trying to avoid.

I've been looking at ThorstenL's unity gain chip amp and am trying to picture it on single rail.

Click the image to open in full size.
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Old 23rd December 2012, 04:03 PM   #4
DUG is offline DUG  Canada
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Input gets cap in series with R2 (Xc= R2/10 at lowest F)

R1 gets a Cap to gnd (Xc= R1/10 at lowest F) (Some opamps don't need R1)

R3 goes to Vcc/2 with bypass cap at Vcc/2 to gnd

Output gets coupling cap (Xc= loadR/10 at lowest F)

Negative opamp supply goes to gnd.

Lowest F is your desired 3dB point...just a guideline.
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Old 23rd December 2012, 04:04 PM   #5
JMFahey is offline JMFahey  Argentina
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Quote:
Had a bit of a search, but haven't found anything on this topology. Is it doable? Am using an lm3875 and trying to get gain as low as possible, (hence inverting).
You can't use it unity gain, it is not compensated as such and will oscillate like crazy.
Minimum recommended gais is 26dB, so if you are trying to use it, as, say, the second "arm" of a bridged amp you'll have to attenuate anyway.
From the datasheet:
Quote:
LAYOUT, GROUND LOOPS AND STABILITY
The LM3875 is designed to be stable when operated at a closed-loop gain of 10 or greater,
Quote:
Normally the gain is set between 20 and 200; for a 40W, 8Ω audio amplifier this results in a sensitivity of 894 mV and 89 mV, respectively.
Quote:
The value for the feedback resistance, Rf1, should be chosen to be a relatively large value (10 kΩ100 kΩ), and the other feedback resistance, Ri, is calculated using standard op amp configuration
If you want an inverting amp, build the regular non onverting version, ground the + input and feed the - input with a 20K resistor from the other amp's output ( or the same R value you used as Rf).
No need for an external inverter.
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Old 23rd December 2012, 04:28 PM   #6
DUG is offline DUG  Canada
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"The LM3875 is designed to be stable when operated at a closed-loop gain of 10 or greater, "

So, a gain of -1 is greater than 10?

Just asking.

Might work. I've thought about this solution but I've not tried it so I can't say one way or the other.

Let us know of the results.

P.S. Don't ground the + input...not on a single supply amp. If you change the position of the feedback shunt resistor (low value R) and its' cap to gnd then you can connect the 20K input R to the top of the Resistor and save a coupling cap in the circuit.

Do you know what I am trying to say?

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Old 23rd December 2012, 05:39 PM   #7
JMFahey is offline JMFahey  Argentina
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Quote:
P.S. Don't ground the + input...not on a single supply amp.
Well, if it's a single supply amp, you still need to ground it (you need a ground reference) but, of course, through a capacitor.
10 to 100uF will be fine.
And yes, running the 20K resistor straight to the first amp output can be done, no capacitors involved, if both are biased from the same 1/2V+ voltage divider.
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Old 23rd December 2012, 05:46 PM   #8
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Quote:
Originally Posted by JMFahey View Post
You can't use it unity gain, it is not compensated as such and will oscillate like crazy.
Minimum recommended gais is 26dB, so if you are trying to use it, as, say, the second "arm" of a bridged amp you'll have to attenuate anyway.
From the datasheet:



If you want an inverting amp, build the regular non onverting version, ground the + input and feed the - input with a 20K resistor from the other amp's output ( or the same R value you used as Rf).
No need for an external inverter.
lm3875 etc have been made stable at, in inverted configuration, at unity gain. This has been much posted about in this forum, so I won't repeat it here. Point is, can it be done from a single rail.

Last edited by Robert Kesh; 23rd December 2012 at 05:51 PM.
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Old 23rd December 2012, 05:50 PM   #9
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Quote:
Originally Posted by DUG View Post
Input gets cap in series with R2 (Xc= R2/10 at lowest F)

R1 gets a Cap to gnd (Xc= R1/10 at lowest F) (Some opamps don't need R1)

R3 goes to Vcc/2 with bypass cap at Vcc/2 to gnd

Output gets coupling cap (Xc= loadR/10 at lowest F)

Negative opamp supply goes to gnd.

Lowest F is your desired 3dB point...just a guideline.
The coupling caps, V++, decoupling caps and things like zobel, are left out for simplicity. But thanks for the rest.
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Old 23rd December 2012, 07:29 PM   #10
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Quote:
Originally Posted by Robert Kesh View Post
Thanks, but attenuate to then amplify is what I'm trying to avoid.

I've been looking at ThorstenL's unity gain chip amp and am trying to picture it on single rail.

Click the image to open in full size.
It just dawned on me that ThorstenL's circuit IS 20dB of attenuation followed by 20dB of amplification.
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