Help with DC Servo for Current Feedback Opamp

[klewis]

I've built a preamp around the LME49713 current feedback op amp. The results so far look very good. I added a DC servo to manage the DC offset - which varies from -2mv to 19mv depending upon the volume pot setting. I used the LME49720 op amp for the servo. I simulated the following circuit and it show the servo correcting the offset. The actual circuit changes the offset to 32mv regardless of the volume pot setting. So, is some respects, it's working in that it's making the DC a constant value, but, it's not the 0 volts expected. Advice and critique would be greatly appreciated.

Regards,

Ken

I used the LMH6723 current feedback op amp model for the 49713 and the op275 for the 49720.

[pacificblue]

Should the DC servo for an inverting amp not also be inverting and act on the non-inverting input?
It is also advisable to use anti-parallel diodes between the servo's input pins to avoid overvoltage.

[Artie]

I've been reading up on CFB opamps too, and there's two things I've picked up on:

1. (This is taken directly from Linear Tech's "CFB Do's and Don'ts") "DO NOT PUT A SMALL CAPACITOR FROM THE INVERTING INPUT OF A CURRENT FEEDBACK AMPLIFIER TO ANYWHERE, ESPECIALLY NOT TO THE OUTPUT." The "all caps" is their form. Note your C4.

2. I don't believe a CFB opamp should ever be used in an inverting mode, due to the difference in input impedances. (This part hasn't been stated implicitly, but no app notes show an inverting circuit.)

Hope this helps.

[klewis]

Originally posted by Artie
I've been reading up on CFB opamps too, and there's two things I've picked up on:

1. (This is taken directly from Linear Tech's "CFB Do's and Don'ts") "DO NOT PUT A SMALL CAPACITOR FROM THE INVERTING INPUT OF A CURRENT FEEDBACK AMPLIFIER TO ANYWHERE, ESPECIALLY NOT TO THE OUTPUT." The "all caps" is their form. Note your C4.

2. I don't believe a CFB opamp should ever be used in an inverting mode, due to the difference in input impedances. (This part hasn't been stated implicitly, but no app notes show an inverting circuit.)

Hope this helps.

Artie,

I read the app notes as well, so, I didn't explain the cap shown parallel with the gain resistor if for the simulation only, it represents the capacitance of the resistor and traces.

Audioman54 who helped develop the 49713 recommended the inverting configuration - like you I had the same impression. Have a look at the thread in the solid state forum "National Op amp inflation" you'll see his comments and my measurements from the preamp.

I tried simulating the it with the servo output going to the non-inverting pin of the preamp opamp and it offset the entire sinwave by about 4v positive.

Other thoughts - thanks.

Ken

[pacificblue]

Originally posted by klewis
I tried simulating the it with the servo output going to the non-inverting pin of the preamp opamp and it offset the entire sinwave by about 4v positive.

With the servo inverting?

[klewis]

Originally posted by pacificblue

With the servo inverting?

I took the output from R21 and connected it to the non-inverting pin of U3 in the simulation and disconnected it from the inverting pin. All of the aforementioned was done with the simulator, not the physical circuit.

[klewis]

Originally posted by pacificblue
Should the DC servo for an inverting amp not also be inverting and act on the non-inverting input?
It is also advisable to use anti-parallel diodes between the servo's input pins to avoid overvoltage.

I think you have a good point here, though the non-inverting input behaves quite differently on a CF opamp compared to VF opamp. I'll post another schematic.

[klewis]

Here's the ouput plot from the first schematic simulation. It show the offset being injected and then corrected.

[klewis]

Here's the simulation schematic with the inverting servo acting on the non-inverting input.

[klewis]

Here's the output plot for the inverting servo configuration. It moves the offset but, just barely, certainly not back to zero. I used resistor values as recommended by national in AN-1192 figure 15. These are for voltage feedback amps.