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Updating a classic: BUF634A
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Old Yesterday, 10:25 AM   #51
jan.didden is offline jan.didden  Europe
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John, I was wondering what influence the quiescent current setting on the BUF634 has on the distortion performance? Intuitively one could think that higher bias towards class A with respect to the load current would decrease THD, but is that always the case?
I have some indication that it's more subtle than that.

Jan
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Old Yesterday, 01:07 PM   #52
johnc124 is online now johnc124  United States
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That's a great question Jan, I've actually been wondering the same thing. I have one of the new BUF634A EVMs in my office and can try to take some measurements at different bias settings this week. Or see if one of my applications engineers has some time to do it.

I could see the intrinsic distortion of the BUF634A not changing significantly with bias. But because its bandwidth does vary significantly with bias, the amount of loop gain that you're able to put around it in the system will be significantly less in low bias mode. Hence the distortion of the full composite circuit might be worse in low bias mode. Unless you started playing tricks with the compensation of the composite amplifier.
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Old Today, 02:20 AM   #53
johnc124 is online now johnc124  United States
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I was able to have an engineer measure the THD of a BUF634A at 4 different bias points today to see how THD changes with the bias current. He measured the THD at 1Vrms, into a 32 ohm load. The bandwidth pin was configured in 4 ways:

1. Open / floating
2. Shorted to V-
3. 1kOhm resistor to V-
4. 10kOhm resistor to V-

This THD measurement is done open loop, so the BUF634A is not inside the feedback loop of another amplifier. Results are attached, there is about a 15 dB difference in THD in the wide bandwidth mode (BW connected to V-) and THD in the low power mode (BW open / floating).

Edit to add: In the graph, the shorted case (wide bandwidth) is marked as 0.1 ohms, and the open case is 1MOhm. I just did this to all all the data points to be plotted vs resistance on a log x-axis
Attached Images
File Type: png BUF634A THDN vs RBW.png (29.4 KB, 26 views)
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New version of the BUF634: http://www.ti.com/product/BUF634A

Last edited by johnc124; Today at 02:27 AM.
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