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BUF634 in parallel: BW pin connection

Victor Levandovsky
Intellectual 575 points
Other Parts Discussed in Thread: BUF634, LMH6723

Hello,

I would like to use two BUF634 in parallel configuration for audio application (headphone amplifier).

I read Application Bulletin "COMBINING AN AMPLIFIER WITH THE BUF634" (sboa065.pdf) and have a questions about connecting BW pins.

In Bulletin BUF634 use maximum frequency bandwidth (BW directly connected to V- rail).

I would like to decrease bandwidth (or have possibility to change bandwidth during testing)

My question is...

1) Should I use individual resistors for each BUF634? Or can I use only One Resistor for all BUF634 (all BW pins connected in parallel and connected with V- with single resistor)?

I think one resistor is better, because multiple resistors (with different value due to tolerance) will provide different currents for BUF634`s and all BUF`s will be in a different mode.

But I`m not sure if this correct.

2) What is a best current (resistor value) in term of signal quality?

  • 0
    TI__Genius 13935 points

    Hello Victor,

    In the application note the buffer was used at maximum frequency because it was inside a feedback loop.  In a feedback loop it is desirable for the buffer to be much faster than the op amp so that the op amp sets the frequency response.  Adding delay through the buffer decreases phase margin. 

    I would give each amplifier a separate resistor.  According to Figure 1. Simplified Circuit Diagram the BW pin is high impedance and the BW resistor is in parallel with an on chip 4k resistor.  Because this is a high impedance node a small current mismatch between the two buffers could create more voltage shift on that node than would be desirable.

    I would consider using this amplifier at the lowest bandwidth for an audio application (no BW resistor).  The reason for this is that audio signals don't need that much bandwidth.  A 44kHz signal at 16Vpp is about 1.5 V/us, according to the datasheet the BUF634 gets 2000V/us even in low bandwidth mode.  

    You may want to consider posting this question in the Audio section as well.  I don't see a plot of noise vs frequency in the BUF634 datasheet.  Sometimes high speed amplifiers are not ideal for audio applications due to 1/f noise. 

    http://e2e.ti.com/support/amplifiers/audio_amplifiers/default.aspx

    Regards,

    Loren

  • 0 in reply to Loren Siebert 1
    TI__Genius 13935 points

    Hi Victor,

    Also, you may want to use some isolation resistance between the two buffers and the load to make sure they share the current evenly.  The graphic below is from the LMH6723 datasheet.

    I would also like to ask what impedance are the headphones?  How much power do  you want to deliver?    

    Regards,

    Loren

  • 0 in reply to Loren Siebert 1
    Intellectual 575 points

    Hello,

    Thank you very much for answers.

    Headphones ~32 Ohm. BUF634 powered by +/-6 VDC (minimum value)

    I have working device with single BUF634, but I would like to test double `634.