LME49830 unstable issue

Hello Po-Chen-san,

I'll suggest you couple of thing to have stable output with your C Load condition 340nF Cap load as follow.

Onyour schematic doesn't have decoupling cap on Vee side. So you should have decoupling cap as close as device pin and Power tr. To get 25V/us of Slew rate, put about 15pF as Cc.  Put 1 ohm resistor in seriese with CLaod as my schematic. And also put 6.8k as Rs to compensate DC offset on + Input pin and GND. Put Pwr Tr AB bias current setting Tr and resistor capacitor ckt ans set Iab on power Tr about 20mA. Then you will have stable output wave form I attached file.  Please visit following URL to check the schematic and output waveform.

0003.Schematic & stability result.xlsx

Best regards

T NAOKAWA

I'd like to chime in here and suggest that a fundamental problem here is that the capacitance load needs a small series ballast resistor.  

An amplifier's output impedance is established by its open-loop Z-out divided by its open loop gain. We know the loop gain falls with frequency, e.g., at -6dB/octave, so the closed-loop Z-out rises with frequency.  In fact, the output looks and acts like an inductor.  Adding a large capacitive load creates an L-C resonance, with a peak at the resonant frequency, F_res.  The O.P. needs to add a series resistor to his capacitor, of about R_s = 1 / 2 pi C_load F_res.  This resistor is located after the ampliifer's feedback connection point, and R_s C_load will set an upper -3dB response rolloff frequency for driving the capacitive load.  If this response-limit frequency is inadequate, other measures will be needed to reduce open-loop Z-out.

Just to add a comment: Comparatively speaking, MOSFETs have a fairly low transconductance, e.g., compared to a BJT.  This creates a fairly high open-loop Z-out, that the LM49830 overcomes with its loop gain.  We shouldn't blame the the LM49830 for any troubles that ensue from the MOSFET's inadequacies.  :-)

Cheers,

  - Win