THS3491: How to Implement a Class AB amplifier by using THS3491 and external MOSFET?

Hi Lionel,

Are you seeing any abnormal results when there is no input signal? Such as ringing or oscillations at the output of the device. The concern is that there could be instability due to the transistors being in the feedback loop of the device due to the nature of current feedback devices. This has been a very common issue when trying to implement current feedback devices such as the THS3491 in this manner.

Best Regards,

Ignacio

Thank you for giving me some feedback. I check with my circuit without signal input, the output is as zreo as input signal. There's no any abnormal ringing there.Also, my circuit will be okay when input sine wave below 100kHz. I use +/-15V to provide THS3491 and power stage apply 10V. Is there any suspected setting?

Hi Lionel,

Thank you for confirming the device seems stable. In simulation there is peaking which is interesting you do not see any signs of instability. In the simulation I was able to get the pmos to bias properly by applying a negative potential to its drain where the schematic shows this node to ground. Is ground the correct setup for this transistor?

Best Regards,

Ignacio

Thank you for giving me feedback.
I set the low-side transistor's drain to ground together with THS3491's supply voltage's ground, and provide THS3491 with +/-15V supply voltage. Do you mean that i need to change low-side drain to negative supply voltage(-15V) instead of conntect it to ground? Or may you please give me a picture or schematic for understanding your point? Thank you.

Hi Lionel,

With the way I was biasing the circuit it was not working until the drain was set to -15V like you mentioned. However, from looking at your images it seems like the circuit is biased correctly as it is outputting the low side of the signal as well. Does setting the drain to the lower potential make any improvement to the circuit or is the output still very distorted? 

Best Regards,

Ignacio