The LM6171 has been very popular with audiophiles. For a voltage op-amp, it has an amazing slew rate of 3000 V/µs at "only" 100 MHz of bandwidth. Distortion performance in the data sheet looks nice, but is unfortunately not given for, say, 1 kOhm load and down to several Hz.
Looking at the simpified schematic, it becomes clear where the slew rate comes from. The non-inverting signal path is just like that of any current feedback op amp. It is the good-old four-transistor emitter follower diamond buffer. The output current gets mirrored to the gain and output stages. The output emitters of this input buffer are what would usually be the inverting input of a CFB op amp. The R_E is connected just as ususal. However, there is another buffer (which I bet is also diamond design) that buffers the inverting input. Hence, a current across R_E get mirrored, converted to a voltage in the gain stage and is current-buffered by the output stage.
My problem: the diamond buffer is essentially an open-loop structure that is outside the overall feedback loop. Due to symmetries, it is surprisingly linear, unless loaded by a low impedance, such as R_E (which will be below 30 Ohms in most CFB op amps). This is why CFB op amps cannot offer really superior THD performance and hence are not popularly used in audio.
I guess this should be true for the LM6171. To the outside, it looks like a VFB op amp as there are two high-impedance inputs. But both buffers will be outside the feedback loop whereas the input stage distortion inside a conventional (long tailed pair) VFB amp gets attenuated by the loop gain.
So how does the LM6171 reach its good distortion rating and acclaim by enthusiasts? Is there a cancellation of distortion because non-linearities are the same in both input buffers? Will this work for even and odd-order harmonics? Will it only work well for some external circuit configurations?
Curious to hear your comments...
Eric
Looking at the simpified schematic, it becomes clear where the slew rate comes from. The non-inverting signal path is just like that of any current feedback op amp. It is the good-old four-transistor emitter follower diamond buffer. The output current gets mirrored to the gain and output stages. The output emitters of this input buffer are what would usually be the inverting input of a CFB op amp. The R_E is connected just as ususal. However, there is another buffer (which I bet is also diamond design) that buffers the inverting input. Hence, a current across R_E get mirrored, converted to a voltage in the gain stage and is current-buffered by the output stage.
My problem: the diamond buffer is essentially an open-loop structure that is outside the overall feedback loop. Due to symmetries, it is surprisingly linear, unless loaded by a low impedance, such as R_E (which will be below 30 Ohms in most CFB op amps). This is why CFB op amps cannot offer really superior THD performance and hence are not popularly used in audio.
I guess this should be true for the LM6171. To the outside, it looks like a VFB op amp as there are two high-impedance inputs. But both buffers will be outside the feedback loop whereas the input stage distortion inside a conventional (long tailed pair) VFB amp gets attenuated by the loop gain.
So how does the LM6171 reach its good distortion rating and acclaim by enthusiasts? Is there a cancellation of distortion because non-linearities are the same in both input buffers? Will this work for even and odd-order harmonics? Will it only work well for some external circuit configurations?
Curious to hear your comments...
Eric
