NFB Rolloff, capacitorless

I'd need to see a schematic to be sure. If the output transistors are in common-emitter -- yea, probably, at the cost of even more crossover distortion.

With the low speed/bandwidth of the opamp I'm not convinced it could have m uch noticeable effect on xover distortion - as f rises, the opamp has ever less control as its [open loop] gain falls ever lower, and its ability to r espond gets ever slower compared to the audio frequency.

Simming the amp previously showed plenty of xover distortion. Yes its commo n emitter pair for the output.

NT

Should be OK.

If you add a base-emitter resistor (or capacitor) the distortion goes down. At low levels, the opamp can drive the load directly.

On Wednesday, 15 April 2015 03:15:38 UTC+1, John Larkin wrote:

small output pair for a watt or 2, and nfb resistors. Zero output pair bia s, lots of crossover distortion. Now... I'm thinking I can skip a nfb rollo ff capacitor by using the opamp's own internal cap to give the rolloff, eg for a 1MHz gbw opamp, if we run it at gain 20, its gain is down in the gutt er above af and no external cap is needed.

imp on a little nfb cap I'm going to.

I did try that in spice, but didnt get any improvement. At 47R it had no no ticeable effect, at 33 ditto plus V_out is reducing, at 22 the whole thing goes horribly distorted and V_out falls massively.

Adding C only makes distortion worse. 0.1uF no noticeable effect, 1uF its g etting a bit nasty.

I'm not seeing how an opamp, with all of 10mA or so I_out would really make much diffence to an 8R load. But I do see how a C or R here would compromi se output tr drive. OTOH I know this technique is used... ?

NT

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SYMBOL res 352 128 R0 SYMATTR InstName R3 SYMATTR Value 10k TEXT -384 664 Left 2 !.tran .01

If the opamp has some spare GBW and current drive capability, that's an easy way to reduce distortion and avoid thermal problems. If not, then not.

g a small output pair for a watt or 2, and nfb resistors. Zero output pair bias, lots of crossover distortion. Now... I'm thinking I can skip a nfb ro lloff capacitor by using the opamp's own internal cap to give the rolloff, eg for a 1MHz gbw opamp, if we run it at gain 20, its gain is down in the g utter above af and no external cap is needed.

skimp on a little nfb cap I'm going to.

noticeable effect, at 33 ditto plus V_out is reducing, at 22 the whole thi ng goes horribly distorted and V_out falls massively.

s getting a bit nasty.

ake much diffence to an 8R load. But I do see how a C or R here would compr omise output tr drive. OTOH I know this technique is used... ?

Of course! Thank you

NT