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NFB Rolloff, capacitorless

Started by Unknown April 14, 2015
Suppose I have a very simple little class B amp, just an opamp driving 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 rolloff 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 gutter above af and no external cap is needed.

Am I right about this? Cost is everything for this, and yes, if I can skimp on a little nfb cap I'm going to.


NT
On Tue, 14 Apr 2015 16:48:39 -0700, tabbypurr wrote:

> Suppose I have a very simple little class B amp, just an opamp driving 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 > rolloff 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 gutter above af and no external cap is needed. > > Am I right about this? Cost is everything for this, and yes, if I can > skimp on a little nfb cap I'm going to. > > > NT
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. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
On Wednesday, 15 April 2015 01:15:04 UTC+1, Tim Wescott  wrote:
> On Tue, 14 Apr 2015 16:48:39 -0700, tabbypurr wrote: > > > Suppose I have a very simple little class B amp, just an opamp driving 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 > > rolloff 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 gutter above af and no external cap is needed. > > > > Am I right about this? Cost is everything for this, and yes, if I can > > skimp on a little nfb cap I'm going to. > > > > > > NT > > 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 much 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 respond gets ever slower compared to the audio frequency. Simming the amp previously showed plenty of xover distortion. Yes its common emitter pair for the output. NT
On Tue, 14 Apr 2015 16:48:39 -0700 (PDT), tabbypurr@gmail.com wrote:

>Suppose I have a very simple little class B amp, just an opamp driving 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 rolloff 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 gutter above af and no external cap is needed. > >Am I right about this? Cost is everything for this, and yes, if I can skimp on a little nfb cap I'm going to. > > >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. -- John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
On Wednesday, 15 April 2015 03:15:38 UTC+1, John Larkin  wrote:
> On Tue, 14 Apr 2015 16:48:39 -0700 (PDT), nt wrote: > > >Suppose I have a very simple little class B amp, just an opamp driving 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 rolloff 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 gutter above af and no external cap is needed. > > > >Am I right about this? Cost is everything for this, and yes, if I can skimp on a little nfb cap I'm going to.
> 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.
I did try that in spice, but didnt get any improvement. At 47R it had no noticeable 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 getting 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 compromise output tr drive. OTOH I know this technique is used... ? NT Version 4 SHEET 1 1136 680 WIRE 816 -112 816 -160 WIRE 592 48 592 16 WIRE 560 64 432 64 WIRE 752 80 752 -64 WIRE 752 80 624 80 WIRE 368 96 112 96 WIRE 560 96 368 96 WIRE 112 112 112 96 WIRE 816 112 816 -16 WIRE 960 112 816 112 WIRE 1104 112 1040 112 WIRE 112 128 112 112 WIRE 368 144 368 96 WIRE 1104 144 1104 112 WIRE 592 160 592 112 WIRE 112 224 112 192 WIRE 432 224 432 64 WIRE 496 224 432 224 WIRE 816 224 816 112 WIRE 816 224 576 224 WIRE 816 240 816 224 WIRE 432 256 432 224 WIRE 752 288 752 80 WIRE 112 320 112 304 WIRE 816 384 816 336 WIRE 112 416 112 400 WIRE 192 416 112 416 WIRE 112 432 112 416 WIRE 112 448 112 432 WIRE 112 576 112 528 FLAG 112 304 +ve FLAG 592 16 +ve FLAG 112 576 -ve FLAG 592 160 -ve FLAG 112 432 0v FLAG 112 224 0v FLAG 816 -160 +ve FLAG 816 384 -ve FLAG 1104 144 0v FLAG 432 400 0v FLAG 368 224 0v FLAG 192 416 0 SYMBOL voltage 112 432 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value 10 SYMBOL voltage 112 304 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V2 SYMATTR Value 10 SYMBOL voltage 112 96 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V3 SYMATTR Value SINE(0 0.1 1000) SYMBOL npn 752 -112 R0 SYMATTR InstName Q3 SYMBOL pnp 752 336 M180 SYMATTR InstName Q4 SYMBOL res 1056 96 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R7 SYMATTR Value 8 SYMBOL Opamps\\LT1001 592 16 R0 SYMATTR InstName U1 SYMBOL res 416 240 R0 SYMATTR InstName R1 SYMATTR Value 1k SYMBOL res 592 208 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R2 SYMATTR Value 47k SYMBOL cap 416 336 R0 SYMATTR InstName C1 SYMATTR Value 33�F SYMBOL res 352 128 R0 SYMATTR InstName R3 SYMATTR Value 10k TEXT -384 664 Left 2 !.tran .01
On Wed, 15 Apr 2015 03:18:43 -0700 (PDT), tabbypurr@gmail.com wrote:

>On Wednesday, 15 April 2015 03:15:38 UTC+1, John Larkin wrote: >> On Tue, 14 Apr 2015 16:48:39 -0700 (PDT), nt wrote: >> >> >Suppose I have a very simple little class B amp, just an opamp driving 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 rolloff 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 gutter above af and no external cap is needed. >> > >> >Am I right about this? Cost is everything for this, and yes, if I can skimp on a little nfb cap I'm going to. > >> 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. > >I did try that in spice, but didnt get any improvement. At 47R it had no noticeable 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 getting 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 compromise output tr drive. OTOH I know this technique is used... ? > >
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. -- John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
On Wednesday, 15 April 2015 16:35:32 UTC+1, John Larkin  wrote:
> On Wed, 15 Apr 2015 03:18:43 -0700 (PDT), nt wrote: > >On Wednesday, 15 April 2015 03:15:38 UTC+1, John Larkin wrote: > >> On Tue, 14 Apr 2015 16:48:39 -0700 (PDT), nt wrote: > >> > >> >Suppose I have a very simple little class B amp, just an opamp driving 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 rolloff 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 gutter above af and no external cap is needed. > >> > > >> >Am I right about this? Cost is everything for this, and yes, if I can skimp on a little nfb cap I'm going to. > > > >> 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. > > > >I did try that in spice, but didnt get any improvement. At 47R it had no noticeable 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 getting 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 compromise output tr drive. OTOH I know this technique is used... ?
> 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.
Of course! Thank you NT