"Class-AA" amp?

I?m no enjineer (c: but I?ve never heard of ?AA?. Is it dry? (That?s a joke?)

It's not a real spec. Searching online found a couple of schematics from people trying to guess what it means.

One design claims that it is a highly biased push-pull amp that limits the opposing transistor's bias at high amplitude. This seems like a bad idea.

Another design has two amps working together, with one amp providing current boosting and another amp providing precision. This sounds a lot like an obfuscated version of a typical high bias push-pull amp with some booster transistors.

On 8.1.16 20:53, DaveC wrote:

Maybe it is alkaline ...

I got curious. There was a schematic, but it was well hidden behind Javascript barriers, and it was drawn in a quite cryptic way.

I made a LTspice model of it, but was too lazy to build the model for NE5532, so I used LT1358 instead.

There is non-inverting amplifier with a gain of 4.3 and an exotic current booster at the output.

For showing function, there is a 1 kHz source with 1 V amplitude in the model.

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Version 4 SHEET 1 1224 992 WIRE 80 48 -16 48 WIRE 512 48 80 48 WIRE 240 128 208 128 WIRE 608 128 320 128 WIRE 80 192 80 48 WIRE 48 208 16 208 WIRE 208 224 208 128 WIRE 208 224 112 224 WIRE 240 224 208 224 WIRE 352 224 320 224 WIRE 816 224 352 224 WIRE 896 224 816 224 WIRE 1136 224 896 224 WIRE -496 240 -624 240 WIRE -304 240 -496 240 WIRE -176 240 -224 240 WIRE -64 240 -176 240 WIRE 48 240 -64 240 WIRE 352 272 352 224 WIRE -496 336 -496 240 WIRE -176 336 -176 240 WIRE -64 352 -64 240 WIRE 352 384 352 352 WIRE 400 384 352 384 WIRE 608 384 608 128 WIRE 608 384 480 384 WIRE 816 400 816 224 WIRE 896 400 896 224 WIRE 512 464 512 48 WIRE -496 480 -496 416 WIRE -176 480 -176 416 WIRE -64 480 -64 416 WIRE 352 480 352 384 WIRE 480 480 352 480 WIRE 608 496 608 384 WIRE 608 496 544 496 WIRE 208 512 208 224 WIRE 480 512 208 512 WIRE -416 576 -464 576 WIRE 16 608 16 208 WIRE 816 608 816 480 WIRE 816 608 16 608 WIRE 896 608 896 464 WIRE 896 608 816 608 WIRE -272 656 -336 656 WIRE 816 672 816 608 WIRE -464 720 -464 576 WIRE -336 720 -336 656 WIRE 80 736 80 256 WIRE 80 736 -16 736 WIRE 512 736 512 528 WIRE 512 736 80 736 WIRE 816 816 816 752 WIRE -464 848 -464 800 WIRE -336 848 -336 800 FLAG -176 480 0 FLAG -64 480 0 FLAG -624 240 In IOPIN -624 240 In FLAG 816 816 0 FLAG 1136 224 Out IOPIN 1136 224 Out FLAG -16 48 +15V IOPIN -16 48 In FLAG -16 736 -15V IOPIN -16 736 In FLAG -496 480 0 FLAG -464 848 0 FLAG -336 848 0 FLAG -416 576 +15V IOPIN -416 576 Out FLAG -272 656 -15V IOPIN -272 656 Out SYMBOL Opamps/LT1358 80 160 R0 SYMATTR InstName U1 SYMBOL Opamps/LT1358 512 432 R0 SYMATTR InstName U2 SYMBOL res -192 320 R0 SYMATTR InstName R2 SYMATTR Value 47k SYMBOL res -208 224 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R1 SYMATTR Value 100 SYMBOL res 832 496 R180 WINDOW 0 36 76 Left 2 WINDOW 3 36 40 Left 2 SYMATTR InstName R8 SYMATTR Value 3.3k SYMBOL res 832 768 R180 WINDOW 0 36 76 Left 2 WINDOW 3 36 40 Left 2 SYMATTR InstName R4 SYMATTR Value 1k SYMBOL cap -80 352 R0 SYMATTR InstName C1 SYMATTR Value 100p SYMBOL cap 880 400 R0 SYMATTR InstName C2 SYMATTR Value 10p SYMBOL res 336 208 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R3 SYMATTR Value 10 SYMBOL res 336 256 R0 SYMATTR InstName R6 SYMATTR Value 1k SYMBOL res 496 368 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R7 SYMATTR Value 3.3k SYMBOL res 336 112 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R5 SYMATTR Value 33 SYMBOL voltage -496 320 R0 SYMATTR InstName V1 SYMATTR Value SINE(0 1 1000) SYMBOL voltage -464 704 R0 SYMATTR InstName V2 SYMATTR Value 15 SYMBOL voltage -336 816 R180 WINDOW 0 24 96 Left 2 WINDOW 3 24 16 Left 2 SYMATTR InstName V3 SYMATTR Value 15 TEXT -648 40 Left 2 !.tran 0 10m

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-TV

Class AA OP Preamp /Headphone Amplifier

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The "current booster" simply measures the current into the load and injects the same amount of current into the output op-amp's node, so that it experiences zero load current.

Sounds like an invitation for oscillation. ...Jim Thompson

That's probably why they call it 'AA': Two class-A amplifiers, one for the voltage, one for the current (as if that doesn't generate a voltage).

joe

Of course, neither op-amp is running class A.

That's a pretty common technique, but it's simpler for the booster (U2) to have a gain of 1 (kill R6, make R7=0), and have R5=R3.

The other trick, for serious power, is to put helper transistors into the opamp supply rails.

Maybe you cannot use it with a double amplifier, like here.

You gotta load the output of the opamp and disconnect it from the collectors to get that to work. R to ground such that Iout from the chip reaches ~50-60% of max. A few pF in shunt with that resistor will speed things up a bit. You can get high slew rates out of garden variety opamps (35v/us with a 741 and 2N2907/2N2219). This is a classic circuit published in a Fairchild App note around 1969.

The load loads the opamp. This circuit works.

R to ground such that Iout from the

A few pF in shunt with that resistor will

Of course, frequency stability has to be considered. My sketch just shows the basic topology. There are variations.

Well, I suppose. I prefer stuff that works without/doesn't care about a load. Save maybe a current pump.

I introduced that circuit into an undergraduate 'Instrumentation for Physics Kids' in 1970 as a power amplifier for driving dc servo motors and whatnot. About an hour into the 3rd lab session, somebody connected a speaker and in about 2 hours there were turntables and speakers hauled in from dorm rooms on every bench. They grabbed every 2N3053 and 2N4037 I had.

Unity gain is a tough problem but anything over 5 was pretty tractable with a 10 pF across a 10k feedback resistor