(Update: Adding useful resource links and theory on not needing to bias, using op-amps as comparators for simplification. Also, stay-tuned for the actual results with a motor, and some potentially interesting side-effects I hadn't expected!).

During my last experiments with a Class-D (PWM/switching) amplifier as a motor-driver, I somehow completely forgot that I once ordered 10 Linear Audio-Amps (Class-B/AB?) with exactly that experiment in-mind (well, with a linear audio-amp)...

And, oddly, it still hadn't come to my attention that I had this tube of linear-audio-amps until I almost pressed "Order" on these: http://www.goldmine-elec-products.com/prodinfo.asp?number=G15382B TA8251AH.

Yep, until Tuesday, that's *14* 4-channel Bridge-Tied-Load audio-amps for $5. Almost impossible to pass-up... If these linear-audio-amp experiments as motor-drivers works, each of these chips could potentially handle *4* DC motors (or maybe even two bipolar steppers?).

It's really hard for me to convince myself not to place that order... but $10-minimum + shipping means something more like $20, which I can't really justify right now, especially when I have a tube of amps to experiment with.

Some Technical comparisons:

TDA1510 (Mine)

• Single-supply
• 24W BTL
• ONE BTL output
• Has inverted and non-inverted inputs for BOTH half-bridges (essentially two high-power op-amps)
• Nice internal pseudo-diagram, but some undocumented pins to fight with...
• Optional bootstrap capacitor inputs

TA8251AH

• Single-supply
• "30W BTL"
• "x 4ch" (that's 4 BTL outputs!)
• Has a single non-inverted input for each BTL output (fixed gain)
• (nice .1in spacing, with a tiny bit of bending!)
• Kinda sparse documentation, but simple-enough "black-box" I/O...
• No bootstrapping necessary

I first wired-up my CA2002/TDA2002 (single-output) amplifier as a quick-experiment... (5 pins, not too complex to interface)... The results were pretty decent. I have two of these, so moving up to a BTL experiment would be easy... But that's when I remembered my TDA1510's, which aren't much more complex (interface-wise) than two TDA2002's...

Again, the results are pretty decent... There's some oddities at certain switching-frequencies/duty-cycles that I can't quite wrap my head around, and they definitely heat up faster than my LMD18200 (MOSFET-based DC-Motor H-Bridge), but the idea seems doable. (Not too different than my results with the L9947, so might drive these audio-amps with PDM rather'n PWM).

So, trying to figure out where these oddities are coming from, now I'm learning about what a Class-B(/AB?) Push-Pull amplifier does... And maybe even *why* they're not (typically) used for motor-control... (Though, I have seen notes stating that they are *sometimes*, so there's that!). This seems like a great resource for the general idea of Class-B amps (discussed more, below): http://engineering.sdsu.edu/~johnston/ME204/Lecture_Notes/Transistors.pdf, check out starting 'round page 23. And this one's a great resource on motor-driving which even has a section on Class-B amps used as motor-drivers: http://hades.mech.northwestern.edu/images/8/84/ProjectW2011.pdf.

Linear amps are sometimes used in motor control when analog control signals and a bipolar power supply are available, and power dissipation and heat are not a concern. ...
You can use a commercial audio amplifier to drive a DC motor...

(One of the few resources I could find that actually said that straight-out!)

But note, I don't see any reason why two single-supply audio-amps couldn't be used in BTL like an H-Bridge, with the exception that input-biasing is maybe a bit more difficult... And note, the quote above comes from a section which discusses using the *linear* aspect to drive the motor with a varying DC voltage (rather'n PWM)... Still haven't seen a resource regarding using PWM with linear-amps... And that's a key-factor that makes these experiments more-easily doable... Theoretically, alls you have to do is make sure the non-inverting input is higher than the inverting input for a full-railed positive output, and vice-versa for a full-railed ground-output. Potentially, no biasing necessary... Treat those op-amps like comparators... (This is the theory I'm basing my explorations on, here).

Another thing, this TDA1510 has "bootstrap" capacitor inputs, much like the Class-D amp experiments from before... But this guy doesn't *require* them, so I'm looking into what "bootstrapping" does with a BJT-circuit (I thought they were for driving N-Channel MOSFETs on the high-side, but I guess "bootstrapping" is a general-purpose kinda all-encompassing term).

Anyways, the highlight (re: bootstrapping), so far, is this document: http://www.learnabout-electronics.org/Downloads/amplifiers-module-05.pdf

I could just be tired, but its explanation is boggling my mind... I mean, it's just mind-blowing, or something. And doesn't seem to match most of the other explanations I've seen... or is coming at it from a completely different approach, or something.

I *think* the end-result is that bootstrapping doesn't really apply to my case... I'm pretty convinced that I'll be using these in full-on/full-off/rail-to-rail mode, something like PWM... Oddly, even though I've seen mention of linear-amps just like these being used for motor-control, most seem to imply using them as *analog* amplifiers. This kinda boggles my mind, as well. Wouldn't the benefits of PWM (nearly full-on/full-off output, and thus less time spent in the "linear" region where the transistor acts as a resistor, wasting power as heat) work just as well in a case like this, even with push-pull configuration where the emitters are tied together...?

Here, I think, I'm mixing up a couple concepts (again, I'm kinda tired). Bootstrapping seems to be ideally-suited for AC input, and (except for the case where I'm running at 100% duty-cycle), that's pretty much exactly what PWM would look like... So, I dunno. It doesn't seem *necessary* right now, in my experiments, but it might help to clean up some of those "oddities" I mentioned, earlier... And, really, since bootstrapping isn't available in most linear-amplifiers I've looked at, these experiments are more general-purpose if I ignore it. Though, it is a learning-experience.

Anyways, onto some of the oddities:

(This is from the TDA2002)

First, obviously, rise/fall times differ (this might be where bootstrapping could cause an improvement?). Further, the delay from falling input to falling output vs rising input to rising output also differ...

Neither of these are huge concerns, as far as driving a motor... Though it might be wise to lower the switching-frequency so less time is spent transitioning (where the transistors are acting like large-value resistors and creating heat... right?)

Now for what I consider a danged-interesting observation... Check out those edges on the output...

There's a *very* distinct "stepping" going on! I can't explain it. Look at the falling-edge, it looks like it's pixellated. And, though it's hard to see in this photo, the rising-edge is even more distinctly-pixellated. Weird.

No friggin' clue what could be causing that... my 'scope was *definitely* in analog-mode at that point.

(TODO: Does the TDA1510 do this, as well...? Does this happen when loaded, as well?)

We're looking at ~2-5us transitions, here... from one rail to the other. This is *way* faster than an audio-amp is *expected* to deal with... Some *vague* idea it might have something to do with "crossover distortion," when the output toggles from being pushed vs. pulled (from the upper to the lower transistor).

The following is *total* hypothesizing, in the moment, based on my likely misunderstanding of dang-near everything... Keep that in mind.

Output Stage ("Single-Ended"):
                V+
                 ^
                 |
                /
              |/
          .---| 
          |   |\
          |     v (NPN Emitter)
          |      |
  IN >----+      +---------------> OUT
from some |      |
*internal*|     /
amplifier |   |v (PNP Emitter)
          '---| 
              |\
                \
                 |
                 |
                 V
                V-

(interesting finding: NPN: "Not Pointed iN" PNP: "Point iN Please")

(Note: This is NOT half an "H-Bridge" as typically used to drive a motor with PWM, this is a portion of a *linear amplifier* circuit).

(This is assuming we've got a split/dual power-supply... my chip is single-supply, but that's slightly harder to think about).

So, (it seems) crossover-distortion is generally associated with input voltages around -0.7V to +0.7V, where the signal "crosses over" the ground-rail.

In that range, *neither* transistor is turned-on (because of the Base-Emitter voltage of ~0.7V). (Note, also, this portion of a push-pull amplifier amplifies *current*, not voltage. When the input is positive, the NPN transistor is turned on, the output tries to maintain the Base-Emitter voltage, so the output voltage is *roughly* 1-to-1 with the input (minus VBE)).

OK, that makes sense to me...

(Note also, typically they insert a slight but constant voltage-difference between the NPN transistor's base and the PNP's to try to minimize crossover-distortion... At that point, both transistors are on, but one only weakly... dunno whether this audio-amp has that, but it would seem surprising if many don't...).

Now, what if there was a capacitor on the output...?

Then crossover-distortion wouldn't occur at -0.7V to +0.7V input voltage, but would occur when the input voltage *minus the output voltage* is somewhere between -0.7V to +0.7V. The capacitor is charged to some level, say 5V, but the input has switched to -5V... The lower transistor is turned on because the input voltage is lower than the output-voltage, current flows through its emitter to its base. Then....

The capacitor discharges until the...

No, we've got to consider the source of the "input" voltage going into this "output stage"... The source of that input-voltage is a *voltage* amplifier... That voltage-amplifier is likely not push-pull, but more like a "common emitter" amplifier...

(Wikipedia)

The key being: The output of this stage is pulled-up by a resistor... It has to overcome internal capacitances, etc. So, by nearly-instantly changing the *whole circuit's* input voltage, the output of the internal voltage-amplifier might be a bit slowed (again, maybe bootstrapping would help here?).

So, then, we have a slew-rate(?) of the internal-amplifier, which might actually be slower than that of the output-amplifier(?).

Then, again, the output amplifier's lower-transistor would turn on, the external capacitive-load might be at 5V, the input to that amplifier might decrease to, say, 4V, the capacitor is briefly shorted to ground through the lower-transistor, until (quite quickly, in comparison to the slew-rate of the internal amplifier) the external capacitor reaches something like 4.5V... at this point the internal amplifier's output has fallen to something like 3.9V, but that's not low enough to keep the transistor on, so the lower transistor is turned-off. The internal amplifier drops slowly a bit more, down to 3.8V, the transistor turns on, the capacitor discharges to 4.3V, the transistor turns off... There's something there that might make some sense, maybe... Plausibly this requires a bit of internal inductance, or internal resistance, as well...?

I'm losing my thought-train...

A couple observations from the 'scope trace:

1) The falling-edge is much quicker... that'd make sense with-regards-to the idea that the internal amplifier is a NPN common-emitter amplifier. The transistor being capable of depleting internal capacitances' charges quicker than the pull-up resistor.

2) The "pixels" on the rising-edge *almost* look like they're occurring at .7V intervals...? I count 18 "blips" over 12... = .67V per step... INTERESTING.

---------------------

Stay tuned for actual results... They're functional but... interesting. Also, thoughts on plausibly *not* needing freewheeling diodes with a Class-B amp?