Not sure what your output current spec is. But if your goal really is 2A then this circuit ain't powerful enough.
BTW, for low frequency power excitation (testing 400Hz aircraft stuff and so on) I use a function generator followed by a 20-year old stereo amplifier. Many of those are DC coupled throughout most of the power stages, just not at the very front. I could imagine that some of RadioShack's Realistic brand amps that run on 12V might be "hackable" for your purpose.
When building from scratch you'd probably be better off with chips like this:
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They are essentially Opamps of the "Goliath" category :-)
If you feel really, really adventurous you could consider class D, or butchering a synchronous buck switcher circuit.
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Regards, Joerg
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Back in my MIT technician days (~1960) I built a dual 400W tube amplifier to drive a two-phase smear-camera motor. Wound my own output transformers on C-cores ;-) ...Jim Thompson
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I love to cook with wine. Sometimes I even put it in the food.
My first tube amp can best that. Solid 1200W output up to about 20MHz, and the power was essentially limited by the 230V/16 breaker in the electirical panel.
I did that a lot, too. But mostly on E-I cores. I think over time my wooden mallet became at least 1/2" shorter from hammering the steel plates back through the bobbin. The last one was always the white-knuckle job, could crack the bobbin.
When I built an audio tube amp as a kid there just wasn't any money for a decent core package for that. So, I decided to take a huge circus speaker and rewind it, to get it's impedance above 200ohms. Worked, but I will never ever do that again. Very frustrating, and I was covered in lacquer.
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Regards, Joerg
http://www.analogconsultants.com/
"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
I used pre-made laminated C-cores, face-to-face C's held together with a big-ass hose clamp (the threaded variety). ...Jim Thompson
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| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
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I love to cook with wine. Sometimes I even put it in the food.
Yes, simple if 10.5Vpp output from a 12V supply is adequate. But what about the rail-to-rail swing?
I attempted the complimetary pair approach, but cannot get the biasing right, probably due to inexperience.
Some readers here have mentioned that 12V audio "op amps" or certain of the TDA series can do DC. If so, how about some reports of specific success? I have found in the past the output degrades below audio.
I am not an EE, but I can't believe this project is really as hard as it seems (to me). A 12V single supply power amp wiith 12V 2A output, discrete output stage and response to DC. This must have been done before.
OP wants bomb proof, but didnt specify how bomb proof. I figure bomb proof means at some time its likely to get connected to some energised supply rather than a passive circuit. With diodes to rails, psu rail crowbar and output fuse it could be made to survive direct connection to the mains.
Significant question here: since you only want up to 5kHz, do you care if there's distortion products above 5kHz? If not you could just leave the class B output pair unbiased and low pass filter the output. A dirty approach, but it does have energy efficiency in its favour.
I'm not clear what you mean by that last sentence. I presume the problem you're having is crossover distortion, which the opamp mostly corrects, but not fast enough to wipe out distortion.
- bias trs to conduct slightly at zero opamp output, this is the classic total solution. There are also less perfect options:
- if it were a 1 off I might just use germanium output trs, they work fine like this without bias. I'd not touch them for commercial projects though.
- using an opamp with higher slew rate can help.
To explain slew rate a bit, a 6kHz max at full swing opamp like the
324 is nowhere near fast enough to produce a clean 5kHz output without output device biasing, it doesnt have enough slew rate. On the other hand with biased trannies it can do the full 20kHz cleanly, though not at full swing.
If it's a unipolar current source, it swings up to close to the +12v rail on positive excursions. How far negative it swings depends on the load; all the amp can do is let go. If it drives a resistive load, the load will pull itself to ground. If it drives an inductive load, it could swing far below ground.
A unipolar current source doesn't need a pulldown. A single PNP transistor or p-fet is all it needs in the output stage.
But still, it won't be simple. Electronics is almost never simple.
Not hard. Most chip amps are op-amps with oversized output stages, so they bias just like any other. They usually include big capacitors for input or output to minimize offset and remove DC from the input (which is important when DC is present, as on so many bad sound card outputs).
Two issues to resolve:
How close to the rails do you want? If >1V "dropout" is acceptable, then that's not really "rail to rail" as the term is usually understood.
R2R or LDO usually means there is no fundamental limiting factor between input and output. This usually precludes an emitter follower, which drops >0.6V. Circuits are usually open-drain (or collector), allowing arbitrarily low voltage drops (usually a resistive curve, tending towards
0V at 0A, and still being good up around rated current).
There are some borderline exceptions, like the popular LM1117 "LDO" regulator, which is still an emitter follower, but it's better than the old LM317, which, I forget if it's darlington (1.2V drop!) or just by design has the dropout it has (~2V).
You could practically use a R2R op-amp (like TLV2372) driving a generic LDO chip. Hmm, that must be a nightmare to keep stable...
If dropout isn't really that big of an issue, an average audio amp like this:
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wouldn't be too bad. The dropout would be around 1.5V as shown. The bottom half (Sziklai pair) is actually better; you could use Sziklai for both output transistors and get closer to 1V dropout. For lower drop, the whole thing needs to be finagled, and you get into issues that emitter followers are free of, like shoot-through.
Alternative approaches include bootstrapped MOSFETs, such as:
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of course this requires an additional supply for the high side circuit. It could be built with a P-channel high side, in which case the whole high-side driver gets flipped (PNP for NPN), and a third current mirror is required to couple it. A somewhat simpler circuit could be used, since you don't need the speed that this circuit gets (good open-loop performance into the MHz!).
Does current reverse? If you don't know, do you know what kind of loads it will drive, and at what rates?
If you'll be driving full swing (0 to 12V) at maximum frequency (5kHz), triangle waveform (or slew-rate limited), you'll have a maximum dV/dt of
0.12 V/us. If the load is R || C, drawing peak rated current at peak output voltage (i.e., 12V / 2A = 6 ohms), the average current draw will be
1A (assuming it's biased in the middle), and the maximum capacitance will be 1A / (0.12 V/us) = 8.3uF. More capacitance and the amplifier will no longer be in control in some points of the waveform, voltage will "coast" down to the actual value.
Tim
--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms
Hmm ...you guys are great, but this R2R criteria in a power stage seems tricky even for advanced designers. I was expecting there to be a standard solution. Guess that's why I never found it out there.
On a positive note, the OP547 power op amp, having somehow solved the "wide output swing" condition, is looking increasingly attractive.
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And I will have a second look at the near DC response of audio power op amps.
Thanks to everyone for sharing some of your holiday time here
I've found that most R2R amps have current mode outputs, which gives you the closes to the rails you can get if we are not talking about C/M/mos here, in which case, would just be R types of outputs via the SS and DD.
IIRC, the LM324 is one that supports that on the low side but not on the high side, since the low side is collector coupled with emitter to the Vee rail.. High side being a emitter follower.. It's been a while since i've looked a 324 lay out but I think that is what makes a LM324 great for sinking signals.
Nah, you're just moving the goal posts--nice try though. Diodes to the mains are useful with inductive loads hanging on drains or collectors, where you can get serious kicks, but you don't need them with emitter followers. And they certainly wouldn't help anything survive a short to AC mains.
A couple of John L's depletion MOSFETs would be the way to make it survive that.
"Jamie" wrote in message news:luIRo.38550$ snipped-for-privacy@newsfe04.iad...
Yes, 324 (quad) and 358 (dual, and I forget if there's a single as well) are "single supply" amps, meaning the inputs and outputs include the negative rail. This usually comes at the expense of weird behavior when Vcm is exceeded (reversal of output polarity) and bad distortion (the output stage is class B or something like that).
The upper range is the traditional Vcc-2V limit for both input and output.
RRIO amps exist, such as the TLV2372. They basically make complementary LMC324s and wire them in parallel: that way, when the N side is biased off by the high common mode voltage, the P side takes over. The two halves are not perfectly matched, and the datasheet emphasizes the change in input voltage offset which accompanies this transition!
Tim
--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms
I'm not sure if it was a misapplication, or the fact that everyone used them, but almost every time I got a small guitar amp in to repair, I found a dead TDA 2030 inside.
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