Audio Amplifier as poor man's power op-amp? - Page 1
Forum section: electronics / projects

author: dany-boy
author_rank: Contributor
timestamp: October 01, 2020, 01:01:03 am
content: Hello!So I have a particular need for a relatively high power op-amp. I'm helping tutor a control engineering lab course, and one of the labs is to apply a simple analog PID to a motor. Key point here ANALOG, digital control is another course. I've been tasked with designing a simple, cheap and "good enough" power stage to drive the motor from an analog signal. I've been thinking about it for a while, and wondered what is stopping me from simply using an audio amplifier? It seems like a very straightforward solution, almost too good to be true. The Amplifier would be something like a TDA2050 DC coupled. The Motor we use is rated at 9V 200mA unloaded (which is the way its used for the lab). Any thoughts?Cheers!

author: Cerebus
author_rank: Super Contributor
timestamp: October 01, 2020, 01:43:54 am
content: Nothing's stopping you, it's a perfectly sane idea. The Crown DC300A (note the DC), once the mainstay audio amplifier of live performance and studio monitoring could also be found sitting in many a physics laboratory driving experiments. Legend has it that the DC300A was originally designed for laboratory use and only later marketed as an audio amp.The only thing that might be a problem is the DC bit. Many audio amplifier designs (both discrete and integrated) aren't expressly designed for operation at DC or at frequencies significantly below the audio band. Bit of a problem if you're trying to servo a PMSM for position, or at low RPM, even worse if you're driving a simple DC motor. You might be well advised to check that any integrated amp you plan to use is genuinely well behaved in the LF/DC region. I doubt you'll have problems, but best to play safe. Not a roadblock, but just a question to be answered.As you're putting things into the hands of students and, potentially worse, lecturers, you might want to make sure that the outputs are well protected from short circuit or over-voltage; the inputs too for that matter. Look on the bright side, you don't have to harden them against Policemen, Soldiers, Actors or Drummers.

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 01, 2020, 01:57:21 am
content: What about a comparator driving a PWM power stage?

author: engineer_in_shorts
author_rank: Regular Contributor
timestamp: October 01, 2020, 02:31:44 am
content: Why avoid using an opamp?I would have thought the classic opamp with a couple of bipolar transistors would work.

author: LaserSteve
author_rank: Super Contributor
timestamp: October 01, 2020, 02:54:50 am
content: LM3886 configured as an inverting opamp with a gain of 20 is good for motors with coils down to 3 ohms. Been using it that way for years with split 24V PSUs. In my case peak currents are an amp or so. You do have to heatsink it.Steve

author: Circlotron
author_rank: Super Contributor
timestamp: October 01, 2020, 03:24:32 am
content: Just watch you don't have the gain set too low. Unlike "proper" opamps, not all audio amplifiers are unity gain stable.

author: dany-boy
author_rank: Contributor
timestamp: October 01, 2020, 03:44:27 am
content: Thanks for all of your replies, in particular to @LaserSteve, @Cerebus and @Circlotron. I'll make some rudimentary simulations to catch any glaring issues regarding stability and start prototyping. For this simple application it seems like a good option indeed.Cheers!

author: David Hess
author_rank: Super Contributor
timestamp: October 01, 2020, 10:28:47 am
content: It depends on the topology since not all amplifiers are "operational" amplifiers with a differential input. But audio power operational amplifiers are very suitable for driving motors.Quote from: Circlotron on October 01, 2020, 03:24:32 amExactly, audio power operational amplifiers are usually decompensated for better high frequency performance, but the usual tricks for stabilizing a decompensated operational amplifier will work on them at the expense of increasing noise.

author: capt bullshot
author_rank: Super Contributor
timestamp: October 01, 2020, 11:18:10 am
content: TCA0372 is a quite low cost power OpAmp that can drive 1A, but has limited power dissipation:https://www.onsemi.com/pub/Collateral/TCA0372-D.PDF

author: emece67
author_rank: Frequent Contributor
timestamp: October 01, 2020, 12:00:03 pm
content: .

author: Dr. Frank
author_rank: Super Contributor
timestamp: October 01, 2020, 12:09:08 pm
content: For that purpose, I like the LM1875, up to +/30V symm. supply, 3A output current , low distortion, used in conjunction (in series) with a chopper Amp.But as with other Power amplifiers, you have to take care about stability.These are mostly stable with gains > 10, so unity gain amplifiers have to be compensated.Frank

author: Cerebus
author_rank: Super Contributor
timestamp: October 01, 2020, 12:13:33 pm
content: Quote from: Circlotron on October 01, 2020, 03:24:32 amWell it's for a lab in "control systems", which I'm assuming is control theory or contains a hefty wodge of control theory, so one sincerely hopes that it ought to be within the wit of the people teaching it to figure out the necessary compensation.Or if they're feeling mean, figuring out the transfer function of the amplifier, publishing it in the lab instructions and leaving the compensation as "an exercise for the student".

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 01, 2020, 03:47:12 pm
content: Agee with the others here. It's a perfectly normal application.In addition:PCB layout is very important for power amplifiers. On the output you have high voltages and currents, while the inputs are sensitive to small voltage differences, and a bit of voltage drop over the wrong PCB track due to the high output and power supply currents can easily be the cause of oscillations and other headache's. Such power opamps also need big elco's for stability, so take care of decoupling.

author: TimFox
author_rank: Super Contributor
timestamp: October 01, 2020, 05:02:07 pm
content: They are not inexpensive, but Apexhttps://www.apexanalog.com/makes good high-power analog op-amps, which work well for driving motors when properly connected. They also make PWM devices for efficiency-sensitive applications.

author: bson
author_rank: Unknown
timestamp: October 01, 2020, 05:20:47 pm
content: Make sure to derate the amplifier's peak power output by a factor of 50-100 or so for a sinusoidal or PWM signal.

author: LaserSteve
author_rank: Super Contributor
timestamp: October 01, 2020, 05:39:05 pm
content: Actually Went home and checked, my Galvanometer scanner amps are drawing +/- 2.2 amps (full deflection left and the full deflection right ) measured at the PSU from LM3886 at DC. With a 3.2 Ohm inductive load. With a modest sheet metal heatsink, they barely get warm.Data sheet says stable at DC as inverting amp at gain of 20 or more.Steve

author: Zero999
author_rank: Super Contributor
timestamp: October 01, 2020, 07:15:54 pm
content: Quote from: dany-boy on October 01, 2020, 03:44:27 amAs mentioned above, stability at low gains can be an issue, but there is a way round that. Configure the amplifier so the noise gain at higher frequencies (above 20kHz, although it can be lower, as you're only interested in DC), is above than the minimum level specified on the data sheet and it will be stable. Refer to the thread linked below.https://www.eevblog.com/forum/beginners/op-amp-spec-noise-gain-configuration/msg2401983/#msg2401983DC performance can be improved by adding a real op-amp inside the loop, but care needs to be taken to avoid oscillation.Just one question: why not use PWM, rather than a linear output stage? It will use much less power and won't require a big heat sink.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 01, 2020, 07:33:54 pm
content: During turn on and short peaks a brushed DC motor can take quite a bit more than the rated current. So it would be a good idea if the amplifier is short circuit proof and with more extra head room for the current. EMI protection of the motor can also be a bit tricky as the amplifiers may not like too much capacitance, at least not without some series resistance.

author: capt bullshot
author_rank: Super Contributor
timestamp: October 01, 2020, 08:28:53 pm
content: Quote from: Kleinstein on October 01, 2020, 07:33:54 pmFor the mentioned TCA0372 (that isn't protected) we (@work) found a pretty simple, effective and cheap solution: Protect its power supply using a LM317. For capacitive load stability, use the usual L || R in its output (as seen in your typical old school audio amp.

author: Zero999
author_rank: Super Contributor
timestamp: October 01, 2020, 09:48:05 pm
content: The TDA2050 mentioned by the original poster should be suitable, but is obsolete so shouldn't be used, unless it's just a one off. The LM1875 would be a suitable alternative. Both ICs are protected against short circuit, so the current surges shouldn't be a problem. Use a bipolar +/-12V power supply, which should give enough headroom, for 9V out.Another question is why not just use an op-amp with some booster transistors? It's fairly simple to implement current limiting.

author: coppercone2
author_rank: Super Contributor
timestamp: October 01, 2020, 10:02:49 pm
content: a old stereo is cheaper then a power op amp (i.e. apex)

author: chris_leyson
author_rank: Super Contributor
timestamp: October 01, 2020, 10:38:54 pm
content: LM3866 or LM1875 as already suggested would be a good choice. LM3866 is popular in laser scanner galvanometer drivers, probably because it's been copied from an old GSI design. Bandwidth and slew rate will be more than enough to drive a 9V DC motor. EDIT: If you need an output current sense for current feedback then a split supply makes the job a lot easier.