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Topic: Which op-amp would you recommend? (Read 1 time) previous topic - next topic

dkl65

Hello!
Which op-amp would you recommend for general-purpose current boosting, but keeping the voltage the same? If possible, make that recommendation out of these op-amps: http://sayal.com/STORE/Action_SEARCH.asp?search_key=op-amp&Submit=Search. How do I use an op-amp (i.e. sample circuit would be appreciated).
Thanks!
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Runaway Pancake

Op-amp outputs are, generally speaking, low current affairs.
State your need ("I have x... and I need to result y.)
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Krodal

#2
Jul 08, 2012, 08:20 pm Last Edit: Jul 08, 2012, 09:07 pm by Krodal Reason: 1
Look for "buffer amplifier" or "voltage follower".

But I'm wondering: how much current ? at what voltage ? for what application ?

wanderson

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dkl65


But I'm wondering: how much current ? at what voltage ? for what application ?

I should have specified that earlier. Lets say I have a low pass filter. It uses a 10k ohm resistor, 100nF capacitor, and a PWM output of greater than 32kHz to create any voltage between 0V and 5V, or a voltage divider. The current produced is very limited, due to the resistor. I would like the current boosted, but the voltage should stay the same. That current would be used to power a device or component. The more current capability, the better. I also will use that current to power an LED (using a 330 ohm resistor to limit current, but change the voltage).

Current: the more the better
Voltage: 0 - 5V
Application: powering components
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SirNickity

Still a bit vague.  "Components" can be anything from a TTL output to the city of New York.  Most op-amps have current limits of about 20 - 40mA.  That's not much.  An LED would be fine.  A DC motor might not be.

Effectively, you're building a switching PSU.  That's fundamentally not an easy affair.

You might have some luck using a power op-amp like the LM3886 for instance.  It's meant for audio, but it's basically a high-power op-amp and can be used as a DC amplifier.

dkl65

#6
Jul 09, 2012, 11:16 pm Last Edit: Jul 10, 2012, 03:04 pm by dkl65 Reason: 1
How will I wire the circuit to the op-amp (pin-outs)? Lets say I am using the low pass filter described above.
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Runaway Pancake

#7
Jul 10, 2012, 01:44 am Last Edit: Jul 10, 2012, 01:48 am by Runaway Pancake Reason: 1

How will I wire the circuit to the op-amp (pin-outs)? Lets say I am using the low pass filter described below.

What where below?

In the link is a hybrid solution, an op-amp with a pass transistor.
Instead of a voltage derived from a pot, as shown, the non-inverting (+) input would be connected to this low-pass filter's output.

**************
Forget the link.. here it is --
http://www.zen22142.zen.co.uk/Circuits/Interface/cmscontrol.htm
"Hello, I must be going..."
"You gotta fight -- for your right -- to party!"
Don't react - Read.
"Who is like unto the beast? who is able to make war with him?"

Krodal

I'm still not convinced you need an analog current driver.
To dim a led, PWM is perfect. As you already know: http://arduino.cc/forum/index.php/topic,113060.msg850201.htm
For a motor, PWM is almost perfect. If the motor is making a whining noise, the PWM frequency can be changed with some tweaking.

Docedison

Does the capacitor go to ground? seems to me it should... If it does then the available current is equal to the voltage divider formed by Xc (100nF) and the 10K resistor.
So we have 1/2Pi X F X C and a 100 nf capacitor has about 50 ohms of reactance @ 32 Khz  If the resistor is going from the PWM Pin to the capacitor and the capacitor goes to ground (classic 2 element Low Pass Filter) then you have a 10K resistor connected to a 50 Ohm Reactance @ 32 Khz at the junction of the resistor and capacitor. The capacitor is a filter in that it will store some current from the PWM pin through the resistor... but there will only be about 500 uA of available current (5V/10,000 ohms).
I've never seen an LM386 used as a DC amplifier... It's not really too ideal a part for that purpose as it's biased to place the output voltage @ 1/2 Vcc and since it is essentially an AC amplifier the drift and Ibias current aren't well specified. It could I suppose be forced to do what you want to do but the earlier suggestion about a Buffer amplifier is right on target. National used to make several different types... the only one that comes to mind is an LH033 100 mA output current But that is an old part... Seems to me there's a way with an NPN transistor inside the feeback loop (Gain Set) that would work very well... It's been 15 years since I did anything like that and it would take me an hour to design and test (simulate)... Time I really don't have now to spend here and now... Ti handles the National line now... I would recommend going "Shopping" for a suitable part on the Ti site... I know you will find what you need.

Doc
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dkl65

Right now, my only concern is about how to use an op-amp (where to plug the ground, Vcc, voltage input / output, etc.) to power a load that takes 20mA or more (e.g. LED). Take a look at Low Pass Filter.png. With the op-amp, the LED would be at the output of the op-amp, not where it is now. Would something like "OP-AMP SINGLE BIPOLAR/8P DIP" or "SINGLE JFET-INPUT OP-AMP 8P-DIP" be okay (from Sayal)? Where How will I wire an op-amp to this circuit?
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SirNickity

Hey Doc, I said 3886, not 386.  :-)  That's not a typo, it's a different animal entirely.  It's still designed ideally for audio, but there's no automatic biasing, so it'll work for DC amplification.  (And does, if you build your amp wrong.  Guess how I know.)

I get what you're trying to do here -- you're using the cap to "smooth" the PWM and turn it into a lower constant voltage.  However, you need some sort of feedback to monitor the voltage and adjust the PWM frequency as necessary to maintain that voltage.  Otherwise, with little to no current draw, you'll eventually fill the cap and the PWM does nothing.  At higher current draw, the cap will empty and you'll just get PWM with a smoother waveform.  Since op-amps are ideally infinite impedance, you'll probably lean toward the former.

Also, you might want a higher-order filter to smooth out more ripple.

Anyway, barring those difficulties, your op-amp would be a simple voltage follower arrangement:

Op-amp Vcc to +5v
Op-amp Vee to ground
Op-amp - input to op-amp output (unity gain feedback loop)
Op-amp + input to the output of your PWM filter (where the LED is now)
Op-amp output to its - input (as noted) and to a current-limiting resistor and LED

dkl65

Do any of the op-amps from the link in the first post (Sayal) provide at least 20mA of current to the load? Would "supply current" be how much current the op-amp can supply to the load?
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SirNickity

You'll have to check the datasheets.  That particular supplier doesn't provide much info for you.  Several of those are listed as "low power," which is opposed to your goal of a current booster.

I Googled one of the parts and found that it has a 6mW dissipation limit.  That's about useless.  I would check another supplier, preferably one that lets you search by design requirements.

JoeN


Hello!
Which op-amp would you recommend for general-purpose current boosting, but keeping the voltage the same? If possible, make that recommendation out of these op-amps: http://sayal.com/STORE/Action_SEARCH.asp?search_key=op-amp&Submit=Search. How do I use an op-amp (i.e. sample circuit would be appreciated).
Thanks!


That's an odd store.  You can check availability but not price, or am I missing something.  What's up with that?  Never hear of these guys before.
I have only come here seeking knowledge. Things they would not teach me of in college.

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