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Driving a Lo-Z load with opamps question

1/14/2000 4:24 PM
Ed Rembold
Driving a Lo-Z load with opamps question
As we know there are a number of Opamps which will drive a 600 ohm load, (some better than others)  
If you had an Opamp which could happily drive a 600 ohm load at 35Ma or so, and you added Another Opamp in parallel  
to the first, would it then be able to drive 300 ohm, while providing 70ma of power? Or am I dreaming?  
Ed R.
1/14/2000 4:27 PM

I've seen some circuits that do this, principally in reverb tank drivers. They usually use a low resistance in series with each output to soften things up a bit.  
Generally they use multiple opamps in the same package just to help the matching as well.  
I haven't tried it myself.
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1/14/2000 4:55 PM

You can try "buffering" the output of the opamp with a pair of transistors in class B (or just a single transistor if you'll only sink or only source current).  
A CMOS inverter can also be used as an output buffer for opamps. It won't have as much crossover distortion as a class-B bipolar buffer. If you use a CD4049 you can parallel inverters to have more current capacity.
1/14/2000 7:25 PM
You could set up the opamps so that they push-pull the load to double the output.  
If you parallel the outputs, I would definitely add a resistor on each output as R.G. said. This will help distribute the load because otherwise one of them would dominate and do most of the work.  
"A CMOS inverter can also be used as an output buffer for  
opamps. It won't have as much crossover distortion as a class-B bipolar buffer."
The CD4007 is also good for this - parallel the 3 pairs of transistors to increase output drive.  
With transistor drivers, to reduce crossover distortion, a resistor could be added between the driver output and their input so that the opamp could supply current at low levels before the transistors switch on. This trick is used in the appnotes for the LM379 and related chips.  
Another idea is to use a power opamp. For instance, an LM380 or maybe even the LM386.  
regards, Jack
1/14/2000 11:41 PM
Ed Rembold
Thanks guys
Thanks again guys, and by the way ,check out the Burr-Brown app. notes when you have time, Found just what I need and more.  
Ed R.
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