I need a high impedance input circuit which will both amplify an input PWM signal and drive a high power circuit with the amplified signal. The input voltage is 1-2 Volts p-p, the output signal must drive a 10 amp 0 - 12 volt load. The 1-2 v p-p has very little current drive capability (less than 0.5 mA). The input signal is a PWM signal with frequency components up to the 100 Khz range.

Can someone point me to a reference circuit or a design you know works. Components should be as inexpensive as possible. Power consumption is not an issue.

Thanks,

Chris Schene

 

Hello,

Your prolem not big but if you are interested in electronics i suggest LM12.

National LM12CL simple power amplifier. This is power opamp (analog) but you can use for PWM amplifier purpose. You can find more info at
www.national.com

Best regards.

Bunalmis.

www.geocities.com/cnc_designer

 

Hi,

Thanks. I checked the price on this one. It's about $16.00 and the gain bandwidth is a little low for my application. It's for an automotive application, so the cost is very important but the power consumption is not.

I was thinking of trying this with a cheap op amp and a high current Darlington pair. I think I can get away with about 2-$3 in parts at high volume.

 

A differential comparitor and a hexfet would take care of this with little fuss or bother. The comparitor would have the reference input held to +1 volt so it would switch at the 50% point on the wave form. As the comparitor would be powered from +- 15 volts, it's output would go from +15 for one state to -15 for the other. This waveform could drive a hexfet directly that switched the load from 0 to -15 volts. With the appropriate choice of the FET, 10 Amps should not be a problem. If you can't visualize this, I can draw a starting point, a real world circuit would clamp the comparitor output to perhaps + .7 - -10 volts and the Fet would be selected so the source was at 0 volts and a negative gate woltage would turn it on. Clamp diodes for the output would be advisable as well.

Regards

cww

 

Perhaps what you really need is two different devices. The first would be a signal conditioner to convert your PWM signal to a conventional 0-5V or 0-10V signal. The second would be a power supply with analogue voltage control.

I can't recommend a particular signal conditioner as you haven't specified what the fundamental frequency of the PWM signal is (I assume the high frequency components are of minor importance). An ordinary DC voltage to voltage converter may work if the input filter frequency is low enough. You should be able to examine the specs of your favourite industrial signal conditioner brand (Action Instruments, Phoenix, Wilkerson, etc.) to find something suitable.

For a power supply various brands are suitable, but I can for example recommend Xantrex. There are others such as Lambda, Kepco, Agilent (formerly HP), etc. These power supplies will take a voltage input and produce a voltage output which is proportional to it, but with the capability to source a high current.

There is a fairly wide price range possible (especially for the power supply), depending upon what features and specifications you need.

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Michael Griffin
London, Ont. Canada
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That's why you use an analog comparator. This is basically an uncompensated Op Amp with speedups for this particular application. The cost is minimal. And the FETs are much easier to drive and much faster with simpler circuitry than a darlington. Heat sinking is also easier because the FET has a very low voltage drop and lower losses in general. Look up an LM311 for the basics and IR has good info on the hexfets although there are lots of competitors now who might be cheaper. This is assuming that you want PWM in PWM out. if you want to convert the PWM to linear, then you need a filter network and some level shifting and possibly inversion. I've got some drawings for that as well. There are probably faster alternatives now for the 311 but that'll get you the idea. These kind a doodads and odds and ends are what I do.

Regards

cww