LM358 too slow!

"Ground sensing"? I think what you mean is that the input common mode range extends to the negative supply rail (that is, to "ground" in a single supply scenario), and the output also can reach the negative rail. That's what the manufacturers typically mean when they call something a "single supply" opamp.

There are plenty of single-supply opamps out there that can do this, as well as plenty of "rail to rail" opamps that can reach both the negative and the positive supply. Just use the parametric search functions for the web site of whatever manufacturer you're fond of.

If you go for a rail-to-rail opamp as opposed to a single-supply opamp, be aware that they can sometimes be less stable than "traditional" opamps. If sensing to the negative rail is all you need, you're probably better off with single-supply.

Thanks for the reply

"ground sensing" is quoted directly from one of the '358 datasheets I've studied.

Rail to rail might be handy as I want to drive a squarewave into the gate of a small MOSFET and it needs to run off a 6V SLA - looking at the '358 O/P stage I doubt it will perform as well as I'd like, who cares what manufacturer as long as I get a part that does what I want.

It's to drive a squarewave and in this application a little ringing isn't that serious a problem - if it rings less than the discrete design I'm using I'll be happy!

A fast comparator with rail to rail O/P might do, but so far I've not found anything suitable in the catalogues I have to hand.

"ian field" wrote in news:rTCch.1964$ snipped-for-privacy@newsfe2-gui.ntli.net:

I've used a CA3140 with positive feedback to make sharp rise/fall times. Fast and accurate enough for feeding into a PLL XOR comparator input in the audio range, anyway. I chose the CA3140 because it will swing fully to zero to allow single supply, use low current input, and be fast.

Can it deliver rail to rail O/P to the gate capacitance of a small MOSFET with single supply down to 5.4V at frequencies upto about 50kHz ?

"ian field" wrote in news:z2Ech.1067$ snipped-for-privacy@newsfe1-gui.ntli.net:

No idea, it's cheaper to try one if you have one around, than to agonise over it, probably. It has 10 mA output drive, a 9V/µs slew rate, and is rated for over 3.5 MHz use, so even at reduced supply voltage it ought to get by at ultrasonic frequencies. The positive feedback will accelerate the slew to overcome momentary lows while charging a capacitance.

It can't do rail to rail, but that wasn't what you asked, at first. It can get within 0.6V though. Whether this is enough I don't know, it's probably best to try unless there is a specific reason not to.

Maybe I'll have a look in my chip tin see if I have one to try, otherwise I'm not even going to bother checking if Maplin carry them until they send me another promotion leaflet with money off vouchers. The discrete design isn't so bad but I ran out of E-line ZTX transistors and TO92 equivalents take more space!

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Don't buy anything from Maplins. My broadband started to go slow and my ISP said I needed a new filter. Maplins said I needed new cable. I bought new cable but the broadband speeded up again - must have been BT. So I took the cable back. But because it had been machine packed there is no way I would return it "in the same condition". The cable WAS "in the same condition". But the idiots said the packing had to be the same -- impossible!

If they want to get things back "correctly" they should use packing it is possible to pack.

Every other company I've taken anything back to, in living memory has accepted it back with full refund.

Except Maplins.

Don't even think of shopping with them.

Don't buy anything from Maplins. My broadband started to go slow and my ISP said I needed a new filter. Maplins said I needed new cable. I bought new cable but the broadband speeded up again - must have been BT. So I took the cable back. But because it had been machine packed there is no way I would return it "in the same condition". The cable WAS "in the same condition". But the idiots said the packing had to be the same -- impossible!

If they want to get things back "correctly" they should use packing it is possible to pack.

Every other company I've taken anything back to, in living memory has accepted it back with full refund.

Except Maplins.

Don't even think of shopping with them.

Why didn't you take the matter up with trading standards?

Having had time to seek out the data sheets, the '3130 looks a much better choice. Maplin don't list it, I suppose now I want some they're obsolete!

"ian field" wrote in news:fUgdh.21$ snipped-for-privacy@newsfe1-gui.ntli.net:

I hope not! I should look at them more, myself. These two IC's in both single or dual form, are very standard devices, it's worth placing an order for a few. Although they won't be ideal for you in this case, check out the LF411/LF412. As a drop-in equivalent to the 417, I find them amazing. Very good general purpose, and in some cases better than more expensive dedicated audio types. If I haven't got any, I make it a priority to get some.

Meant replacement for 741..

Thanks - always open to suggestions, I'll have a look for the sheets for those as well.

How about a plain ol' LM393 dual comparator (or the quad LM339)? Cheap as dirt, supply voltage is 2V to 36V, supply current is low, it should be fast enough for your 50kHz requirement without being so insanely fast that you can't keep it from oscillating, input goes to *below* ground if you want, and output is an open collector, so it'll saturate down to 0.3V or so.

If you're running an H-bridge rather than a single MOSFET it might not be ideal, because you won't have equal charge and discharge currents so your on and off times will be different and you'll get shoot-through. But for a single MOSFET that shouldn't be an issue.