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opamp problem

I suspect your feedback resistor is drawing a significant current from the output stage, which loses most of its pull down capability when the output swings below a volt or so. Above about .6 volt output, the pull down is performed by a PNP follower, but below that, there is only a very low current single transistor acting as a current source and losing gain rapidly below one diode drop as the collector base junction goes forward biased.

Take a look at the graph of Output Characteristic Current Sinking at:

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The approximate schematic (including the output transistor shown as a

50 uA current source in parallel with the PNP follower is on page 19.

True rail to rail opamps usually have a higher current mosfet output transistor that looks like a low resistance when it is on, instead of like a current source.

Reply to
John Popelish
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Hi

I think I must be misunderstanding something about the 358 opamp. I am using this part because I understand that it works down to 0V in a single ended supply configuration. Vcc=12V.

Sure enough, when I set it up as a simple voltage follower the output voltage will follow the input voltage right down to gnd. very nice.

But really I want to use it as a inverting amplifier, unity gain. The input signal is 3.3 Vpp. So I wire amp up in standard inverting configuraiton, R1 = R2 = 22k, so gain = 1. The non-inverting input connects to Vb = 3.3/2 = 1.65 through 10k.

The idea is the opamp will take the input signal, level shift it to around Vb and output it with no gain. This circuit works fine with a rail-to-rail op amp. But i figured I could use a 358 since i have such a high Vcc (no worries about input signal getting to close to vcc) and since this part can swing right down to 0 v.

But the sine wave output is clipped at 0.5 volts. Why doesn't it swing all the way down to ground?

Thanks for any advice/ideas?

--
Chris
Reply to
Chris B

I suspect the 358 has trouble sourcing any current when close to ground potential.

Try a pull-up resistor to 12V on the output so it's only ever sinking current. Say 10k.

Graahm

Reply to
Pooh Bear

BBBBut.. the top popped off LMV824 and my AD8515 is sending smoke signals....

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

I suppose I could go looking for one that's rated at 16V, but the fun is over here. ;-)

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

On 07/12/2005 the venerable Chris B etched in runes:

I have always found it better to provide opamps with a small negative supply if the output is to include 0V. Use one of the miniature 12V->5V DC->DC convertors such as Traco TME2405S.

+12V and -5V supply rails are perfectly acceptable for any op amp. 
--
John B
Reply to
John B

On 08/12/2005 the venerable Spehro Pefhany etched in runes:

OK, OK, I'll finish the sentence. ... any op amp that can take a 15V or higher supply.

--
John B
Reply to
John B

Yes the negative rail is an option and was actually used on a previous design. The other option is rail to rail op amps. The goal of this new design is all cost cutting. So the idea of using 358 with single rail is quite attractive.

Chris

Reply to
Chris B

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