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Clamp circuit help

You could use a voltage divider to set the opamp DC operating point, instead of depending on the bias current. And redistributing the gain isn't a bad idea anyhow.

I seldom try to match opamp-input network impedances, unless it's demonstrably necessary. Bias currents are low these days, and more resistance makes more noise.

The Q1-to-C1 thing doesn't make much sense to me. It's charging C1, adding some DC offset, which is not the same thing as clamping the signal swing. Maybe it does what you want, but it's not the same dynamics as what you originally did.

Oh, C1-R1 is a highpass with corner frequency about 72 Hz. So it's costing gain at 50 Hz. Do you only want this to work at 50 Hz?

John

Reply to
John Larkin
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I've now started thinking about using a really inexpensive op amp like the 324 for this circuit, which brings new problems. The 324 has a worst case full range bias current of 500 nA, so even with a lower stage gain the voltage divider is going to have to be low-valued to prevent big errors at the output, even with an attempt to match the impedance of the feedback network. The worst case offset current over temperature is

150 nA. A low-valued voltage divider bias network means an electrolytic coupling capacitor to get the required highpass 3dB point, and higher power consumption. Eeech.

The circuit I'm designing on is one I'm hoping other hobbyists will be able to build if it works out, perhaps in places where "exotic" single supply op amps are not available. I'd like it to work properly with the funky-junkiest Radio Shack parts if possible.

It may be a moot point, in as I've continued working on the project I've found that I actually need the clamp to be somewhere else!

7.2 Hz cutoff, I think if I did the math right?
Reply to
Bitrex

The posted question concerned the clamp behavior, which I readily solved. John "Cluck-Cluck" Larkin's comments on the clamp were off-base.

However, I wouldn't do any of the project as originally posted ;-) ...Jim Thompson

--
                  [On the Road, in New York]

| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

Bitrex is trying to PEDESTAL the signal... DUMB-DUMB ;-)

7.2Hz is correct. ...Jim Thompson 
--
                  [On the Road, in New York]

| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

If you spec'd performance instead of "patches" needed we could get to a solution in minutes. Particularly if you ignore ol' cluck-cluck behind the curtain ;-) ...Jim Thompson

--
                  [On the Road, in New York]

| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

he's

--
Although he's killfiled you, he hasn't shunned you, and since he
replies to you from posts to which you reply, from authors which he
hasn't killfiled, you should expect retaliation when you post insult.

In my experience and, I think Jim's,

(although, of course, I can't speak for him)

our desire to debate your errors honestly has almost always been met
with chicanery.

As far as obsession goes, if one isn't obsessed with eliminating evil
and railing against it, one might as well suck the Devil's cock.

You, apparently, have.
Reply to
John Fields

with

NPN

he's

Bwahahahahahahaha ;-)

BTW, I sent you an E-mail inquiring if you avoided the fires. No response. Have you dropped austininstruments.com?

It might be also that I'm using gmail as a relay on the road :-( ...Jim Thompson

--
                  [On the Road, in New York]

| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

amplify

with

NPN

up

he's

--
No.

I didn't get your email, but everything else here seems to be OK,
thanks. :-)
Reply to
John Fields

with

NPN

he's

How's the gay thing working out for you?

John

Reply to
John Larkin

Oh, I did the rolloff calc after I scaled the resistors down 10:1, noted earlier, to kill the burst oscillation.

John

Reply to
John Larkin

amplify

with

NPN

up

he's

--
Well, since I'm not taking any dick up the ass or sucking any cock,
I'd guess that, according to you, it's not working out well.
Reply to
John Fields

amplify

with

NPN

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You sure talk about it a lot. Graphically. Longingly, almost.

John

Reply to
John Larkin

Bitrex

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Got any popcorn left?

John S

Reply to
John S

with

NPN

I've succumbed to a kind of circuit-design writers block where I'm having trouble keeping focused on what's important amidst all the different issues - perhaps someone else's approach will snap me out of it. In theory this should be a pretty straightforward circuit.

I found my original circuit is not going to work properly with the rest of the system, so forget about it. Let's not worry about clamping, etc for now and break the gain stage section of the new circuit into the basic requirements: I have an AC signal with a large DC offset, and I need a gain of about 1000. Single low voltage supply, say from a couple of AA batteries. With a gain of 1000 applied to the signal of interest the output should be about 0.3V p2p, riding on some DC level that's not really important so long as the signal isn't clipped.

There should be a 3dB highpass point of no greater than 10 Hz, and the upper bandwidth should be around 100 Hz. The circuit should function properly with "funky-junky" single-supply opamps like the LM324 with high input bias and offset currents, but also consume as little power as possible. The 10 Hz 3dB point requirement is "sacred", the other requirements may be bent somewhat as needed. Noise performance is not really relevant.

Is that a detailed enough spec?

Reply to
Bitrex

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There is such a thing as AC-feedback which gets rid of input offset and other DC effects- your BW is so small you can use a single amplifier with the 60dB gain, AC-coupled all the way. Even a little 1 MHz GBW type will roll you off at 1000 Hz so you have plenty of room for the programmed 100Hz rolloff. Make the amplifier inverting to eliminate positive stray feedback... this is a non-project.

Reply to
Fred Bloggs

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Reply to
Fred Bloggs

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Hold it, do I understand you correctly, you have a 300uv p-p at the input and you state this, =93 Noise performance is not really relevant=94.

Is it expected that this circuit operate anywhere near line voltage or equipment being powered by such?

60dB of gain is a bunch not to worry about noise performance.. but then maybe the operating environment is a screen room for all I know.

Aside from all the above, what's wrong with your current gain stage? Btw, do you just have a schematic you can post? I don't care to fart with your spice file.

Reply to
meg

amplify

with

NPN

up

Then why all the hassle about clippers???!!!

What's the load? An ADC?

John

Reply to
John Larkin

with

NPN

Senile old hen can't even invent his own insults! He has to copy mine.

John

Reply to
John Larkin

amplify

with

NPN

clamp

the

amp.

up

10

Well, I am amplifying a white noise source. The point of the gain stage is to get the noise in the bandwidth of interest up to a level that I can use it. I'm not using it for cryptography or anything, so I guess a little more noise can't hurt?

Essentially, the problem is now that I don't think it will work with op amps with high bias currents because of offset issues.

Reply to
Bitrex

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