Opamps with inversion problem.

I wonder if perhaps I could make good use of the inversion bug, using it to extinguish outputs without the usual extra circuitry required when input i s out of range. I've read mixed reports on the TL06/07/08 series, LF356 & M C34081 so question is which cheap common opamps can I /rely/ on to invert o utside of a narrow input voltage range? An opamp where V_in needs to go clo se to the rails won't really do it.

thanks NT

Reply to
tabbypurr
Loading thread data ...

They don't _invert_, exactly. What happens is that when the positive CM range is exceeded, the output goes to the negative rail. This happens regardless of which input is higher, which is why it's not really inversion.

The confusion arises because that situation only occurs in low gain noninverting circuits, especially followers.

What exactly did you have in mind?

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
 Click to see the full signature
Reply to
Phil Hobbs

Looking at the detailed schematic for e.g. the TL082 I don't think anything about the particulars of the states where one or both inputs exceeds the negative common mode range will be reliable or predictable, in part due to the way the bases of the input stage current mirror loads are biased, the operating point in that situation not well-defined and what exactly happens and when be dependent on manufacturing tolerances and vary from device to device.

All the manufacturers seem to guarantee in the data sheets is if it does invert it won't latch up.

Reply to
bitrex

t to extinguish outputs without the usual extra circuitry required when inp ut is out of range. I've read mixed reports on the TL06/07/08 series, LF356 & MC34081 so question is which cheap common opamps can I /rely/ on to inve rt outside of a narrow input voltage range? An opamp where V_in needs to go close to the rails won't really do it.

sure, it's just often called inversion

Hence I'll be using a noninverting circuit to get it to happen. AIUI the problem results from input exceeding the opamp's input v range, th ereby switching off one of the input transistors. So it should still happen even if the opamp is operating open loop, which is what I plan.

circuit-wise do you mean? I want the output to flip to a rail if V_in goes a couple of volts outside it's usual range. The couple of volts spec there is vague, but needing the input to approach a rail before flipping is not u sable. The opamp will run like a comparator with no nfb so there should be no latch-up regardless of anything. So it does have to function when output s get railed in a quad pack eg. I don't care if it hangs for upto about 0.

2 seconds, or if GBW nosedives etc, it's only running a bargraph display. T he result of this 'inversion' will be that lower segments will go out when higher segments light.

The issue is that I need to know which common cheap opamps can be relied on to do this inversion trick.

NT

Reply to
tabbypurr

t to extinguish outputs without the usual extra circuitry required when inp ut is out of range. I've read mixed reports on the TL06/07/08 series, LF356 & MC34081 so question is which cheap common opamps can I /rely/ on to inve rt outside of a narrow input voltage range? An opamp where V_in needs to go close to the rails won't really do it.

I don't expect manufacturer guarantees obviously. And the inversion thresho ld will vary some, but not hugely. No problem there.

NT

Reply to
tabbypurr

On Wednesday, October 31, 2018 at 11:44:19 PM UTC-4, snipped-for-privacy@gmail.com wrot e:

to extinguish outputs without the usual extra circuitry required when input is out of range. I've read mixed reports on the TL06/07/08 series, LF356 & MC34081 so question is which cheap common opamps can I /rely/ on to invert outside of a narrow input voltage range? An opamp where V_in needs to go c lose to the rails won't really do it.

Lack the skill set to make it happen using a more conventional circuit topo logy...

Reply to
bloggs.fredbloggs.fred

t to extinguish outputs without the usual extra circuitry required when inp ut is out of range. I've read mixed reports on the TL06/07/08 series, LF356 & MC34081 so question is which cheap common opamps can I /rely/ on to inve rt outside of a narrow input voltage range? An opamp where V_in needs to go close to the rails won't really do it.

pology...

I don't want the extra components.

NT

Reply to
tabbypurr

Red LED "dotgraph" (apologies for the "quality" of schematic, the second transistor is also grounded and not connected to non-inverting input):

Reply to
bitrex

you can't make a silk purse out of a sow's ear but you should be able to make a nice faux sow's-ear purse out of a billygoat's ass

Well that's the idea of the joke at least maybe needs development

Reply to
bitrex

e:

it to extinguish outputs without the usual extra circuitry required when i nput is out of range. I've read mixed reports on the TL06/07/08 series, LF3

56 & MC34081 so question is which cheap common opamps can I /rely/ on to in vert outside of a narrow input voltage range? An opamp where V_in needs to go close to the rails won't really do it.

topology...

How do you know the "extra" components are excessive unless you sketch up t he conventional circuit. And it's not the number of components so much as t he number of different part numbers on the BOM that make the circuit diffic ult to populate. It may not be wise to use that inversion property for anyt hing safety related as there may be as much as 2:1 variation in the CM rang e ( as measured from the supply rail).

Reply to
bloggs.fredbloggs.fred

it to extinguish outputs without the usual extra circuitry required when in put is out of range. I've read mixed reports on the TL06/07/08 series, LF35

6 & MC34081 so question is which cheap common opamps can I /rely/ on to inv ert outside of a narrow input voltage range? An opamp where V_in needs to g o close to the rails won't really do it.

topology...

What is it?

Reply to
bloggs.fredbloggs.fred

attempt at meeting the OP's requirements as stated in another post in the thread without any "exotic" components and low-cost, a "bargraph" display where instead of the LEDs illuminating progressively and simultaneously as the input signal voltage increases, the previous LEDs are extinguished and only the highest LED is lit.

Looked at it again though and that circuit won't work as drawn, needs fixin'

Reply to
bitrex

it to extinguish outputs without the usual extra circuitry required when in put is out of range. I've read mixed reports on the TL06/07/08 series, LF35

6 & MC34081 so question is which cheap common opamps can I /rely/ on to inv ert outside of a narrow input voltage range? An opamp where V_in needs to g o close to the rails won't really do it.

topology...

that uses extra parts though. I'm doing minimum possible cost stuff. Opamps with a small differential V_in would get me a useful extra feature.

NT

Reply to
tabbypurr

ote:

ng it to extinguish outputs without the usual extra circuitry required when input is out of range. I've read mixed reports on the TL06/07/08 series, L F356 & MC34081 so question is which cheap common opamps can I /rely/ on to invert outside of a narrow input voltage range? An opamp where V_in needs t o go close to the rails won't really do it.

t topology...

the conventional circuit.

there are several ways to do it. Regardless though if I can get a power sav ing feature for zero extra parts that's good.

part numbers on the BOM that make the circuit difficult to populate.

both make it cost more IRL

ated as there may be as much as 2:1 variation in the CM range ( as measured from the supply rail).

of course :)

NT

Reply to
tabbypurr

The usual way to do that is to wire the LEDs in series. That way the only one that lights is the one whose top comparator is at VDD and whose bottom comparator is at 0. (See the LM3914 datasheet.)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
 Click to see the full signature
Reply to
Phil Hobbs

I noticed it doesn't even work as a matter of fact ;) The general topology could work but would require still more parts mostly a lot of resistors. Meh.

I use the TL431 as constant-voltage-source dropper for mutliple LEDs-trick all the time though with e.g. 7 segment common cathode red LED displays, might consider that whatever topology you end up with, it can save a lot of current limiting resistors and board space

Reply to
bitrex

to extinguish outputs without the usual extra circuitry required when input is out of range. I've read mixed reports on the TL06/07/08 series, LF356 & MC34081 so question is which cheap common opamps can I /rely/ on to invert outside of a narrow input voltage range? An opamp where V_in needs to go c lose to the rails won't really do it.

Feedback time. I reckoned it could be done cheaper with discrete darlington s, no comparator. So am looking at that currently to see if I can make it w ork.

NT

Reply to
tabbypurr

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.