Opamp current clamp outside common mode range

[Flyback]

Hi,
To what current, if any, would the attached current clamp regulate the current, given that it is being operated outside of its common mode input range?
LTspice and jpeg attached.
The opamp is actually a TL084C.

TL084 family datasheet
https://www.ti.com/lit/ds/symlink/t...l=https%3A%2F%2Fwww.ti.com%2Fproduct%2FTL084H

 

[ronsimpson]

I thought the TL084 input range included the + supply.

 

[Flyback]

Thanks, thats only the TL084H.

 

[ronsimpson]

If you are just outside the common mode input range, it will function with less gain and slow.
At some point the output is unpredictable.
Many spice models do not watch for CMIR.

 

[Diver300]

The data sheet seems to say that the common mode range goes up to 4 V less than the supply. Running at 1.25 V less than the supply may well mean that the op-amp will have zero gain, and will have stopped reacting to one input or the other.

 

[Diver300]

The input voltage to the op-amp will be 18.75 V. It's 1.25 V below the supply, and not 18.25 V as the diagram says.

 

[Flyback]

Thanks, this is always the situation, when someting is outside of limits, and what then happens......regarding the NON TL084H versions, ie the TL084CDT version...this is what a "Guru" on the offficial Texas instruments forum says...

QUOTE >>>>>>
The datasheet says that the common-mode input voltage range typically goes up to V+. So I'd expect the worst that would happen is that the electrical characteristics are not met, i.e., that the offset voltage could be somewhat larger than 6 mV.

Just use the TL084H (or TL081H for a single channel) instead, which is guaranteed to go up to V+ (and is cheaper, and has a smaller offset voltage).
<<<<<< UNQUOTE

So, if we dont use the TL084H, but other TL084x , then, now with this extra info..... what would be the worst that could happen may i ask?

 

[Flyback]

From what i can see, this different TL084C by st.com would be able to handle input voltages tight up to the vcc+
(Our Vcc+ is +20V, our Vcc- is 0V)
Would you agree?

https://www.st.com/resource/en/datasheet/tl084c.pdf

Current clamp in jpeg and LTspice attached

 

[Roff]

Is the load actually a 100 ohm resistance? If not, what is it? I have a circuit for you that should work if the maximum voltage across the load is not more than about 12 volts.
EDIT:
The 5.1v zener could be replaced by a 47 ohm resistor.
You might not need the Darlington pair, or the 0.62 ohm resistor.
You might be able to use the same technique on your existing circuit.

 

[danadak]

Zip the spice model then attach.

 

[Roff]

Zip the spice model then attach.

Done. Thanks!

 

[Roff]

Is the load actually a 100 ohm resistance? If not, what is it? I have a circuit for you that should work if the maximum voltage across the load is not more than about 12 volts.
I have updated the schematic. See attached.

 

[Tony Stewart]

why does question seem repetitive recently?

I answered before the TL07xx handles Vcm toVcc+

TL07xx Low-Noise FET-Input Operational Amplifiers
1 Features
• High slew rate: 20 V/μs (TL07xH, typ)
• Low offset voltage: 1 mV (TL07xH, typ)
• Low offset voltage drift: 2 μV/°C
• Low power consumption: 940 μA/ch (TL07xH, typ)
• Wide common-mode and differential
voltage ranges
– Common-mode input voltage range
includes VCC+
•

"guaranteed"

 

[Flyback]

why does question seem repetitive recently?

I answered before the TL07xx handles Vcm toVcc+

Thanks Tony, only the TL074H and TL084H offer CM input range up to Vcc+.
As we discussed before, the datasheet is somewhat misleading......and comes tantamount to down right contradicting itself.

It seems to be playing on peoples belief that people think that JFET input opamps tend to have CM input range up to the POS rail...which they dont.

Its the marketing people...they are basically mugging people in to wrongly using the TL074(not H) on thier PCB...then when they find it doesnt work....they push them into the TL074H....which does work....but its the only sub $3 opamp in the world that has CM input to the pos rail, and can also do 20V pos rail.......so people of course, would not usually choose such a one-off part in the first place (second source problems).......but this is how people are being mugged over.....its a clever little trick i confess.

 

[ZipZapOuch]

What is the reason for not using the better AND cheaper op amp - TL074H?

 

[Flyback]

thanks, TL074H and TL084H are the only sub $3 opamps in the world which do 20V...and CM in to vcc+.
So they are too rare.

 

[Tony Stewart]

Thanks Tony, only the TL074H and TL084H offer CM input range up to Vcc+.
As we discussed before, the datasheet is somewhat misleading......and comes tantamount to down right contradicting itself.

It seems to be playing on peoples belief that people think that JFET input opamps tend to have CM input range up to the POS rail...which they dont.

Its the marketing people...they are basically mugging people in to wrongly using the TL074(not H) on thier PCB...then when they find it doesnt work....they push them into the TL074H....which does work....but its the only sub $3 opamp in the world that has CM input to the pos rail, and can also do 20V pos rail.......so people of course, would not usually choose such a one-off part in the first place (second source problems).......but this is how people are being mugged over.....its a clever little trick i confess.

false assumptions lead to false concluisions

 

[ZipZapOuch]

false assumptions lead to false concluisions
View attachment 144400

Umm

From what i can see, this different TL084C by st.com would be able to handle input voltages tight up to the vcc+
(Our Vcc+ is +20V, our Vcc- is 0V)
Would you agree?

https://www.st.com/resource/en/datasheet/tl084c.pdf

Current clamp in jpeg and LTspice attached

 

[Tony Stewart]

Umm

that refers to his comment on the 84C and not my reaction to the 74 which does have a 2nd source with TI that typically works Vcm=Vcc+ but ST does not guarantee all will do this, just typ.

so it is a yield issue which may or may not affect costs and shipments on-time. But it is a 2nd source.
Also I didn't know TL084C exists at ST
. I only found TL084I/AI/AC/BI/B at
and
TL081, TL081A, TL081B, TL081H at TI
yet
I only recommended the 74 from either TI and ST although suffixes are not all the same. TL074I, AI, BI TL074C, AC, BC from ST
With TI's drop-in improved replacement TL074H

A better design may be useful.

Furthermore since TI recently improved the 74 to be more cost-effective in the "H" version, with EMI hardening and improved performance, one should not assume this product will cease production anytime soon!.

umm!