OP-Amp maximum input voltage

[toninlg]

amp max input

Hi,

For the majority of OP-Amp, the maximum input voltage is equals to the power supply voltage. What happens if the input signal is different of 0 while the OP-Amp power supply equals 0?

Thanks a lot.

 

[FvM]

op-amp maximum input voltage

Hello,

the said amplifiers mostly have diodes connected between input and both supply nodes, serving as input protection against ESD (electrostatic discharge). These diodes have a limited current capacity, could be e. g. 10 to 30 mA DC. Connecting an input voltage with missing supply could result in damaging the diodes. In many applications, where this situation can't be excluded, series current limiting resistors at the inputs should be considered. For high frequency applications, they could be bypassed with capacitors. Many OP data sheets are discussing the topic.

Regards,
Frank

 

[toninlg]

op amp can input exceed supply voltage

I know that there are diodes in microcontrollers for example but is it the same for op-amp? In the datasheet of ua741
https://www.ortodoxism.ro/datasheets/texasinstruments/ua741.pdf
it is said that "The magnitude of the input voltage must never exceed the magnitude of the supply voltage or 15 V, whichever is less." The schematic is in the datasheet but there are any diodes in input stage. Perhaps is the limitation due to the transistors layout? But if so what is the cause for op-amp with JFET input stage?

Thanks a lot.

 

[FvM]

Hello,

you are right to mention LM741. It's an example of an OP with input voltage limited to supply in maxim ratings without any internal protection. The same with a lot of bipolar OP from the 60th and 70th. These devices probably can be damaged by input voltages exceeding the supply range.

Many devices, bipolar and FET, eg. LM358, TL071 have an allowed maximum input voltage ranging from Vb- up to a positive limit independent of supply, allowing the input voltage to swing above Vb+, also with no supply. They typically have a substrate diode from input to Vb-. Also if not specified, it may be able to carry off a small negative input current. With higher negative currents, there is danger of causing excessive supply current and even latchup effects, possibly destroying the device.

Protection diodes for both polarities with specified current capacity can be found with newer devices. Utilizing specified data, you can avoid external protection diodes with these OP.

If behaviour is unspecified in datasheets or specified protection is insufficient, external protection is advisable.

Regards,
Frank

 

[JDJ]

"The magnitude of the input voltage must never exceed the magnitude of the supply voltage or 15 V, whichever is less."

That should be your clue. The only opamps I have seen that operate with voltages above or below their supply voltages are specially made high side current senses (there may be others).

If you think about the input circuit used (differential pair), assuming the positive supply is connected with low impedance to ground (discharged supply caps), if you bring either of the inputs above that voltage (>0), you are clamping the input to ground through the forward biased BC junction of the input transistor AND the collector resistor. Also most ICs are essentially built on a substrate that is, in normal operation, reversed biased. If you reverse that bias (now forward), you no longer have separate components. Unfortunately, outside of normal operating specs, (reversed supply etc.) an IC is totally different circuit than the equivalent discrete circuit.

A side note is that early MOS SINGLE SUPPLY logic devices (the earliest chips required external negative supplies) used internal negative voltage generators to keep the substrate reverse biased; look at early single supply EPROMs (NMOS); that little chip beside the memory is the bias generator. If you really want to prove it: connect power only to the chip and put a meter on the pins; several will have a diode drop NEGATIVE voltage on them (that's the protection diode clamping the voltage). CAUTION, not recommended for a chip you want to use again, as the floating inputs could be damaged.

This may not hold true with today's IC designs as I know some types actually use insulating layers between circuit parts and the substrate, instead of just reverse biased PN junctions (anyone know?).