Confusion with a text about input protection diodes and resistor

Question

Here is the section from a book I came across online:

If I don't get it wrong, it says the diodes D1 and D2 prevent currents to flow into the opamp in case of over voltage. And the reason they are Schottky type is because this type of diodes have low voltage drop which is preventing internal opamp diodes to conduct.

It then says the didoes also "have degrees of freedom". I guess it means the diodes have possibility that they must not act they supposed to be in some occasions. And therefore a series resistor Rlimit limits the current in such occasions.

Here is my confusion:

First of all I used to think that Rlimit is there to protect the diodes not the opmap input. But the text says the Rlimit is there to limit the input current of the opamp in case the external diodes fail.

But why would one needs external diodes if Rlimit already is capable of limiting the opamp input currents? Wouldn't the existence of Rlimit make D1 and D2 redundant already?

Answer 1

The extra degree of freedom the text is mentioning is that the external diodes can be chosen to handle different currents.

So with just the op-amp diodes and input resistor, for a given EOS (electrical over-stress) event, the only thing that you can change is the resistor. So if the op-amp diodes can only handle the 5mA, and you know that the max EOS voltage is 10V, your resistance value is fully constrained.

With the external diodes, you effectively have one constraint (EOS voltage), and two uncontrainted values (resistance, and max diode current). So if you would like to use a smaller resistance value, you are able to, you just have to chose diodes with higher current ratings.

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First of all I used to think that Rlimit is there to protect the diodes not the opmap input. But the text says the Rlimit is there to limit the input current of the opamp in case the external diodes fail.

Rlimit works with the diodes to reduce the overstress current. So if you apply a -2V input, and the diodes drop 0.7V, the voltage drop in Rlimit will be -1.3V. If the diode can only handle 1.3mA of continuous current, the resistor must be 1.3kΩ.

But why would one needs external diodes if Rlimit already is capable of limiting the opamp input currents? Wouldn't the existence of Rlimit make D1 and D2 redundant already?

External diodes can have higher current ratings, but have to ensure that they are the ones turning on, don't want to use the internal EOS structure. Basically the internal one is there, but may not be sufficient in your situation, so you can supercede it with external diodes.

Answer 2

The diodes are there to protect the op-amp. R1 is there to protect the diodes.

"Degrees of freedom" is just a fancy way of saying the designer has more choices -- as the author gives in his example of being able to use diodes with higher current-carrying capabilities than the internal diodes in an op-amp.

Answer 3

The misconception

But the text says the Rlimit is there to limit the input current of the opamp in case the external diodes fail.

What it suggests. This is misleading text because it gives the impression that currents are constantly flowing into the op-amp inputs. An op-amp works only with input voltages (within the limits of the supply voltages) and not with input currents because its input resistances are very high. There are indeed some very small bias currents but they are created not by the external input sources but by an internal bias current source.

What it actually says. The text refers to external input currents that would pass through the op-amp inputs but internally are immediately diverted through the power sources. This only happens when an input voltage goes outside the limits of the supply voltages. The "diverting" diodes do not actually belong to the op-amp, and therefore it is not correct to call the currents flowing through them "op-amp input currents". Rather, they are "protecting-diode input currents".

The problem

In the following lines, I will use a lot of effort to convince you of my thesis that although an op-amp is a voltage device and does not consume input current, we still protect it from an excessive increase in its input current that would result from a breakdown in the input stage... just because the current is what can damage an op-amp, not the voltage.

The basic idea

It is quite paradoxical and can be formulated as follows: Because we fear a breakdown, when we go beyond the permissible limits we deliberately cause such a breakdown. A typical inventive idea...

The implementation

Genuine Zener diode. To do this, we need to somehow divert the input current to ground. We can do it with a Zener diode which has the property of sharply reducing its resistance at a certain voltage... but it is difficult to implement integrally and consumes considerable power.

Artificial Zener diode. Then we decide to do it with an ordinary diode by artificially increasing its forward voltage by connecting an additional voltage source (the power supply) in series. This simply means connecting the diode not to ground but to the power source. This is the well-known shunt limiter configuration.

The author's idea

It is simple and obvious - to buffer the built-in limiter with another more powerful limiter that turns on in parallel with the internal one a little earlier than it.