TI LME49710-series Differential Input Overvoltage Diodes

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mediatechnology
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TI LME49710-series Differential Input Overvoltage Diodes

Post by mediatechnology »

I really like the LME49710-series (LME49720 dual; LME49740 quad) and its higher-voltage cousin the LME49860.
They're great replacements for the NE5532 and NE5534.

Like the NE5532 and NE5534, the LME49710-series do have one undocumented limitation the designer needs to be aware of:
There are differential input overvoltage protection clamp diodes between the inputs.
For the majority of audio applications feedback holds the inputs in null and these diodes are invisible.
In some traditional, classic op amp circuits they become visible.

I discovered the LME-series differential protection diodes while substituting an LME49720 for an MC33078 in a diode-based active clamping circuit.
For the record both the MC33078 and LME49720 have PNP inputs.
In this particular clamping circuit the MC33078 worked correctly; the LME49720 did not. A 5532 would also not work.

A typical application for an unused op amp might be a peak overload or signal present indicator.
When using spare sections of dual bipolar op amps to perform peak detection, clamping or comparator functions be aware of these diodes in some part numbers.
The LME-series datasheets do not mention it but the NE5534 schematic shows diode-connected transistors.

Image
NE5534 Internal Schematic showing input clamp diodes between inputs.
Image Courtesy of Texas Instruments
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JR.
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Re: TI LME49710-series Differential Input Overvoltage Diodes

Post by JR. »

Yup, it is pretty common practice to use clamp diodes across low noise transistors to prevent them from zener reverse breakdown (roughly 7V backwards across emitter-base junction).Transistors will lose beta and get noisy when allowed to zener. With an opamp there may also be issues with output state being affected.

I regret that I have little experience with the modern crop of opamps, so nothing specific to contribute.

Some opamps also get funny when inputs approach one rail of the other. This is often specified in data sheets. .

JR
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