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input bias current of a current-mode opamp

We think we've observed bias current drift on some THS3062 current-mode opamps, on the non-inverting input, roughly 10 uA in 6 months, systematic in one direction across multiple amps.

A regular opamp has, say, an NPN differential pair, identical transistors run at relatively low emitter and base currents.

A current-mode opamp has PNP and NPN transistor bases on the ni input, with fairly high emitter currents for speed. So the net Ib is the small difference between the largish base currents. We're conjecturing that PNP and NPN transistors might age differently, and change the net Ib.

Something like this:

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin
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Sounds like high voltage damage. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

In a built-up circuit, the only thing I know about that might plausibly cause that would be zenering one of the base-emitter junctions. In an IC, that ought to be taken care of by the input protection network--maybe that's getting damaged.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
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Reply to
Phil Hobbs

I agree with Jim. What are you applying to the non-inverting input? Is it possible you are going so fast (or applying an input with a power supply absent) that one or the other of the base-emitter junctions is reverse-biasing? High-speed transistors can't tolerate much reverse Vbe.

Reply to
Steve Goldstein

The amp gain is +4, and we are applying +-2.5 volts, from a lowpass filter output. The amp is maybe 100x faster than we are driving it.

The offset drift may have some other cause, but I was wondering if the PNP-NPN asymmetry might have something to do with it.

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John Larkin         Highland Technology, Inc 
picosecond timing   laser drivers and controllers 

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin

That terminal "inv input" can be very useful as an output "simulating" a gain of one buffer.

Reply to
Robert Baer

...and one can anneal out the damage.

Reply to
Robert Baer

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