Are leaky amps sinking your design?


My amplifier “leaks” a low level signal at the output with the power supplies off. What gives…is my circuit sunk?

RAQ:  Issue 24


Ah yes, I've come across this "plumbing" problem once or twice before, it truly is one of those "rarely asked questions." Upon further discussion with the customer, I found out the amplifier is connected as a voltage follower, the power supplies are off and there is a 2-V p-p signal at the amplifier input... a key piece to the puzzle. The problem appears to be output related, but the source of the leak can be traced back to the input. More specifically, the input protection circuitry for the differential input stage.

Processes used to fabricate high-speed (>50 MHz) amplifiers utilize transistors with very high ft. While these devices operate at very high frequencies, their breakdown voltages can be quite low, hence the need for protection. Input protection can be as simple as a few series diodes connected in anti-parallel between the non-inverting and inverting input of an amplifier. The diodes limit the amount of voltage applied across the base-emitter junctions of the input differential pair and prevent reverse breakdown. The number of series diodes determines the differential input voltage rating. Voltage ratings can range anywhere from ±0.8 V to ±VS. The value is easily determined by checking the Max Ratings Table in the datasheet, under Differential Input Voltage.

This particular customer was using an amplifier with a ±0.8-V differential input rating. What happened was the protection diodes became forward biased when the 2-V p-p signal was applied to the input. The input signal then "leaked" through the protection diodes to the output via the feedback resistor. Fortunately, you don't need to call a plumber to fix this type of leak. Instead, reduce the input signal level to below ±0.8 V p-p, or simply use an amplifier with a higher max differential input voltage rating.

So, next time you come across a "leaky" amplifier, check your differential input voltage rating. And, remember, if you can’t lower the input voltage to solve the problem, then, just like a plumber who needs to replace a washer to stop a leak you might have to replace the amplifier.

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John Ardizzoni

John Ardizzoni

John自2002年开始在ADI公司工作,担任高速放大器部门应用工程师。 加入ADI公司之前,他曾在IBM的RFIC应用部门和M/A-COM公司工作了20年。 John还是ADI公司“非常见问题解答”(RAQ)栏目的共同作者。 他拥有30多年的电子行业工作经验,曾撰写过许多文章和设计构想。