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Old 17th November 2011, 04:41 PM   #1
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Default How do the protection diodes on an opamp's input exactly work?

Hello! I am designing a small gadget of mine. Because of this, i've been reading through some datasheets and noticed that many opamps (mostly the ones which are considered more or less 'hi end') have a pair of antiparallel diodes or transistors with their base-collector junctions shorted across the input pins. I assumed that these would clip any input signal whose amplitude is greater than the threshold voltage of the diodes. But is that how it really works, or am i missing something? Because that would mean that these 'hi end' opamps won't bear an input AC voltage greater than, say, +-0.7V, which isn't quite fine with my design. On the datasheets, it says 'common mode input voltage' somewhere between +-12V, which would be perfectly fine.. but doesn't 'common voltage' mean voltage, that appears simultaneously on both inputs, i.e. not the AC audio signal.. Please, explain. Thanks in advance!
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Old 17th November 2011, 05:01 PM   #2
jcx is offline jcx  United States
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FET, CMOS input op amps can often survive more V of input diff than BJT input pairs

but if you want a comparator you should buy a chip designed for that use

op amps in normal operation only have mV of input diff V, even if they survive larger V they can be slow in recovering form the severe overdrive, output saturation

low noise BJT are particularly sensitive and may need external protection too if expected to see large diff V
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Old 17th November 2011, 06:35 PM   #3
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Here's a brief but informative article from Analog Devices addressing the issue:
Amplifier Input Protection... Friend or Foe?
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Old 18th November 2011, 10:45 PM   #4
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Quote:
Originally Posted by sofaspud View Post
Here's a brief but informative article from Analog Devices addressing the issue:
Amplifier Input Protection... Friend or Foe?
Hot carriers from Zener breakdown of the input devices can create recombination centers and degrade beta, so the clamp is sort of a necessary evil in most cases.
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Old 18th November 2011, 11:00 PM   #5
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by ribolovec2 View Post
Because that would mean that these 'hi end' opamps won't bear an input
AC voltage greater than, say, +-0.7V, which isn't quite fine with my design.

Please, explain. Thanks in advance!
Hi,

It doesn't mean that at all. The feedback forces the difference in voltage
between the two pins to be miniscule under normal operation conditions.

The op-amp can handle any AC level up to its clipping point down to unity gain.

rgds, sreten.
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Old 18th November 2011, 11:27 PM   #6
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The diodes are usually connected to the voltage rails,Thus any 'spikes' will be clamped to about ~0.7V of the supply voltage.

Last edited by DigitalJunkie; 18th November 2011 at 11:32 PM.
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Old 21st November 2011, 05:38 PM   #7
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Thank you all very much! Now i finally understood it!
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