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Old 19th May 2012, 10:19 AM   #1
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Default Input bias current and impedance of an op amp?

Hello! I have a question regarding the input impedance of an op amp in a non-inverting configuration. I need to make a voltage follower with an input impedance of 500kohms. I was looking at the datasheet for NE5532 and it says that the input impedance is typically 300k. What does that exactly mean? I thought that the input impedance in a non-inv configuration is determined by the value of the bias resistor. According to the specs the bias input current is 200nA, so this shouldn't be a problem with a bias resistor of 500k? (200nA * 500kOhms = 0.1V drop across it. I think that's low enough or is it?) With a bias resistor of 500 k, what would the actual input resistance - 500k or 500k in parallel with the op amp's 300k?
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Old 19th May 2012, 10:32 AM   #2
godfrey is offline godfrey  South Africa
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The input impedance will be 500K. The effect of the 300K is reduced by a factor of 10000 or more by the feedback, so it is insignificant.

You may need to put a small capacitor in parallel with the 500K for stability.
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Old 19th May 2012, 10:45 AM   #3
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I take it that the input impedance quoted is the differential one - between + and - inputs. It seems about right because LTPs in input stages typically run with collector currents in the range of 100's of uA. This would give an Re (equivalent emitter resistance) of the order of 100R or so. With the input transistors having Hfe around 1000 (estimated from the bias current) it would mean each is looking like 100k into the base terminal.

In a non-inverting configuration the feedback means you end up with the common-mode input impedance which is at least two orders of magnitude higher, because the Re becomes the tail current source's dynamic impedance. So to all intents and purposes, you'll just see the bias resistor as the input impedance. There is the CM input capacitance to take into account though at higher frequencies.
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Old 19th May 2012, 10:52 AM   #4
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Thank you.
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Old 19th May 2012, 11:48 AM   #5
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If you want a high input impedance you would be better with a FET input device like a TL071 or OPA134.
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