My dim recall and shaky understanding tell me that the use of feedback dramatically reduces the impact of the input impedance of the device itself: the overall input impedance of an amplifier will be something like that 300k multiplied by the loop gain, which is usually a large number. I have used these for sample and holds in the past, in a pinch, when I had run out of LF444 (my preferred opamp for S&H applications, with its typical bias current of 10pA). ![]() ![]() Incidentally, the TL064 has typical input bias current of 30pA, less than half the 072. At 300k input impedance, that seriously affects how you design, because this sets up a divider with the input resistor which changes the gain significantly, I should think. I would say that the first "golden rule" of opamps (the inputs admit no current) barely applies to the 5532. So, the 5532 has input impedance 3 million times lower, and bias current about 3000 times higher, than 072. Here is how the two stack up ("typical" values from datasheets): Well, I'm not sure, but I'd guess it's the input impedance. Is it the input impedance that's the trouble, or the input bias current? (Not a rhetorical question - I'd love to understand this properly.) ![]() Sometimes, the most important aspect of an opamp is its input impedance. Sketch-n-Etch wrote:A friend recently tried to replace TL072 with NE5532 in a filter circuit, and the circuit basically didn't work.
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