Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!thunder.mcrcim.mcgill.edu!snorkelwacker.mit.edu!think.com!samsung!sdd.hp.com!spool2.mu.edu!uunet!world!jon_sree From: jon_sree@world.std.com (Jon Sreekanth) Newsgroups: sci.electronics Subject: Re: Impedance of a diode ? Message-ID: Date: 23 Jan 91 17:23:02 GMT References: <5170101@hplsla.HP.COM> Sender: jon_sree@world.std.com (Jon Sreekanth) Organization: The World Lines: 29 In-Reply-To: tomb@hplsla.HP.COM's message of 22 Jan 91 17:27:13 GMT In article <5170101@hplsla.HP.COM> tomb@hplsla.HP.COM (Tom Bruhns) writes: Thanks to Tom and to others who replied in this newsgroup. I also received several replies by email. The summary is that, from data books, and from theoretical calculation, the impedance is better than a few megohms. The leakage current is, according to one email response I got, 25nA max at 20 volts, 5000 at 75 volts (from a National Semi databook) Assuming a linear slope, the dynamic impedance is 55V/5uA = 11 Mohm. The actual dynamic impedance will be better because the exponential curve will lie below the straight line approximation. The capacitance of the reverse biased junction, according to one posted response, is 1.5 pF at 2.5V. In that case, Xc is 5.3Mohm, for 20KHz. Actually I only care about telephone line bandwidth (4KHz, max), so the impedance is greater. In my application there are 2 of these reverse biased diodes, in parallel, sharing voltage with a 220K resistor. So it seems that the parallel combination of two 1N4148's is negligible compared to 220K. Thanks again, / Jon Sreekanth Assabet Valley Microsystems Fax and PC products 346 Lincoln St #722, Marlboro, MA 01752 508-562-0722 jon_sree@world.std.com