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I was wondering if there exist any relatively simple circuit which would have an I-V characteristic similar to that of a diode (i.e. exponential but small before the cut-in voltage) but with a higher cut-in voltage. I know I could just have several diodes in series but this quickly becomes impractical as I need the cut-in of around 5V which means around 8 diodes in series.

EDIT: Actually, what is important for me is that diode to have a smoother knee in the I-V characteristic. The more evidently exponential the thing the better.

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    \$\begingroup\$ Connect a battery in series with the diode \$\endgroup\$ – nidhin Feb 21 '15 at 14:14
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    \$\begingroup\$ A transistor with voltage divider might do it. Do you really need a specific curve when the diode comes on. Just basically on/off at some voltage allows for more options. \$\endgroup\$ – Olin Lathrop Feb 21 '15 at 14:24
  • \$\begingroup\$ Yes, I do need a specific curve as the setup is to be used in the feedback path of an opamp to create a log amplifier. I just want to extend the 'exponential' region of the response of the amplifier. \$\endgroup\$ – Sanuuu Feb 21 '15 at 14:33
  • \$\begingroup\$ LED or Avalanche diode have higher cut-in voltages. But I am not sure about the curve. \$\endgroup\$ – nidhin Feb 21 '15 at 14:47
  • \$\begingroup\$ Which part of a standard diode's I-V curve is not exponential enough for you? \$\endgroup\$ – Dave Tweed Feb 21 '15 at 15:37
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How important is the exponential part of the I-V? As @nidhin suggested, 2 LED's in series could work. or a 5V zener.. or a 2 2.5V zeners in series.
The low voltage zeners are somewhat exponential, but not as good as diodes.

EDIT: Actually, what is important for me is that diode to have a smoother knee in the I-V characteristic. The more evidently exponential the thing the better.

OK then try 2 LED's.

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The Maxim application note "Integrated DC Logarithmic Amplifiers" covers principles including temperature compensation and reports over 5 decades dynamic range using a MAX4206 i.c.

See http://www.maximintegrated.com/en/app-notes/index.mvp/id/3611

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