I've noticed that in all resources on diodes and rectifiers, they show the output voltage as the positive half-wave of the input signal. However, that seems wrong.

I understand that there's a voltage drop across the diode, and if the total voltage is below this level, the diode is closed. Therefore, it'd only seem logical, if the diode didn't open right away, but only after the input wave reaches this voltage.

Here's my illustration - first, input. Second, my idea of output. Third - output as shown in textbooks.

enter image description here

If I am wrong, how is it possible that there's no "flat area" in the output signal, when the input is below the diode's opening level?

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    \$\begingroup\$ most textbooks assume ideal components for their initial analysis (capacitors without parasitics, diode with ideal characteristics, ideal opamp.) If the analysis was to push real-life results what characteristics do they choose? schottky? Si diode? Power Si diode... all have different fwd drop BUT an ideal has zero fwd drop \$\endgroup\$
    – user16222
    Feb 10, 2015 at 10:48
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    \$\begingroup\$ 120 V - 0.7 V = 120 V \$\endgroup\$
    – Greg d'Eon
    Feb 10, 2015 at 12:08
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    \$\begingroup\$ great question. It shows that you're not just learning what you're reading, but thinking about it too. You'll make a good Engineer :) \$\endgroup\$
    – Fuzz
    Feb 10, 2015 at 14:28
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    \$\begingroup\$ @Kynit is that an abuse of significant figures? \$\endgroup\$
    – Cole Tobin
    Feb 10, 2015 at 19:59

2 Answers 2


Yes, you are right, have a look at this ltspice simulation of a simple full wave rectifier (click to enlarge):

enter image description here

Textbooks like to simplify things before they go in depth (if at all). How many text books have you seen to talk about diode drop at that point at all?

Its an application of wittgensteins ladder.

Note that at higher frequencies things like the diodes recovery time will start to play an important role too, but even less textbooks talk about that. Both things are not immediately important to understand the concept that should be learned at that point.

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    \$\begingroup\$ And, purely as a meliorative quibble, the textbooks never quantify the magnitude of the applied voltage. Obviously, the higher the AC voltage the less noticeable the discrepancy will be. And since the voltage could be arbitrarily high, the conceptual drawing could be arbitrarily accurate. Quibble, quibble, quibble. \$\endgroup\$ Feb 10, 2015 at 12:45
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    \$\begingroup\$ @WhatRoughBeast Most mendacious amelioration of minor material merits my mention - +1. \$\endgroup\$
    – Russell McMahon
    Feb 10, 2015 at 21:22

Yes you are absolutely right. The diode , in a practical case should start switch ON only after the input voltage has gone above a certain voltage level(0.7 V for Si diode and 0.3 V for Ge).

But in most of the textbooks the things are given in a pretty simple way just to make sure that the reader is able to grasp the concept and you will find written somewhere in the text that they have considered ideal diodes i.e they assume that voltage required to switch ON the diode is almost zero volts.

  • \$\begingroup\$ I was with you right up until the last sentence. Yes, they should specify somewhere what assumptions they are making. However, I've seen a lot of textbooks in my lifetime, and have yet to see even a single one of them that actually did this, in this or any other field. \$\endgroup\$ Feb 10, 2015 at 16:38
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    \$\begingroup\$ @Matthew Najmon, you might be right about this. But i have read one or two books on this topic and i remember it ,even though vaguely, that somewhere around this topic they have written that the analysis of the rectifier carried below is for an ideal diode.And in one text book, both cases were considered , the one with ideal diodes and the one with non ideal diodes. \$\endgroup\$ Feb 10, 2015 at 16:46
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    \$\begingroup\$ This may well vary by country, or at different levels of study, or by some other variable. It's also possible I just happened to get all poorly written books. The books I've seen aren't nearly a representative sample of all textbooks ever written, for me to be able to say that getting it right can never happen, or even that it's necessarily exceptionally rare. It is, however, a big enough sample for me to be fairly confident in saying that getting it right shouldn't be assumed as a universal, or as nearly universal. \$\endgroup\$ Feb 10, 2015 at 19:29

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