tell me what you know about diode clipping

[frank_p]

It's been used. This is a simple form of a diode-clamp wave shaper, which was common in desiging waveform generators at one time.

Anybody know titles of old books that had coverd the theory of signal shapers ?  This does not seems to be covered a lot in modern books...

There is a chapter in "microelectronic circuits" by Sedra and Smith that has some info on using diodes + opamps to shape signals, although IIRC it does not have anything as fancy as the triangle to sine conversion in the falstad applet listed in this thread.

The book is in its 6th edition now so you can find really cheap 3rd or 4th eds on line on alibris or other used text sites. Its a pretty good book in general as well. (i think the author was or is the dean of engineering at Waterloo).

Yeah, I have bought the fourth edition used at thriftbooks.com.  It's in chapter 12. I was looking for more configurations...  Thanks for pointing out anyways.  Thats a good book. 

[slotbot]

like what sort of shape changes are you trying to achieve?

you could play with something like a mcu using wavetables to generate many shapes easily or use it to change shapes by taking an analog sample then having the value of the sample being the index of a lookup table that you make. Like mapping an amplitude at the input to a new amplitude on the output. Otherwise maybe an OTA would do something interesting since you can have square or logarithmic relations between the input and output?

[Quackzed]

diode ladder... ok. i get it.

if it´s higher than 700mv, the first pair conduct, so the 2K resistor to ground and the pot make a voltage divider (e.g. if the pot is set to 2kohms, the divider is 2k-2k, then the gain is divided by 2, that´s -6dB, the higher the pot, the lower the gain--> more limiting)
- when the second pair kicks in , the voltage divider is even lower, so more limiting is applied.. the relation between the diode resistors defines the "compression rate".

--so at each intersection of diodes is an opportunity to reduce or divide the signal and have an 'increasingly'  soft-clipped signal? i've been using 2 diodes (.7) and a resistance then another 2 diodes (1.2) and a resistance, one after the other in parallel to ground, but essentially it seems i've missed that the  diode ladder is basically a stepwise (voltage drop by voltage drop)..er ladder!
-so if a signal gets through the first pair, it then becomes twice as hard to get through the second pair.as well as reducing volume to the output .

[Quackzed]

o.k. diode ladder... say 3 antiparallel diode pairs in series to ground .each have a threshhold v of .6   and a signal thats say 4v so it can clip... 2 nodes ; at each node there is a resistor 1k to ground(no cap, for sake of simplicity)   
+<=>-<=>-<=>-g
         \       \_r__g
          \_r______g
at .6v first pair begin to conduct, and untill the signal gets above the second pair's threshold the only active part of the ladder is this.
+<=>
         \_r_______g
once the signals reaches 1.2v it looks like this
+<=>-<=>--r--g
         \_r______g
is there anything actually different between that and doing it in parallel?
+<=>---r---g
+<=>-<=>---r---g
other than saving the cost of extra diodes?

[dschwartz]

o.k. diode ladder... say 3 antiparallel diode pairs in series to ground .each have a threshhold v of .6   and a signal thats say 4v so it can clip... 2 nodes ; at each node there is a resistor 1k to ground(no cap, for sake of simplicity)   
+<=>-<=>-<=>-g
         \       \_r__g
          \_r______g
at .6v first pair begin to conduct, and untill the signal gets above the second pair's threshold the only active part of the ladder is this.
+<=>
         \_r_______g
once the signals reaches 1.2v it looks like this
+<=>-<=>--r--g
         \_r______g
is there anything actually different between that and doing it in parallel?
+<=>---r---g
+<=>-<=>---r---g
other than saving the cost of extra diodes?

yups! that´s how it works..
is not the same to put it in parallel, unless you change the value of the r´s..

try simulating it on ltspice or livewire

[frank_p]

like what sort of shape changes are you trying to achieve?

you could play with something like a mcu using wavetables to generate many shapes easily or use it to change shapes by taking an analog sample then having the value of the sample being the index of a lookup table that you make. Like mapping an amplitude at the input to a new amplitude on the output. Otherwise maybe an OTA would do something interesting since you can have square or logarithmic relations between the input and output?

Diode ladder configurations.  As far as I can see there are not a lot of stuff on that topic in the Sedra. I don't know why but I have an impression that there are many types of ladders that can be done and that the theory had been discussed in older books. But I may be mistaken.

[Quackzed]

first, thanks dsshwartz for the insight into diode ladders as i had never really dug into them, being a bit alien to me and most of the circuits i am familiar with.
i did a flastad sim of the diode ladder setup as well as the parallel setup, but i didn't see any differences.
i tried using different limiting resistors to get a better visual reference for any differences, and so long as i kept the resistances the same as well as the diode conduction voltages, the 2 signals appeared to be equally effected.
at first i thought that having one pair of diodes passing signal for both diode/resistors to ground would be where the difference came in...
as opposed to the parallel setup, where the signal is split before the diode pairs/resistors to ground, but i couldn't find any difference.
i suspect that it's another 'black box' where you can swap around parts of a series string but the end result is the same.
it does seem that a diode ladder has all the advantages of less parts and more exact conduction voltage matching, without any drawbacks.
i DO feel like I'm learning something as well, a picture is worth a thousand words.. sadly i crashed falstad before i could save it to post as a reference...    I'll have to try ltspice some time! falstad is a bit light. but good for fast analysis

[matt239]

How do you determine resistor values for such a diode ladder?   

[Quackzed]

basically you can think of it as a voltage divider...
so if you use a 10k resistor from the output, and a 10k between the diodes and ground, half of the signal above the diode threshold will be clipped...half will be unaffected, sort of a 50/50(10k/10k) clean/clipped blend...
--->----[  10k  ]-----,--->
                             /
                           ---
                          ^ v
                           -,-
                            l
                           10k
                            l
                            G
with a ladder setup, whatever is over the first diode pairs threshold will be clipped 50%(10k/10k)...as mentioned but...
whatever makes it over the second diode pairs threshold will be clipped an additional 50% ...

so below the first diodes threshold you have 100%signal
above the first diode pair threshold you get 50% signal
now from that 50% that is left,whatever part makes it over the 2nd diode pairs thresh will be half clipped again, leaving just 25% of the original signal above THAT threshold, and on and on...
  --->----[  10k  ]-----,--->
                             /
                           ---
                          ^ v
                           -,-
                            /---[10k]---G
                           ---
                          ^ v
                           -,-
                            l-------[10k]---G 
if we leave the top 10k but use 20k resistors after each diode pair then
instead of 50% clipped then another 50%clipped -10k/10k
we'd get 1/3rd clipped then another 1/3 of that 1/3rd clipped...   -10k/20k like a voltage divider...
--i hope that makes sense, not an in depth explanation, but at least the basic idea of whats happenin'
thanks again to R.G. for taking the time to explain it to me, i hope i 'm not mucking it up ...
                         
               

                           

[matt239]

Sounds good to me.
Thanks!

[Bill Mountain]

I'm fairly positive that I've seen a diode ladder in one of Fred Nachbaur's circuits but I can't seem to find it.  I always wanted to try it.  He called in a limiter circuit.

[sault]

http://www.tubecad.com/2006/05/blog0066.htm

It took me a little while to find this one again, I remember looking at it years ago and trying to wrap my head around the idea. The circuit is interesting to me - in this case how it uses a balanced signal instead of just dumping everything to ground. The principle is the same, though. Has such a nice graph illustrating the effect!

http://gilmore2.chem.northwestern.edu/projects/limiter_prj.htm

This is another link, again from the perspective of Hi-Fi. Covers a few different schematics, too.

http://sound.westhost.com/articles/soft-clip.htm

What I like about this link is that it brings up the idea that I didn't initially "get"... that diodes have resistance that decreases as the applied voltage goes up. It has numbers, more colorful graphs, and again talks about the concept of limiting power via soft clipping (diodes) to protect the power amp/headphones/etc.


Saul t

[ct_anthony]

Hi all

Resurrecting an old thread

hi quackzed.. i have been designing a simple power amp simulator using a gain stage with an opamp (or a 4049 inverter) and a simple diode ladder..

the diode ladder is formed by 4 back to back pair of diodes in series to ground and some resistors and caps. The first pair lower end is tied to ground by a 2K resistor and a 33nF cap, and connected to the second pair, which is tied to ground through a 1K resistor..

the signal line before the diodes have a 220nF cap and a 10kpot in series, so:
- when the signal doesnt clip, it´has a fairly flat response.
- if it´s higher than 700mv, the first pair conduct, so the 2K resistor to ground and the pot make a voltage divider (e.g. if the pot is set to 2kohms, the divider is 2k-2k, then the gain is divided by 2, that´s -6dB, the higher the pot, the lower the gain--> more limiting)
- when the second pair kicks in , the voltage divider is even lower, so more limiting is applied.. the relation between the diode resistors defines the "compression rate".
- also, when the first pair starts conducting, a band- pass filter is made by the 220nF & 2K, and the pot resistance with the 33nF cap, this should give a "transformer saturation" type of response..the higher the saturation, the lower the bandwith...
- the pot controls the hardness of the limiter

Is such a diode ladder supposed to be in the inverting loop of the opamp or tied to ground after the opamp like as an hardclipper would be?

Regards, anthony

[R.G.]

It depends a lot on the impedance driving the diode ladder and the impedance to be driven by the ladder.

The advantage of using a diode ladder in a feedback loop is that the input impedances are known quantities on each end of the ladder. Anything in the feedback loop of an opamp is effectively driven as though the output of the opamp was a current source and the inverting input ground/0 ohms.The opamp acts to drive the output voltage to make the current through the feedback impedances cancel any currents at the inverting input.

You can still do diode ladders to ground, but getting what you want for clipping up front gets trickier because whatever drives the ladder has to be capable of driving it, but not so capable that it can force the ladder voltage to be equal to the signal voltage. As a practical matter, that means putting some resistance between the driving source and the ladder to ground.