Diodes as resistors?

[kalel]

I'm wondering whether for any purpose, diodes could be used as resistors. In their normal function, as far as I know, the voltage drop is not a desirable effect. Also I realize that the voltage drop will depend a lot on the amount of current so it might not be predictable/easy to work with, especially since it's not a linear curve with diodes. But could they theoretically be used to provide resistance (and are they ever used that way), and in that function, will they have equal efficiency as that of a resistor (a resistor providing equal resistance)?

[mdszy]

No. Diodes have a nearly-constant voltage drop across them when they're conducting, but they act like a short circuit. If you simply put a voltage source across a diode, you will get a voltage drop across the diode, but will also get as much current through it as the supply can give, which will likely burn out the diode.

[w2aew]

I'm wondering whether for any purpose, diodes could be used as resistors. In their normal function, as far as I know, the voltage drop is not a desirable effect. Also I realize that the voltage drop will depend a lot on the amount of current so it might not be predictable/easy to work with, especially since it's not a linear curve with diodes. But could they theoretically be used to provide resistance (and are they ever used that way), and in that function, will they have equal efficiency as that of a resistor (a resistor providing equal resistance)?

Actually, yes.  There are at least two major applications where diodes are used as resistors, in a sense.

The first application is when diodes are used as switches.  In this case, they're essentially switched between a high impedance (switch open) to a low impedance (switch closed).  Ordinary switching diodes are used like this all of the time.  Also, PIN diodes are used like this for RF applications a lot too.

The second application that comes to mind is more about PIN diodes specifically.  PIN diodes have a property where their broadband RF impedance looks nearly like a pure resistance over a wide frequency range, and the value of the resistance is proportional to the forward bias.  This is exploited in RF circuits to make things like voltage controlled attenuators, voltage controlled phase shifters, etc.   PIN diodes also make very good RF switches (see first application).   

Here are a couple of videos I did on the topic of diodes as switches:

[DaJMasta]

I sort of agree with both replies, there's a yes and a no side to using them like this.

They certainly do not act as resistors, but if your application doesn't really need the specific characteristics of a resistor, then they can serve a function that you'd normally expect a resistor to be used for.  There are some power supply and voltage shifting applications that can be done with diodes, but it's generally not as common now because of how cheap regulators and switching converters are.  As mentioned, they're used as attenuators and switches in AC applications because with known characteristics, you can use them to only allow signals above a threshold or to partly attenuate a large signal.

[danadak]

Not to confuse but -

1) Varicaps, diodes optimized for use as variable capacitors when reverse
biased.

2) Zeners, if you want your R to drop V this is one way of doing it mostly independent
of current.

Some ref material -


https://www.diodes.com/design/support/collateral-and-downloads/


http://mmiloslavsky.narod.ru/Dimmer2/Files/References/TVS_and_Zener_Theory_and_Design_Consid.pdf



Regards, Dana.

[PauloConstantino]

Every I/V characteristic is a resistor locally. If you keep the voltage swing small, it becomes locally linear like a taylor series. A diode with a constant positive bias applied to it (the operating point), will act like a linear resistor if you apply a small enough AC voltage around the operating point.

[xfs]

Diode bridge compressors in audio. There were many circuits using that principle, today there are better and more linear approaches, mostly VCAs. One example is attached, it appeared in Elektor magazine. Looking at the circuit it was probably intended for guitar use.

[w2aew]

Diode bridge compressors in audio. There were many circuits using that principle, today there are better and more linear approaches, mostly VCAs. One example is attached, it appeared in Elektor magazine. Looking at the circuit it was probably intended for guitar use.

I also did a video where a diode was used as dynamic load in an audio limiting circuit:

[MLXXXp]

If you consider a resistor to be something that provides resistance and dissipates heat when current flows through it, then a diode can be considered to be a resistor, just not a constant one. If you don't take into account the capacitance or other non-resistive aspects of a diode, then yes, a diode will, as you say: have equal efficiency as that of a resistor (a resistor providing equal resistance).

One example of where I've used a diode for this purpose is sharing the power dissipation load with a linear regulator. If I want drop 5V down to 3.3V using a linear regulator, I'll sometimes put a standard diode forward biased between the 5V source and the regulator. The diode will provide "resistance" to drop approximately 0.7V  across it, leaving the regulator to only have to drop the remaining 1V.  This way, the heat that needs to be dissipated will be shared between the two components.

Sure, you could use a higher power regulator or use a (bigger) heat sink, but this way the heat can be distributed over a larger area. If I'm using a "wall wart" to provide the 5V, I'll sometimes put the diode in the cable (with some heat shrink tubing over it), before feeding it into the case. This way, some of the heat is dissipated outside of the case containing the regulator, so it doesn't heat up so much internally.

[T3sl4co1l]

What is a resistor?

A resistor exhibits useful resistance, which is a way of saying it obeys Ohm's law.

"Useful" meaning it obeys Ohm's law over a useful range of frequencies and voltage or current.

No resistor is perfectly ohmic.  All real resistors exhibit dominant capacitance, inductance or more complicated impedance responses at high frequencies (typically MHz to GHz).  (The frequency response is linear, so it's not strictly non-ohmic, because V(f) = I(f) * Z(f) and V and I are linear in the usual sense, but Z(f) is not a constant resistance R, rather a complex function of f itself.)  Likewise, they aren't ohmic in the static sense: excessive voltage or current will cause overheating, changing the value, or burning it out, or causing arcing.  Also, on a very precise scale (parts per million), resistors are nonlinear for a variety of poorly understood reasons.

So with our terms defined, let us consider the diode.

A diode is strongly nonlinear, and also has complicated time-dependent properties, depending on type.

As w2aew has illustrated, PIN diodes exhibit useful resistance, at high frequencies, that is variable with bias.

Statically, a diode would not usually be said to have useful resistance: examples like the diode VGA (variable gain amplifier) or compressor that xfs mentioned are very limited in scope.  The change in signal current must be a small fraction of the bias current, otherwise the signal is distorted terribly.  This is using calculus, in spite the properties of the diode, not for any benefits of it.  Simply put: for suitable considerations (differentiability), for an ever-smaller signal, the V(I) curve looks ever flatter (linear).  But this does not change the fact that, on the whole, the diode has an I ~= exp(V) curve, which is one of the most nonlinear functions regularly encountered in math and science.

At high currents, it is true that a diode exhibits resistance: there is parasitic resistance, from the leads, and the bulk semiconductor leading up to the junction itself, which behave ohmically.  At high currents, where the voltage drop across this resistance dominates over the junction voltage drop, the diode can be said to be resistive.  This is important under surge conditions, where the resistance acts to limit peak current (and also acts to dissipate power, causing the diode to fail).  It is not part of normal operation, because the current level is normally much higher than the diode's ratings.

Some exceptions include old fashioned germanium point-contact diodes (where the point contact has so little area that the resistance is large), and SiC schottky diodes (where the bulk semiconductor simply cannot be made as conductive as other materials, and therefore the resistance dominates at currents within ratings).

Because of these special-case conditions, it might not be genuine to say that a diode never exhibits resistance, but a diode definitely cannot be said to be a generic, unequivocal resistor.

Tim

[David Hess]

There is an old application note from PMI or Burr-Brown or Analog Devices where diodes replace the otherwise very high value feedback resistors to make an inverter which operates at picoamp current levels.

[homeless_peep]

Another application for diode resistors, audio voltage controlled filters. This is the "steiner parker" filter, now present in Arturia's synthesizers (all of them i believe).
http://yusynth.net/archives/ElectronicDesign/N-Steiner-VCF-1974.pdf
Depending on the input of that differential pair, the diodes are biased differently and present a different ohmic resistance, thus changing the resonant frequency.