No germanium diode available for small crystal radio -- can active components handle the task?

As others (@Kaz) have noted, a Schottky diode may be a simple and cheap solution. I personally haven't seen a crystal radio made with them, but that can very well be because I have really never checked for such a circuit. By all means that should be your first try.

A germanium diode is best known for two properties:

• Low threshold voltage
• Relatively high resistance in contrast to silicon diodes, resulting in a more curved characteristic.

The low threshold voltage (essentially 0V!) can easily be reproduced with an active half wave rectifier as shown in the image below (found on Elliott Sound Products).

The operational amplifier is used to eliminate the (rightmost) diode's threshold voltage by inserting the diode within the feedback loop. The positive halve waves are amplified by -1 (\$A = -\frac{R2}{R1}\$), so essentially it is an inverting rectifier. With a sine wave you won't notice the difference as both half waves are symmetric.

The leftmost diode prevents the opamp from being driven in saturation (low rail) during the positive halve input wave. Subsequently the inverting input will act as virtual ground (V- = V+) which stabilizes the circuit.

This circuit only works reliably with a dual power supply as the opamp's output will be driven about 0.6V below ground.

Note that the crystal-radio's Ge diode was required for listening to extremely weak signals from distant stations using no power supply.

To pick up the closest few AM stations, usually the diode need not be germanium. Well, unless you're down in the basement, or way out in the country far between cities. Or, if you're not using a ground with a longwire antenna.

Heh, you could always add an adjustable 0-1V battery supply using a 100K voltage-dividing pot, and place it in series with your 1N914 diode for forward biasing, then adjust the volts to maximize RF reception (perhaps 0.6 volts?) Add a 0.1uF bypass cap to route the RF past this DC bias supply? A little coin-cell should be more than enough here.

If a 1N914 diode doesn't do it, and if you don't want to use a ground+antenna, often you can fix things by using a ferrite loop antenna with extra-long ferrite core ...or by winding an old-style loop antenna, 1-meter diameter, need roughly 250uH inductance to match a 365pF tuning capacitor for 550KHz-1.5MHz . In a city with an AM transmitter within miles, such a resonator can develop several volts RF amplitude. Sometimes you can even charge a capacitor and use it to flash an LED. One guy in Chicago said he was seeing several volts at a couple amps, and could use a silicon diode and run DC solar-cell motors (this from an AM station less than 1KM distant.)

Cheat: watch the LC resonator's output with an oscilloscope. Tune it to maximize the RF amplitude, and if it's well above 1V p-p, then your detector diode doesn't need to be germanium.

Finally, is a professional signal generator available? Set it to 1MHz sine output, turn on AM modulation at about a KHz or so, and connect the output to a few-turns loop inductor perhaps a foot across (Heh, or string a 1-turn loop around the lab, or even out the window and around the entire building.) Use this "transmitter" to provide RF for designing your crystal radio. When you can receive a strong signal, crank the transmitter output way down, then redesign your radio to bring it back up again. After sufficient cycles of design improvements, shut it down and tune for ambient signals.

PS
Don't fall for a misconception being propagated by crystal radio sites: they say that LC resonator is just a bandpass filter. Nope, wrong, and its purpose isn't to block other AM stations while passing just one. Instead, the resonator is part of an "electrically-short resonant antenna" configuration, where the 'EA' effective aperture is immensely enhanced by resonant coupling to incoming EM waves. In other words, disconnecting the LC resonator doesn't cause your crystal radio to receive all AM stations at once. Instead, it goes silent, because the antenna-wire's "electrical diameter" has decreased to nearly zero. With no resonator present, the too-short antenna no longer couples strongly to nearby EM fields, and has stopped absorbing EM energy. (The same antenna wire, whenever a high-Q resonator is connected, can intercept vastly increased milliwatts. It completely alters the fields surrounding any antennas shorter than 1/2 wavelength. It focuses the EM waves onto itself, somewhat like the "director" elements in a Yagi antenna.) Very cool physics, a classical analog of gas absorption lines, particle-collision resonances and even of Stimulated Emission (heh, does it display Rabi Oscillations when given sudden pulses?!!) See products based on this piece of little-known EM physics: Select-a-tenna, and Terk AM antenna. Check it out:

• https://scholar.google.com/scholar?cites=12255458343984770096
• https://scholar.google.com/scholar?cites=477284299795847297
• http://amasci.com/tesla/nearfld1.html#flux

So, everyone always assumed that crystal radios were too simple to spend time investigating? They're too simple for engineer post-doc "science fair projects?" Guess again!

You are talking Active circuits here .This means that power is available .The Opamp active rectifiers would need a good fast opamp .Jellybeans like LM324 are far too slow.If you apply some foward bias current on the diode you can overcome the foward drop issue .When this is done the common Si diode say 1N4148 will work as well as the rare say OA81 Ge diode .This prebiasing has been done on early solid state radios before I was born .If you do not prebias you get nonexistant weak signal performance and horrible distortion at medium signals .The old vacuum tube detector diodes were high impedence devices that did not need prebias. It can be said that the contact potential did the prebias. Sure I have lots of Ge devices but this is a non commercial site and I recommend you prebias your diode .If you get this detector right you can get very good sound quality .Operation is better at high impedence due to lower distortion and less tuned circuit loading .If you have some old small signal RF Ge transistors floating around they should make very good diodes.