It is quite obvious that we can't build an amplifier out of resistors (because of their linearity). But is there a circuit theorem that states the impossibility of building an amplifier out of diodes (semiconductor or vacuum tube ones) without using transistors, relays and other 3-terminal devices?

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    \$\begingroup\$ I guess a phototransistor is a kind of two-terminal amplifier. \$\endgroup\$ – DKNguyen Oct 14 at 0:56
  • \$\begingroup\$ There's conservation of energy. Without some power source other than the input signal, you'll never get power amplification. If you consider diodes with a negative resistance region (like tunnel diodes) you can make an amplifier circuit from them. \$\endgroup\$ – The Photon Oct 14 at 1:09
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    \$\begingroup\$ @the-photon Of course, such a hypothetical "diode-only amplifier" will also have a power supply lines - just as all existing amplifiers do. So is it possible to construct such a device? \$\endgroup\$ – Mr_Tusk Oct 14 at 1:30
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    \$\begingroup\$ Yes, a Parametric Amplifier uses a diode as a gain element, in concert with LC passives. However, it must be pumped by an AC source, which would require a power supply, and some kind of amplifying device. Just as @ThePhoton says. \$\endgroup\$ – glen_geek Oct 14 at 1:46
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    \$\begingroup\$ The most fun type of amplifier I know of is a magnetic amplifier, as it uses only a transformer. However, it's a special type of transformer, designed to saturate, and it requires a pump signal. It's very robust, and was often used as a power output stage to drive servo motors for WW2 gunnery platforms. Go look it up \$\endgroup\$ – Neil_UK Oct 14 at 5:48

Gunn diodes have a region of negative resistance, i.e., at certain input voltage the current flowing through the diode is increased when the voltage is decreased. These diodes can be used to amplify RF, but they only become practical at microwave frequencies.

Wikipedia: Gunn diode



You can make an amplifier from tunnel diodes, but that's probably not what you are talking about, since ordinary diodes do not exhibit a negative resistance characteristic, at least not in the normal range of operation.

It's possible to make a parametric amplifier from an ordinary diode because of the capacitance change with voltage. That involves a pump oscillator at a higher frequency than the signal you want to amplify. It has been used for very high frequencies and more moderate ones (eg. Philbrick P2 amplifier).

However, this is an incomplete answer without some kind of reference to a theorem that says the resistive nonlinearity of a diode cannot possibly provide voltage or current gain.

  • \$\begingroup\$ given the gm of a diode is 0.026ma/volt at 1mA, and 1amp/volt at 26mA, can that be used. Still no power gain, of course. \$\endgroup\$ – analogsystemsrf Oct 14 at 4:13

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