Negative dynamic resistance
With a normal resistor the current flowing through the resistor increases with voltage. With a negative dynamic resistor the current decreases with increasing voltage at least over some portion of the input voltage range. Figure 1 shows a little known negative dynamic resistance circuit. As voltage V2 increases the current at first increases as per a normal resistor. However at slightly over 1 volt the current starts to fall in a very linear fashion with increasing voltage (see figure 2) until you reach 1.7 volts where the negative resistance region ends.
Simplifying application of the circuit
A slight disadvantage of the circuit in figure 1 is that you must supply an additional voltage V1. If you are willing to accept a lower degree of linearity (figure 4) and a reduced frequency response then you can run the entire circuit using a supply voltage of about 0.7 volts. This is easily obtained by forward biasing a standard silicon diode and results in the circuit show in figure 3. You can experiment with different diodes to put the supply voltage in the middle of the negative resistance range.
Negative dynamic resistance can be used to create oscillators, Q multipliers and regenerative radio circuits. It can also be used in feedback control systems. Figure 5 shows how to easily make a radio frequency oscillator. This is idea for creating equipment like grid-dip meters and similar devices. The amount of negative resistance can be increased by reducing R2 in figure 5. It is also possible to inductively couple negative resistance to LC circuits. Causing the LC circuit to resonate.