The tunnel diode circuit (see diagram) is an example. The tunnel diode TD has voltage controlled negative differential resistance. The battery V b adds a constant voltage (bias) across the diode so it operates in its negative resistance range, and provides power to amplify the signal. Suppose the negative resistance at the bias point is Δ Δ v / Δ Δ i = − − r . For stability R must be less than r . Using the formula for a voltage divider, the AC output voltage is v o = − − r R − − r v i = r r − − R v i so the voltage gain is G v = r r − − R In a normal voltage divider, the resistance of each branch is less than the resistance of the whole, so the output voltage is less than the input. Here, due to the negative resistance, the total AC resistance r − − R is less than the resistance of the diode alone r so the AC output voltage v o is greater than the input v i . The voltage gain G v is greater than one, and increases without limit as R approaches r .