Understanding Negative Resistance Characteristics

Negative resistance is a property where, over some operating range, an increase in voltage across a device causes a decrease in the current flowing through it, or vice versa. This is often visualized as a region on the device's Voltage-Current (V-I) characteristic curve that slopes downwards from left to right.

Analyzing Device Characteristics

Let's examine the options provided to determine which device does not typically exhibit negative resistance:

• DC Motors: DC motors operate based on the interaction between magnetic fields and current-carrying conductors. Their primary V-I characteristic generally shows increasing current with increasing voltage (considering factors like back EMF). They do not possess an inherent negative resistance region in their standard operating modes.
• Single Phase AC Motors: Similar to DC motors, AC motors work on electromagnetic principles. While their V-I characteristics can be complex due to inductance, capacitance, and varying loads, they do not fundamentally rely on or exhibit a negative resistance characteristic.
• Tunnel Diode: This semiconductor device is specifically known for its prominent negative differential resistance region. This characteristic is due to quantum mechanical tunneling and makes it useful in oscillator and amplifier circuits.
• SCR (Silicon Controlled Rectifier): An SCR is a thyristor that exhibits negative resistance between its forward blocking state and its fully conducting state. It triggers into conduction when a gate pulse is applied (or when the anode-cathode voltage exceeds the breakover voltage), and once conducting, it can maintain conduction even if the gate signal is removed, displaying a characteristic dip in voltage (effectively negative resistance) as current increases rapidly.

Conclusion on Negative Resistance

Based on the typical V-I characteristics:

• Tunnel diodes clearly exhibit negative resistance.
• SCRs also show a negative resistance characteristic during their switching process.
• DC motors and Single Phase AC motors do not possess this negative resistance property as a fundamental part of their operation.

Therefore, a DC motor is a device that does not exhibit negative resistance characteristics.