Input resistance refers to the resistance seen by a signal entering a circuit or device. It is an important parameter in electronics, especially in amplifiers, sensors, and measurement systems. Input resistance is typically calculated as:
where: • is the input voltage, • is the input current.
Calculation
Pure Resistive Circuit
Just use resistance transforms to calculate the total resistance
Circuit with Source Powers
The input resistance is the Thevenin equivalent resistance, which is calculated by deactivating all independent sources (setting voltage sources to short circuits and current sources to open circuits) and calculating the resistance seen from the input terminals
High Input Resistance
• Amplifiers: In devices like operational amplifiers (op-amps), a high input resistance is desirable. It ensures that the amplifier draws very little current from the signal source, preventing it from affecting the original signal. • Sensors: In sensors or transducers, high input resistance helps maintain the integrity of weak signals without significantly loading the sensor.
Low Input Resistance
• Measurement Devices: In some cases, low input resistance might be necessary, such as when designing circuits that require accurate current measurements.
Impact on Signal
The value of the input resistance affects signal quality. A high input resistance ensures minimal signal loss, whereas a low input resistance can load the signal source, leading to signal attenuation or distortion.