Compute precise electrical input impedance values effortlessly today.
Depending on the selected amplifier configuration, different electrical formulas determine the total input impedance ($Z_{in}$):
Input impedance ($Z_{in}$) is a critical parameter in electronic circuit design, representing the opposition that an amplifier presents to an incoming signal source. Proper calculation and matching of input impedance ensure that maximum signal power transfers efficiently from the source to the amplifier without unwanted attenuation, distortion, or loading effects. Whether working with discrete bipolar junction transistors (BJTs), field-effect transistors (FETs), or integrated operational amplifiers, understanding how biasing networks and feedback loops alter $Z_{in}$ remains essential for hardware engineers.
When an audio or radio frequency source connects to an amplifier, the source resistance forms a voltage divider with the amplifier's input impedance. If the input impedance is significantly lower than the source impedance, a substantial portion of the signal voltage drops across the source, leading to signal loss. High input impedance is particularly desirable in instrumentation amplifiers and sensor front-ends to prevent the loading of weak transducers. Conversely, impedance matching in high-frequency RF transmission lines requires precise resistive matching to eliminate signal reflections.
Different active configurations exhibit vastly distinct input impedance profiles. For instance, a common-emitter BJT amplifier provides an input impedance limited by the parallel combination of its biasing network resistors and the reflected base resistance ($\beta \cdot r_e$). On the other hand, operational amplifiers configured in non-inverting modes offer exceptionally high input impedances due to the high input impedance of differential input stages, which can be further boosted using positive or negative feedback topologies.
A low input impedance loads down the preceding signal source, causing voltage attenuation, signal distortion, and loss of overall system gain.
Voltage-series feedback increases the input impedance of an amplifier, whereas current-shunt feedback typically decreases the input impedance.
Yes, at higher frequencies, stray capacitance and input capacitances of active devices introduce reactive components, making input impedance complex.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.