Input impedance and output impedance are two basic but critical parameters in circuit design. They determine how a circuit receives a signal, how much load it places on the previous stage, and how effectively it can drive the next stage. In practical electronic systems, correct impedance planning improves signal accuracy, reduces power loss, and prevents unstable or distorted operation.
Input Impedance
Input impedance refers to the equivalent impedance measured at the input terminal of a circuit. A common way to understand it is to apply a known voltage source U to the input terminal, measure the input current I, and calculate the impedance as Rin = U / I.
For voltage-driven circuits, a higher input impedance is usually preferred because it draws less current from the signal source. This reduces loading on the previous stage and helps preserve the original signal amplitude. If the input impedance is too low, the previous circuit must deliver more current, which may cause voltage drop, distortion, heating, or reduced bandwidth.
For current-driven circuits, the requirement can be different. A current source is designed to provide a stable current, so the load impedance affects the voltage developed across the load. In some current-output applications, a lower input impedance can improve current transfer and reduce unwanted voltage swing. However, practical design still needs to consider compliance voltage, noise, bandwidth, and power dissipation.
Voltage-Source Driven Circuit
In a voltage-source driven circuit, the signal source is ideally a constant-voltage source with very low internal resistance. The load receives energy according to the voltage applied across it and the current it draws. If the load impedance is represented by Rin, then the current is approximately I = U / Rin.
The input power consumed by the load can be expressed as P = U x I = U^2 / Rin. This equation shows that, for a fixed voltage source, a larger input impedance draws less current and consumes less power. Therefore, voltage amplifiers, sensor inputs, ADC inputs, and measurement instruments often use high input impedance to avoid disturbing the measured circuit.
Current-Source Driven Circuit
A current-source driven circuit can be understood as a source that attempts to provide a constant current. When a current I flows through the load impedance Rin, the voltage across the load becomes U = I x Rin, and the load power becomes P = I^2 x Rin.
From this relationship, a larger load impedance produces a higher voltage and higher power consumption for the same current. This is useful in some transducer, LED, and current-loop applications, but the voltage must remain within the allowable output range of the current source. If the required voltage exceeds the source compliance voltage, the current can no longer remain stable.
Output Impedance
Output impedance is the equivalent impedance seen when looking back into the output terminal of a circuit. It represents the internal resistance or equivalent resistance of the signal source, amplifier output, power supply, or driver stage.
For an ideal voltage source, the output impedance should be 0 ohms. This means the output voltage remains constant even when the load current changes. In real circuits, output impedance is never exactly zero, so load current creates an internal voltage drop and reduces the voltage delivered to the load.
For an ideal current source, the output impedance should be infinite. This allows the current to remain constant even if the load voltage changes. In practical current sources, finite output impedance causes current variation as the load changes.
Voltage Source with Output Impedance
In a practical voltage source, output impedance Rout is connected in series with the load resistance R. If the source voltage is U, the load voltage is reduced by the voltage drop across Rout. The current can be estimated as I = U / (Rout + R).
The load voltage is Ur = I x R, and the load power is P = I^2 x R = U^2R / (Rout + R)^2. The smaller the output impedance, the stronger the source's ability to drive the load and maintain a stable output voltage.
Current Source with Output Impedance
In a practical current source, output impedance is effectively connected in parallel with the ideal current source. Part of the current may flow through the internal output impedance, while the remaining current flows through the external load. This reduces current accuracy when the load changes.
For maximum power transfer in a simple resistive model, the load resistance can approach the internal resistance of the source. For most signal and power delivery applications, however, the design goal is not always maximum power transfer. Voltage interfaces usually prefer low output impedance and high input impedance, while current interfaces require enough output impedance and compliance voltage to keep current stable.
Practical Design Guidelines
- Voltage signal transmission: use a low output impedance source and a high input impedance load to reduce signal loss.
- Power transfer: evaluate source resistance, load resistance, thermal limits, and efficiency instead of only matching impedances.
- High-frequency circuits: impedance matching is important for reducing reflection, improving bandwidth, and maintaining signal integrity.
- Measurement circuits: use high input impedance instruments so the measurement device does not significantly affect the tested node.
- Power supplies: lower output impedance generally improves load regulation and transient response.
Summary
Input impedance describes how a circuit loads the previous stage, while output impedance describes how strongly a circuit can drive the next stage. In voltage-driven systems, high input impedance and low output impedance are usually desirable. In current-driven and high-frequency systems, impedance requirements depend on the source type, load behavior, operating frequency, and power-transfer target. Understanding these relationships helps engineers choose components, design stable interfaces, and improve overall circuit performance.