Input impedance is the resistance a device presents to any signal trying to enter it. Measured in ohms, it determines how much current a device draws from the signal source connected to it. A high input impedance draws very little current, preserving the original signal. A low input impedance draws more current, which can weaken or distort the signal before the device even processes it.
How Input Impedance Works
Every electronic device with an input, whether it’s an amplifier, a measurement instrument, or a logic gate, forms a simple relationship with whatever is feeding it a signal. The signal source has its own internal resistance (called output impedance), and the receiving device has its input impedance. Together, these two impedances form a voltage divider. The higher the input impedance relative to the source impedance, the more of the original signal voltage reaches the device.
Think of it like water pressure in a pipe. If you connect a wide-open valve (low impedance) to a garden hose, the pressure drops because water rushes through freely. If you connect a nearly closed valve (high impedance), almost no water flows, and the pressure in the hose stays the same. In electronics, “pressure” is voltage and “flow” is current. A high input impedance keeps the voltage intact by limiting current flow.
Why High Input Impedance Matters
When you connect a measurement tool like an oscilloscope to a circuit, you’re adding its input impedance in parallel with the circuit. This extra load pulls current from the circuit and can change the very voltage you’re trying to measure. A standard oscilloscope probe in its 1x setting presents 1 megohm of input impedance with about 56 picofarads of capacitance. Switching to the 10x setting raises the effective input impedance to 10 megohms while dropping the capacitance to roughly 13 picofarads, which disturbs the circuit far less, though at the cost of a weaker signal reaching the scope.
This principle, called the loading effect, applies everywhere. Any time you tap into a signal, you change it slightly. The goal is to make that change negligibly small by ensuring the measuring or receiving device has an input impedance much higher than the source impedance.
Input Impedance in Amplifiers
Amplifiers are designed with specific input impedance values depending on their purpose. An ideal operational amplifier (op-amp) has infinite input impedance, meaning zero current flows into its inputs. Real-world op-amps come close: FET-input op-amps draw less than one picoamp of input current, making their input impedance extremely high. Bipolar high-speed op-amps are less ideal, with input currents in the tens of microamps range.
In medical devices, input impedance requirements are especially strict. ECG amplifiers traditionally used a minimum input impedance of 10 megohms, but research has shown this isn’t enough. With modern adhesive electrodes and dry electrode designs, a 10-megohm amplifier input can distort the heart signal, creating false waveform features and depressing segments of the trace that doctors rely on for diagnosis. Current recommendations push the amplifier input impedance to 10 gigohms, a thousand times higher, to avoid these artifacts.
When Matching Impedance Is the Goal
High input impedance isn’t always the right answer. In radio frequency (RF) systems and power delivery, the goal flips: you want the input impedance to match the source impedance. When a transmission line carrying an RF signal meets a device whose input impedance doesn’t match the line’s characteristic impedance, part of the signal bounces back as a reflection. The reflection coefficient is zero only when the load impedance equals the line impedance, meaning all the power gets absorbed and none is thrown back toward the transmitter. Reflected power wastes energy and can damage transmitting equipment.
This is why RF cables and connectors are standardized at 50 ohms (for test equipment and antennas) or 75 ohms (for video and cable TV). Every component in the signal chain is designed to present the same impedance, minimizing reflections at every connection point.
The tradeoff is efficiency. When load impedance perfectly matches source impedance for maximum power transfer, exactly half the total power is dissipated inside the source itself. Efficiency tops out at 50%. For applications where efficiency matters more than maximum power delivery (batteries, power supplies), you actually want the source impedance as low as possible relative to the load.
Audio and the Bridging Approach
Audio engineering splits the difference between voltage preservation and impedance matching using a technique called bridging. Instead of matching impedances, audio preamps are designed with an input impedance roughly ten times the output impedance of the microphone or instrument connected to them. A typical dynamic or condenser microphone has an output impedance of 150 to 200 ohms, so preamps usually offer an input impedance around 1,200 to 2,000 ohms.
This 10:1 ratio ensures that nearly all the microphone’s voltage reaches the preamp without significant signal loss, while still providing a stable electrical connection. Going much higher than 10:1 offers diminishing returns and can make the input more susceptible to picking up noise.
Digital Circuits and CMOS
In digital electronics, CMOS logic gates have input impedances around one trillion ohms (10¹² ohms). This is so high that the input current is essentially zero, just picoamps. The practical benefit is enormous fan-out: a single CMOS output can drive more than 50 other CMOS inputs without any signal degradation under static conditions.
The catch is capacitance. Every CMOS input adds a small amount of capacitance that must be charged and discharged during each logic transition. The more inputs connected to a single output, the longer each transition takes. So while the DC input impedance of CMOS is nearly infinite, the effective impedance at high switching speeds drops because of this capacitive loading. Circuit designers balance fan-out against speed requirements to keep signals clean.
Choosing the Right Input Impedance
The right input impedance depends entirely on what the circuit needs to do. For voltage-sensitive measurements and signal preservation, you want it as high as possible to avoid stealing current from the source. For RF power transfer, you want it matched to the transmission line. For audio, a 10:1 bridging ratio keeps signals clean without inviting noise. The common thread is that input impedance is never just a spec on a datasheet. It defines the relationship between a device and everything connected to it, and getting it wrong means losing signal quality, wasting power, or both.