 Impedance bridging

Impedance bridging, also called voltage bridging, connects a source to a much higher load impedance so that little signal electric potential difference is lost across the source's output impedance. It is useful when the receiver should observe a signal while drawing little electric current from its source.

Represent a linear source at one frequency by open-circuit phasor $V_s$ and series electrical impedance $Z_s$. With receiving input impedance $Z_L$, the voltage divider givesThus $|Z_s/Z_L|\ll 1$ makes $H$ close to one as a complex number, controlling both amplitude and phase loading. For positive electrical resistances, the relative voltage loss is $R_s/(R_s+R_L)$. A $1\mathrm{k\Omega }$ source feeding $99\mathrm{k\Omega }$ retains $99\%$ of its open-circuit voltage; feeding $9\mathrm{k\Omega }$ retains $90\%$.

For a tolerance $\eta$ on that resistive relative loss,The often convenient ten-to-one ratio is therefore a chosen accuracy compromise, not a universal definition. A one-percent limit requires at least a ninety-nine-to-one ratio in this ideal resistive model. Voltage divider loading error gives the related loading problem when the source is itself a divider.

Impedance bridging with capacitive input explains why a large DC electrical resistance is not enough at every frequency. With source resistance $R_s$ and receiving $R_{\mathrm{in}}$ in parallel with capacitance $C_{\mathrm{in}}$,At $R_s=1\mathrm{k\Omega }$ and $R_{\mathrm{in}}=1\mathrm{M\Omega }$, the DC ratio is about $0.999$. With $C_{\mathrm{in}}=100\mathrm{pF}$, its magnitude at $1\mathrm{MHz}$ is only about $0.846$. The capacitive path loads the source despite the high DC input resistance.

Texas Instruments' three-amplifier presentation motivates high-input-impedance buffering by explicitly showing that source current and nonzero source output impedance degrade accuracy.^[ref] A buffer changes the load seen by the signal source; its own finite input and output behavior still matter.

Impedance matching has a different objective in maximum-power-transfer applications. For a resistive source, choosing equal source and load resistances gives half the open-circuit voltage, whereas bridging seeks a ratio near one. Signal fidelity, delivered power and high-frequency transmission behavior require their appropriate models.