Why Non-Inverting Amplifiers Have Higher Input Impedance

In operational amplifier (op-amp) design, the non-inverting amplifier configuration inherently provides a vastly higher input impedance than the inverting amplifier configuration. This article explores the fundamental electronics principles behind this difference, specifically focusing on how input signals are routed to the op-amp terminals and the effects of the virtual ground phenomenon. Understanding these circuit topologies helps engineers prevent signal loading and select the right amplifier stage for sensitive electronic applications.

The primary reason a non-inverting amplifier offers superior input impedance lies in where the input signal is applied. In a non-inverting configuration, the input signal connects directly to the op-amp's non-inverting (+) terminal. Because internal op-amp inputs are constructed using either Field-Effect Transistors (FETs) or Bipolar Junction Transistors (BJTs) designed to draw minimal current, the intrinsic impedance at this terminal is naturally massive. It typically ranges from several megaohms (\(M\Omega\)) to teraohms (\(T\Omega\)).

Furthermore, the non-inverting configuration utilizes series-shunt negative feedback. The feedback network is connected to the inverting (-) terminal, entirely separate from the input signal path. This arrangement actually multiplies the already high intrinsic input impedance of the op-amp by a factor of one plus the loop gain, pushing the effective input impedance toward infinity in ideal scenarios. As a result, the non-inverting stage draws virtually zero current from the signal source.

Conversely, the inverting amplifier configuration suffers from a much lower input impedance due to the "virtual ground" effect. In an inverting stage, the non-inverting (+) terminal is connected directly to physical ground. Because an op-amp with negative feedback works to keep both of its input terminals at the exact same voltage potential, the inverting (-) terminal is forced to sit at zero volts as well, creating a virtual ground.

In the inverting topology, the input signal is not applied directly to the op-amp pin. Instead, it must pass through an input resistor (\(R_{in}\)) that leads to the inverting (-) terminal. Because the inverting terminal is held at virtual ground (0V) by the feedback loop, the input signal source only "sees" the resistance of \(R_{in}\) going straight to ground. Therefore, the total input impedance of an inverting amplifier is strictly limited to the value of that single input resistor:

\[Z_{in} = R_{in}\]

To achieve a high input impedance in an inverting amplifier, a designer would have to use a very large resistor for \(R_{in}\). However, using high-value resistors introduces significant thermal noise and increases errors caused by the op-amp's input bias currents, making it an impractical solution for precision electronics.

Ultimately, the physical routing of the feedback loop dictates the impedance. By isolating the input terminal from the feedback network, the non-inverting amplifier preserves the massive natural impedance of the op-amp's internal transistors. The inverting amplifier sacrifices this high impedance by placing the feedback summing point directly in the path of the input signal, clamping the input impedance to the value of a single external resistor.