Superdiode

The superdiode is a precision half-wave rectifier circuit, used when the input signal is smaller than the voltage drop of a conducting diode (0.7 V). It’s meant to act as a “perfect diode” using an operational amplifier, where:

$$v_o=\{\begin{array}{ll}{v}_i& v_i\ge 0\\ 0& v_i<0\end{array}$$

Regular rectifiers work well when $v_i>0.7$, i.e., to get a significant $i$ we need a significant $v_i$. Certain applications require very small input signals, so we use op amps to tune the transfer characteristics in a more precise way.

What’s going on here? When $v_i$ is negative, the op amp output $v_A$ will also go negative, reverse-biasing the diode, leaving open loop configuration. No current will flow through the resistor, so $v_o=0$ in this case. $v_i$ only needs to exceed a very small amount for the op amp to output something. This results in a negative feedback loop and the current will flow from $v_A$ to ground, causing a voltage that essentially follows the input.

This is contrasted from a voltage follower circuit in that it only follows for positive $v_i$.