the transimpedance amplifier
Most op-amp circuits take a voltage in and give a voltage out; the transimpedance amplifier takes a current in and gives a voltage out. It is the circuit you use whenever a sensor produces a current rather than a voltage, the classic example being a photodiode. The name means trans-impedance because its gain has units of ohms: volts of output per amp of input.
The circuit grounds the plus input and feeds the input current straight into the minus node, with a feedback resistor Rf from output to that node. The virtual ground holds the minus input at 0 V, so the current source barely sees any voltage and is not loaded. All of its current flows through Rf, and the output sits at minus the input current times Rf. For example, a photodiode delivering 1 microamp into a 1 megohm feedback resistor produces an output of minus 1 V.
Transimpedance amplifiers read photodiodes, light meters, optical communication receivers, and current-output sensors of every kind. Holding the source at a virtual ground keeps the source's own capacitance at nearly 0 V, which keeps it fast. The honest caveat: that source capacitance together with Rf forms a pole that can make the amplifier ring or oscillate, so a small capacitor in parallel with Rf is almost always needed to stabilize it.
A photodiode in the dark passes only nanoamps, but under light it passes 1 microamp. With a 1 megohm feedback resistor the output reads minus 1 V, a clean, easy-to-measure voltage proportional to the light.
The feedback resistor sets the volts-per-amp, turning a tiny photocurrent into a usable voltage.
The feedback resistor sets the gain, but together with the source capacitance it can make the amp ring or oscillate; a small parallel feedback capacitor is usually required for stability.