Log-Antilog Amplifier Explained | Applications in Audio Circuits

In this video, we break down how a log–antilog amplifier works using an op-amp-based circuit. This single-quadrant log–antilog converter demonstrates how exponential relationships can be implemented in analog design. The circuit uses two matched NPN bipolar junction transistors along with two FET-input op amps, chosen for their low noise and minimal drift. We show how the output voltage relates to the inputs through an exponential function: Vo = V1 · exp(V2 / VT) This behavior closely resembles the Shockley Diode Equation, where VT (thermal voltage) is approximately 25 mV at room temperature (~290 K). We also discuss how proper DC biasing and negative feedback allow the op amps to maintain stability, enabling the virtual short condition between their inputs. Finally, we review practical component choices for building this circuit, including matched transistor pairs such as: THAT 300 Quad Matched Transistor Array THAT 340 Matched Transistor Array onsemi NSS40301 Dual NPN Transistor Analog Devices MAT14 Quad Transistor For the op amps, we highlight low-noise FET-input options such as: Texas Instruments OPA1656 Texas Instruments OPA2607 This video is ideal for students and engineers interested in analog circuit design, signal processing, and audio applications like VCAs.