Linear-in-dB variable gain amplifier using geometric ladder circuit

摘要

A variable gain amplifier uses a geometric ladder circuit that produces a transfer function having substantially uniform steps measured in dB. Where the ladder has a plurality of substantially identical resistor rungs of a first resistance, one stile that is a conductor connecting the rungs, and another having a series of substantially identical resistors of a second resistance, then for identical inputs at different rungs, the output signal at an end of the ladder is attenuated by a number of substantially equal steps, one for each rung between input and output. For a ladder with a base rung R, an output at an end opposite the base rung, stile resistors of resistance alphaR, and other rungs all of resistance (1+(1/alpha))R, the step size is 20 log₁₀(1+alpha). By using such ladders in op-amp feedback loops, chaining different stages with different values of alpha, coarse and fine gain adjustment can be provided.