A CD4049 CMOS inverter wired as an analog amplifier sweeps from clean preamp to fuzz

The CD4049 Hex Inverter, normally a digital logic chip, can be configured as an analog audio amplifier by connecting a feedback resistor (RF) from output to input plus an input resistor (RI). Gain equals RF/RI: with RI = 10 kΩ and RF = 100 kΩ, gain = 10, a clean preamp suitable for contact mics, coils, or guitar pickups; raising RF (e.g. toward 10 MΩ) raises gain into harmonic distortion, heavy fuzz, and eventually oscillation. A small feedback capacitor (CF) across RF rolls off high frequencies, mellowing the harsh square wave — larger CF is darker and bassier, absent CF is bright and flat. The input capacitor (CI) blocks DC and sets bass roll-off. Cascading two high-gain stages produces tube-like overdrive through to aggressive fuzz. The 74C14 Schmitt Trigger cannot substitute for the 4049 here: only the standard non-hysteretic CMOS inverter works for linear amplification.

Examples

Preamp: RI = 10 kΩ, RF = 100 kΩ (gain 10), no CF. Distortion sweep: RI = 10 kΩ, RF = 100 kΩ in series with a 5 MΩ pot — vary the pot from clean to extreme distortion. Cascade two stages for heavier fuzz; add CF ≈ 100 pF across RF to soften it into smoother overdrive.

Assessment

Why can’t a 74C14 replace a CD4049 as a linear preamp? What do the input capacitor CI and feedback capacitor CF each do? A student wants a contact-microphone preamp with a slight bass roll-off to reduce handling noise — which components should they adjust, and in which direction?