4069UB Build Guide

The CD4069UB is an unbuffered CMOS hex inverter — six independent gain stages in a single 14-pin IC that costs about $0.50. It's one of the best components for exploring multi-stage distortion design on a breadboard.

This guide assumes you've read the Distortion Circuit Design document, which covers the conceptual framework: transfer functions, bias points, symmetric vs. asymmetric clipping, and the stage archetype table. Here we put those ideas into practice with real circuits.

1. Why the 4069UB?

Each of the six inverters is a complementary MOSFET pair (one NMOS, one PMOS) that forms a voltage amplifier. The UB (unbuffered) designation is critical — it means the inverters have a smooth, analog transfer curve rather than the sharp digital snap of the buffered version.

When biased at the midpoint of the supply voltage, each inverter acts like a small amplifier with a voltage gain of ~40× at 5V supply and a transfer curve that looks like a steep S-curve — essentially a tanh-like function. The gain increases at higher supply voltages, but so does heat dissipation.

2. Power Dissipation

Higher supply voltage means more gain but also more heat. Keep this in mind when cascading multiple stages:

The chip's absolute maximum is ~500 mW total. At 5V you're nowhere near this even with all 6 stages. At 9V (a single battery) you have plenty of margin for 3-4 stages. This is another reason 5V or 9V is the sweet spot for breadboard experiments.

3. Single-Stage Circuits

Self-Biased Inverter Amp (Full Gain)

The simplest useful circuit — one inverter stage biased at its midpoint:

The feedback resistor Rf provides DC feedback that stabilizes the quiescent point at half the supply voltage — right in the middle of the S-curve. The input coupling cap blocks DC so the audio signal swings symmetrically around that midpoint. At low levels this is nearly linear (clean gain of ~40×). Turn up the input and the S-curve's soft corners start to round the peaks — odd-soft pre-stage behavior. Push it harder and the output clips against the supply rails — odd-hard clipper.

The low-frequency cutoff is set by the coupling cap and the input impedance: f = 1 / (2πRC). With 100nF and ~25KΩ input impedance, cutoff is around 64 Hz — fine for guitar. Use a larger cap (1μF) for bass or full-range audio.

Reducing Gain with Negative Feedback

Full open-loop gain (~40×) is a lot — it'll distort almost anything. To set a controlled, lower gain, split the feedback resistor into two parts and use negative feedback, just like an op-amp inverting amplifier:

This gives you a clean gain stage with predictable behavior — the odd-soft pre-stage archetype at moderate levels. Input impedance is now set by R1, which is much easier to control. The tradeoff: you're using the gain for linearity rather than distortion. To get back to clipping, either increase the input level or cascade into a second, full-gain stage.

4. Multi-Stage: Cascading Inverters

This is where it gets interesting. Chain two or three inverter stages, each with its own bias resistor and coupling cap:

Now you have independent control at each stage:

• Stage 1 — Rf1 = 1MΩ (centered bias): odd-soft pre-stage. Wakes up harmonics gently.
• Stage 2 — Rf2 = 1MΩ (centered): another symmetric stage. The already-rounded signal gets rounded further. Odd harmonics compound — 3rd × 3rd starts generating 9th, etc. Things get buzzy.
• Stage 3 — Replace Rf3 with a voltage divider to shift bias off-center: now this stage is even+odd soft. The asymmetry introduces 2nd harmonic warmth into the already harmonically rich signal.

Inter-Stage Coupling Caps

Between stages, the coupling cap value acts as a simple post-shaper — a high-pass filter with cutoff f = 1 / (2πRC) where R is the next stage's input impedance (~25KΩ at full gain):

This is one of the most powerful tools in multi-stage design: by filtering between stages rather than at the end, you control what gets distorted by the next stage. A 10nF cap between stages 2 and 3 means only treble gets the third stage's clipping — the bass from stages 1 and 2 passes through unmolested. This is how you get tight, defined distortion rather than a wall of mush.

The 4069UB vs. the Fuzz Face

The Fuzz Face is a finished instrument — beautiful, specific, hard to improve on for what it does. The 4069UB is a laboratory — it lets you test the archetype framework hands-on and hear exactly what happens when you change one variable at a time.

5. The 4069UB as Sound Source

The 4069UB isn't just for processing audio — you can use it to generate waveforms too. Combined with a 40106 (Schmitt trigger hex inverter), a handful of resistors, caps, and diodes, you can build oscillators that produce triangle, sawtooth, and square waves — then distort them through more 4069UB stages on the same breadboard.

Triangle Wave Oscillator (40106)

The 40106's built-in hysteresis makes it a natural oscillator. A resistor and capacitor on one inverter stage create a relaxation oscillator — the cap charges through the resistor until it hits the upper threshold, the output flips low, the cap discharges until it hits the lower threshold, the output flips high, and so on. The voltage on the cap is a triangle wave — the charge/discharge cycles are linear because the resistor provides constant current into the cap.

You get two waveforms for free: the square wave at the output, and the triangle wave at the input (the cap voltage). The triangle is the raw charge/discharge ramp; the square is the oscillator's switching output.

Sawtooth from Triangle: Add a Diode

To convert the triangle into a sawtooth, add a diode in parallel with the feedback resistor. The diode conducts in one direction only, so:

• Charge cycle — diode conducts, cap charges quickly through the diode's low resistance
• Discharge cycle — diode blocks, cap must discharge slowly through the full resistor value

The result: fast rise, slow fall (or vice versa depending on diode orientation) — a sawtooth wave. The asymmetry ratio depends on the resistor value vs. the diode's forward resistance (~100Ω for a 1N4148). With a 10KΩ resistor, the fast edge is about 100× faster than the slow ramp.

From Oscillator to Fuzz: A Complete Signal Chain

Here's where it comes together. Use one 40106 and one 4069UB to build a complete synthesizer voice with shaped distortion — all on one breadboard:

This architecture — oscillator → buffer → overdrive → fuzz — is essentially Craig Anderton's "Tube Sound Fuzz" circuit from Electronic Projects for Musicians (1980), one of the first published designs to exploit the 4069UB's analog behavior for deliberate distortion. Anderton recognized that the CMOS inverter's S-curve, when cascaded, produces a progressive saturation that mimics the stage-by-stage compression of a tube amp.

6. Parts List

Everything you need for the breadboard experiments above:

Further Reading

• Distortion Circuit Design — the conceptual framework: transfer functions, bias, symmetry, and stage archetypes
• CMOS Gate as Analogue Amplifier — detailed technical reference on 4069UB bias, gain, power dissipation, and feedback configurations
• Logic Noise: Sawing Away with Analog Waveforms — Hackaday series on CMOS oscillators, waveform generation, and the Craig Anderton "Tube Sound Fuzz" lineage
• Craig Anderton, Electronic Projects for Musicians (1980) — the original "Tube Sound Fuzz" 4069UB circuit and many other CMOS audio designs
• Transfer Function Lab — draw your own waveshaper and hear the result
• Digital Octavia Lab — interactive stage-by-stage Octavia simulation