4. Pulldown/up resistors
These resistors are often needed in CMOS circuits. They are useful when we want to force a high or low output state when the input is not changing, but still leave the input able to accept a input signal. They are typically a high value from 10K - 100K and goes to ground (pulldown) or V+ (pullup) at the input of the gate. For example, if we have a gated oscillator on the input and we want to make sure the output always goes to ground when the oscillator is off we need a pullup resistor at the input (assuming the gate is inverting). We will use a few pratical examples later on.
5. Lets breadboard the "CMOS'ifier", a simple square wave fuzz.
We will use this circuit as the front end for the guitar signal, so we can do other things later on, such as octave down. But it also makes a pretty cool fuzz on it's own.
For this we will use a CD4069 chip, simply because it has a easier pinout than the CD4049 and takes less space on the breadboard. First, lets take a look at the pinout of this chip. Lets connect input/output jacks, power and ground to the breadboard and the chip like this (these connections will not be shown later):
Stage 1. Preamp
Our first stage of our circuit will be boosting the guitar signal into logic levels. This is a gainstage similar to many CMOS based distortions, such as the classic Tube Sound Fuzz by Craig Anderton. This is one of the exceptions to the digital logic and can only be done with the CD4069/CD4049. It gives a very pleasing distortion, but has the drawback of being rather noisy.
Lets put it up on the breadboard. Notice the green wires in the first image that are used to disable all the unused inputs. This will not be shown later, but it's a good pratice to always disable any unused input. The second image shows the same circuit with a trimmer for gain control. Put several gainstages in series for a sweet sounding distortion. I usually have two gainstages in my designs when I have one extra inverter left over.
We can use any circuit to boost the signal into logic levels, transistor-based or op amp, but the advantage with this method is that we're only used 1/6 of one chip. So the rest of the inverters can still be used for other things, oscillators, LFO, filters ect. and this will save us some space. Another easy way is to use a LM386 amplifier chip and boost the signals into a squarewave (common in many Tim Escobedo designs), or a op amp comparator.
Next part: CMOS workshop part 3