analog - Which CMOS logic families can safely be used to construct linear circuits?

I've just learned that digital CMOS inverters can be configured to perform analog functions (most notably oscillators and amplifiers). However, many of the examples tend to favor old CD4000-series devices. In addition, this application note mentions in Section 3 that the use of buffered inverters can cause stability issues.

1. Which logic families can be reliably configured to perform linear operations? Which families should be avoided?


2. Will "special" protection circuitry such as the 5V-tolerant I/O for AHC and LVC cause additional stability issues or prevent linear operation?


3. What would happen if I tried to build a linear circuit using a TTL-compatible device (HCT, ACT, AHCT)?



4. Is it considered bad practice to use digital ICs in their linear region?

Answer

All logic families like to use buffered inverters, because those are more reliable and use less power in digital applications. However, unbuffered inverters are useful to build crystal oscillators, so they exist in many families; search for 74xx1GU04.

A 5 V-tolerant I/O has no ESD protection diode to VCC, so it tends to have less capacitance, and distorts the signal less if it exceeds VCC.

TTL-compatible inputs have a lower switching threshold, so they are no longer symmetric between VCC and ground.

Unbuffered gates are meant to be used in linear circuits; buffered gates are unlikely to work at all.

Another useful application note: Understanding (un)buffered CD4xxx characteristics.