Fairchilds Ua709 a Second Widlar Design and Its One Persistent Flaw

# Fairchild's uA709: a Second Widlar Design, and Its One Persistent Flaw

## 1. An Improved, Widely Adopted Amplifier

Widlar's second operational-amplifier design, the uA709, improved substantially on the uA702's input characteristics and gain, and it became a genuinely popular catalog part through the mid-to-late 1960s, used across a wide range of instrumentation and general-purpose analog applications that had previously required discrete designs.

The uA709's success established something important for the commercial case of linear integrated circuits broadly: a monolithic amplifier could be good enough, reliable enough, and cheap enough that circuit designers would default to it rather than building a discrete equivalent, which is the same kind of commercial threshold-crossing Part A described for MOS logic in calculators.

## 2. The Flaw That Kept It From Being the Final Word

The uA709 required external frequency-compensation components — a capacitor and sometimes a resistor, wired in by the circuit designer outside the integrated circuit's own package — to keep the amplifier stable at the gain and bandwidth settings a given application needed. Get that external compensation network wrong, and the amplifier could oscillate instead of amplifying cleanly, a failure mode that put a real design burden back on every engineer who used the part, and one that the device this series turns to next was built specifically to remove.