Integrated dual tone multi-frequency telephone dialer
Michael J. Callahan, C. Johnson · 1977
A CMOS LARGE-SCALE integrated circuit that generates the familiar telephone touch-tones* will be described. The design contains both MOS and bipolar transsistors, high speed digital circuits, ion-implanted resistor ladder networks, and an operational amplifier. Two complete digital to analog converters are realized on chip. Overall design philosophy is to reduce system cost by eliminating numerous precision discrete components, designing out required factory and maintenance adjustments, and by reducing total parts count.