Currently, there is an ever-growing demand for low-power mixed signal integrated circuits for such applications as mobile or wired communications and other portable systems. In these applications, the supply voltage is being scaled down to reduce overall power consumption. As for the fabrication process, digital CMOS is always the preferred technology due to its efficient economic costs. As a result, contemporary analog circuits must not only operate with low supply voltages, but should also be realizable in typical digital CMOS processes. However, this concession leads to significant performance degradation of the analog circuits. Since op amps are the most critical building blocks in all analog systems, the objective of this report is to study the theory and design of low voltage op amps for digital CMOS processes. Two categories of low voltage op amps have been identified as being suitable for low voltage applications [1]. The first category operates with 2-3V power supplies. The distinct features of this class of op amps are that they utilize N-P complementary, input pairs and the gain stages use active gain enhancement to boost the gain of the overall

model. Nested G m -C Compensation (NGCC) N th -Order Analog & Mixed-Signal Center (AMSC) i V 0 V 1 m G 2 m G 1 mf G 1 m C i V out V dd V ss V 1 m g 1 b V 2 b V 2 m g 12 M 22 M 21 M 1 Mf 1 mf g 13 M 14 M 11 M (a) Representation (b) Transistor Level How to Implement a Positive G m ? M22. to parallel in r transisto PMOS a add and M21 Remove , G G If current. additional provide to M21 add then , current Mf1 current M22 , G G If 1 Mf G g g , 22 M 21 M G g g , 14 M 11 M G 1 mf m2 1 mf m2 1 mf 2 m mM22 2 m 1 m mM11 1 m > . > > . = - = - Analog & Mixed-Signal Center (AMSC) dd V ss V 1 m g 2 m g 3 m g 3 M 5 M 4 M - V + V 1 m C 2 m C 3 m C out V 4 m g 2 mf g 3 mf g 3 b V 2 b V 1 b V 1 mf g Four stage operational amplifier with NGCC topology Design Example of a Four-Stage Amplifier Analog & Mixed-Signal Center (AMSC) Measured Performance of the 4-Stage NGCC Op Amp. 2 2 o o 0.22mm 0.22mm Area //20pF 10k 20 // 10k Condition Load 1....

• Design Considerations • Existing approaches • Proposed Approach (1)

• Good voltage gain can be obtained using cascode stages. But these stages are not amenable for LV power supply. • Under LV conditions, large voltage gain can be obtained using cascade amplifiers. That is growing horizontally, rather than vertically. • Direct Cascade of simple (inverting) stages could give the required voltage gain without any control of poles and zeroes. Potential stability problems. • Dynamic behavior for optimal performance requires feedback (and feedforward) circuits. This can solve the stability problems.