Design of integrated analog CMOS circuits-a multichannel telemetry transmitter [single-stage differential amplifier]

A single-stage differential amplifier, implemented in a standard metal-gate process, serves as a basis for consideration of specific characteristics of analog CMOS circuits. The dependence of important circuit parameters like open-loop gain and gain-bandwidth product on transistor geometries, and biasing current are derived. Furthermore, it is shown how to improve the amplifier characteristics by optimizing the layout. The differential amplifier stage is compared with an equivalent one based on a standard bipolar process. This comparison shows that the CMOS stage exceeds in the low-current region (nanoamperes) not only the possible voltage gain but also the gain-bandwidth product of a bipolar stage. The results are applied to the design of a multichannel telemetry transmitter for patient monitoring. The transmitter chip incorporates a digital part for the multiplex control and a precise analog part for the modulation unit. The modulator is a voltage controlled oscillator and consists of two parts: a voltage-to-current converter and a current controlled oscillator. The linear range of the voltage-to-current converter is limited to at most three decades due to the offset of MOS transistors. The current controlled oscillator, however, has a linear range of more than seven decades. This was made possible by applying a new design principle, which is specific for CMOS technology. Besides the large linear range the multivibrator has excellent temperature stability. The chip area is 4 mm/SUP 2/.