1. Field of the Invention
This invention relates generally to differential amplifiers, and more particularly to a metal-oxide-semiconductor (MOS) differential amplifier exhibiting high differential gain over a wide common mode range.
2. Description of the Prior Art
The advantages offered by NMOS technology are well known; e.g. higher density, greater yield, etc. Thus, smaller NMOS device geometries permit a greater number of devices to be produced per unit area or, stated another way, a single NMOS device will occupy less space. This characteristic is extremely important in the design and fabrication of complex digital integrated circuits; for example, single chip microprocessors.
Whereas digital circuitry is generally characterized by its "ON/OFF" or "ONE/ZERO" nature, most measurements in the real world are inherently analog; e.g., temperature, pressure, speed, voltage, etc. Therefore, it is necessary that microprocessors and other digital circuitry communicate or interface with analog circuitry such as amplifiers, buffers, comparators, etc., in order to permit digital processing of the analog signals. The required interfacing may be accomplished by providing analog components which are external to the microprocessor chip. However, such arrangements generally require more current, a larger power supply and commonly present more opportunities for design and manufacturing errors. To avoid these disadvantages, complex analog circuits such as differential amplifiers are being manufactured integrally with the digital circuitry; e.g., on the microprocessor chip itself, and due to the complex nature of microprocessors, the inclusion of analog devices on the same chip requires that the same manufacturing process be employed. Thus, a differential amplifier included on an NMOS microprocessor chip must be fabricated in accordance with NMOS processing techniques, and the design of the differential amplifier must be tailored to such processing techniques.
Previous MOS implementations of differential amplifiers have been very similar functionally to bipolar implementations. That is, both employ a current source, level shifters, integrator output buffers, etc. Furthermore, the prior art devices generally require an input stage having a high differential gain and a low common mode gain since each additional stage amplifies the common mode signal; i.e., errors produced in the first stage are further amplified in subsequent stages. Thus, the device is complex, requires a large number of components and is both costly and more difficult to fabricate. In addition, the use of enhancement devices at the inputs limits the maximum swing of the amplifier since these devices will turn off if the input becomes too low.
A detailed discussion of such prior art differential amplifiers can be found in the "IEEE Spectrum," February, 1979; pages 24-32 and in the "IEEE-Journal of Solid State Circuits"; December, 1978, pages 760-766.