Enhancement-load invertor


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In this section:

The enhancement load invertor

  1. The load line
  2. The transfer curve
  3. Noise margins
  4. Eye diagram
  5. Circuit layout

The enhancement load invertor

A circuit diagram of an enhancement load invertor is shown in the figure below. It consist of two enhancement mode (normally off) transistors, one used as the driver whose gate forms the input of the invertor and a second transistor whose gate is connected to the drain and acts as a load device:


enhload1.gif

The enhancement load invertor is the basic logic element of the first transistor-only technology. The historic importance of this technology is that only one type of transistor was used which led to a simple process while being very space efficient since no resistors are needed. This technology was quickly replaced with depletion load circuits followed by complementary MOS circuits. The simple process technology and the fact that all MOS circuit issues are very similar to that of a full CMOS circuit technology makes the enhancement mode circuit technology a useful topic of study.


The loadline

The load line of the enhancement load is shown in the figure below. Shown are the I-V characteristics of the driver transistor in addition to the load line of the enhancement load device. Since the currrent through each device must be the same as they are connected in series, the output voltage can be otained at the intersection of the I-V curve at a specific gate voltage of the driver and the load line. The result is shown in the inset and the detailed procedure is described in the next section.


enhload6.gif

The transfer curve

The detailed construction of the transfer curve which is obtained by plotting the output of the invertor as a function of the input is illustrated with the figure below:


enhload7.gif

The eye diagram

The eye diagram contains the transfer curve as well as the transfer curve which is plotted with the input voltage on the vertical axis and the output voltage on the horizontal axis yielding the mirror image of the transfer curve mirrored by the y = x plane.


enhload3.gif

Bart J. Van Zeghbroeck, 1998