Step 4: The Enable Circuit

Picture of The Enable Circuit
Ideal FET.jpg
Real FET.jpg
The first thing we have to tackle is the Enable Transistor.  The schematic shows that all you need is one N-channel MOSFET to be the enable.  That, however, is an ideal MOSFET.  An ideal MOSFET is a symmetrical device that can switch currents in both directions.  The 2N7000* is not an ideal MOSFET, there is a small diode built into it that is parallel with the actual FET.  This means that current can be switched on and off when flowing in one direction, but it will flow freely through the other direction whether the FET is on or off. I don't know why the MOSFETs are made that way, it is probably a manufacturing thing.  My guess is that for some reason, it is easier to make MOSFETs with a diode than without.

To counter this, we put 2 FETs together with their diodes facing opposite directions. That way, current can't flow in either direction freely UNLESS the MOSFET is switched on.  This is why our SRAM will need 8 transistors instead of 6.

*It should be noted that most discrete MOSFETs have a body diode, as its called, in them.