SUPER BUFFER

Regular CMOS inverter has pull-up and pull-down transistors which do not have have equal charging, discharging times when dealing with output capacitor due to inherent carrier mobility. Thus, having an inverter to drive any off-chip large capacitors or some other critical circuitry is not a reliable solution (asymmetric rise,fall delays).

In contrast to this, super-buffers have good drive strength and provides symmetric driving behavior (equal rise and fall times ). A super buffer has 4 internal transistors whose W/L ratios and connections gives better characteristics of charging/discharging output node cap with symmetrical rise,fall delays.

Following are the two configurations of a super buffer - inverting and non inverting type:

Direct control of 3 and 4 transistors with Vin is the key here, which gives double effect for them either to on or off so that Vout can drive more number of effective loads.

Let’s see how an inverting super - buffer works:

    When Vin is changing from 0 → 1 : transistor 1 becomes on ; since 3rd transistor becomes on immediately as it has direct connection to Vin. At the same time; gate voltage of 4th transistors gets pulled down to 0 and due to Vout node its goes to off state immediately. Here the responses are instantaneous as output transistors are connected to Vin.

    When Vin is changing from 1 → 0 : 1 and 3 devices goes off. This gives a higher (approx VDD ) at 4th device input there by giving higher conductivity and faster charging to output node.