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CMOS Clamp Diodes for Improved Full-Swing Fully-Complementary MOS/Bipolar Logic Circuits

IP.com Disclosure Number: IPCOM000099924D
Original Publication Date: 1990-Mar-01
Included in the Prior Art Database: 2005-Mar-15
Document File: 3 page(s) / 105K

Publishing Venue

IBM

Related People

Shin, HJ: AUTHOR

Abstract

Disclosed are two new CMOS clamp diode circuits for full-swing, fully- complementary MOS/bipolar logic circuits. The new diodes reduce parasitic capacitances at critical nodes and improve the circuit speed.

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CMOS Clamp Diodes for Improved Full-Swing Fully-Complementary MOS/Bipolar Logic Circuits

       Disclosed are two new CMOS clamp diode circuits for
full-swing, fully- complementary MOS/bipolar logic circuits. The new
diodes reduce parasitic capacitances at critical nodes and improve
the circuit speed.

      Fig. 1 shows the first new CMOS clamp diode (combination of MN4
and MP4) applied to the full-swing MOS/bipolar logic circuit with
feedback.  The drains of MN4 and MP4 are connected to the output node
O rather than tied to the nodes X and Y, respectively.  Therefore,
the new diode reduces the parasitics at X and Y by as much as the
drain junction capacitances of MN4 and MP4.

      When both inputs (A and B) are '1', since MN1, MN2, MP5, and
MN3 are ON and MP1, MP2, MN5, and MP3 are OFF, Y and O are
shorted to VSS (= GND).  Thus, MP4 functions as a MOSFET diode and
discharges X down to  VTp , where VTp is the p-MOSFET threshold
voltage with the back bias VDD -  VTp .  If  VTp  is smaller than
VBE, QN1 is clamped OFF.  In this state, MN4 is ON because the
gate-source bias (=  VTp ) is larger than its threshold voltage VTn0.
 But no current flows in MN4 as the drain-source voltage is zero.

      If A is changed to '0', MN2 turns off and MP2 becomes ON.  Most
of the MP2 current flows into the base of QN1 because MP3 remains OFF
until O and Z change their logic states.  Since Y is shorted to O via
MN3 and MN4 and QN1 is active, voltage between X and Y is VBE and
a negligible fraction of the MP2 current leaks through MP4 to the
output. Thus, before the output crosses the inverter logic threshold,
its rise benefits from a full base drive to QN1 and zero crossover
current.

      When O rises above the logic threshold, O and Z change their
logic states, MN3 becomes OFF, and MP3 becomes ON. Now the base of
QP1 is clamped by MN4, which is still ON. As O and Y are pulled
up, the body effect increases VTn of MN4.  On the other hand,  VTp
of MP3 and MP4 is reduced. Therefore, MP3 and MP4 get stronger and
bypass more of the MP2 current to O. MP3 and MP4 must be made weak
to prevent a premature cut-off of QN1.

      Finally, X reaches VDD and the threshold voltages of MP3 and
MP4 return to VTp0...