BiCMOS tri-state output buffer

In a tri-state output buffer for selectively providing a high signal level, a low signal level and a high impedance output to an output node, the buffer being of the type that includes an input port for receiving an input signal, an enable port for receiving an enable/disable signal, and first and second bipolar output transistors connected in series between first and second power terminals, the first transistor having a collector electrically coupled to the first power terminal, an emitter electrically coupled to the output node, and a base, the second transistor having a collector electrically coupled to the output node, an emitter electrically coupled to the second power terminal, and a base, an improvement comprising:

• a circuit for establishing an electrically-resistive path between the base of the first transistor and the emitter thereof, the circuit including means for selectively protecting the first transistor against harmful bias, the protecting means including:
• a transmission gate for adjusting the resistance of the path to a low resistance if a disable signal is present at the enable port or if the output node undergoes a high-to-low signal transition at a time when an enable signal is present at the enable port and to a high resistance if the output node undergoes a low-to-high signal transition at a time when an enable signal is present at the enable port;
• an inverter stage;
• and
• a first and a second FET in parallel connection between said first power terminal and a first control node, the first FET having a gate connected to receive the input signal and the second FET having a gate connected to receive the enable signal, the control node being connected to one gate of said transmission gate, the output of said inverter state being connected to the other gate of said transmission gate whereby said control node controls said transmission gate.

A method for operating a tri-sate output buffer of the type that includes an input port for receiving an input signal and an enable port for receiving an enable/disable signal and first and second bipolar transistors in series between first and second power terminals, the first transistor having a collector electrically coupled to the first power terminal, an emitter electrically coupled to an output node, and a base the second transistor having a collector electrically coupled to the output node and an emitter electrically coupled to the second power terminal, the method comprising:

• electrically coupling the base and emitter of the first transistor to prevent any reverse bias if the enable/disable signal assumes a "disable" value;
• electrically decoupling the base and emitter of the first transistor if the enable/disable signal assumes an "enable" value;
• electrically coupling the base and emitter of the first transistor to prevent any reverse bias during a high-to-low transition of the output signal if during said transition the enable/disable signal is at the "enable" value;
• and
• electrically coupling the base and emitter of the first transistor to prevent any reverse bias if the input signal is maintained at a low level and the enable/disable signal is maintained at the "enable" value.

A tri-state output buffer comprising:

• a first bipolar output transistor having a collector electrically coupled to a first power terminal, an emitter electrically coupled to an output node, and a base;
• a second bipolar output transistor having a collector electrically coupled to the output node, an emitter electrically coupled to a second power terminal, and a base;
• signal means, electrically coupled to the bases of the output transistors, responsive to an input signal to cause the output transistors to provide at the output node an output signal corresponding with the input signal;
• enable means, electrically coupled to the bases of the output transistors, responsive to an enable signal to override the signal means and cause the output node to assume a high impedance state when the enable signal assumes a "disable" value;
• and
• circuit means operative to prevent reverse bias between the base and the emitter of the first transistor whenever the output node is in the high impedance state, the circuit means comprising:
• a variable-resistance current path between the base and the emitter, the current path comprising a transmission gate;
• and
• an inverter stage and first and second transistors in parallel connection between the first power terminal and a first control node, the first transistor having a gate connected to receive the input signal and the second transistor having a gate connected to receive the enable signal, the control node being connected to one gate of the transmission gate and to an input of the inverter stage, the output of the inverter stage being connected to another gate of the transmission gate whereby the first and second transistors control the transmission gate;
• the circuit means operative to cause the current path to assume a low resistance and thereby prevent reverse bias when the enable signal assumes the "disable" value.