A circuit and device including a transistor and diode

A circuit comprising:

• a transistor;
• a signal source;
• a base of the transistor being switchably connectable to the signal source;
• a first diode connected between the base of the transistor, and a reference voltage;
• wherein the circuit is arranged such that when the signal source is not connected to the base of the transistor, a voltage applied at an emitter of the transistor causes a current flow through the base of the transistor and through the first diode such that the transistor is in an ON state;
• the impedance of the signal source is lower than the impedance of the transistor through the emitter and base;
• the first diode provides a current limiting function such that when the signal source is connected to the base of the transistor, current flow through the base reduces such that the transistor switches to an OFF state;
• and in which
• the circuit comprises a second diode, the second diode comprised from a semiconductor region that provides the base of the transistor and a further semiconductor region in direct contact with the semiconductor region that provides the base of the transistor;
• and in which the signal source is connectable to the base of the transistor through the second diode.

A method of operating a circuit, wherein the circuit comprises a bipolar transistor, a diode having a first terminal connected to a base terminal of the transistor;

• and a second terminal of the diode connected to a reference voltage, and wherein a voltage is applied across an emitter of the transistor and the second terminal of the diode, the method comprising:
• switchably connecting the base terminal of the transistor and the first terminal of the diode to a signal source having an impedance lower than the impendence across an emitter and base terminals of the bipolar transistor such that:
• when not connected causing a current flow through the base terminal of the transistor and through the diode to turn the transistor ON;
• or
• when connected, current flow through the base of the transistor reduces to switch the transistor OFF.

An integrated circuit comprising:

• an NMOS transistor;
• a gate of the transistor being switchably connectable to a signal source;
• a first diode connected between the gate of the transistor, and a reference voltage;
• the integrated circuit arranged such that when the signal source is not connected to the gate of the transistor the transistor is in an ON state;
• a resistor device connected between the signal source and the gate of the transistor;
• and
• a second diode connected between the gate and the drain of the transistor;
• the second diode selected to limit a difference between a voltage at the gate and source of the transistor when the transistor is in the ON state.