Analyses and design of bias circuits tolerating output voltages above BV/sub CEO/

Abstract

Due to the inevitable tradeoff between speed and breakdown voltage, the spectacular speed improvement of modern SiGe processes in recent history has partially been achieved at the cost of a reduction in breakdown voltages. Because supply voltages have hardly been reduced however, circuits operating at a supply voltage above the collector-emitter breakdown voltage (BV/sub CEO/) are common practice today and collector-base avalanche currents are therefore of major concern. Transistors that need to handle a collector-emitter voltage above (BV/sub CEO/) are typically found as output transistors in output driver stages and in bias current circuits. Such circuits can be designed to tolerate collector-emitter voltages above (BV/sub CEO/) by driving the base terminal with a relatively low impedance. This paper analyzes various conventional as well as two new bias current circuits supporting operation at collector voltages above (BV/sub CEO/). In the new circuits, feedforward and feedback avalanche current compensation techniques are introduced that obtain a substantial increase in output breakdown voltage of the bias circuits and improve the accuracy of the current mirror at output voltages above (BV/sub CEO/). With the feedback technique, a measured increase in output breakdown voltage by more than 2 V is demonstrated while the accuracy of the current mirror ratio at output voltages of 2 to 3 times (BV/sub CEO/) is improved by an order of magnitude.