Method to increase frequency of digital circuits

Details Method to increase frequency of digital circuits Method to increase frequency of digital circuits

1999-12-07

2001-09-11

Tran, Toan (Department: 2816)

Miscellaneous active electrical nonlinear devices, circuits, and

Signal converting, shaping, or generating

Current driver

C327S110000, C327S111000

Reexamination Certificate

active

06288579

ABSTRACT:

TECHNICAL FIELD
The present invention relates to shaping load impedance to maximize the operating frequency of electronic circuits, and is particularly applicable to digital circuit design and integrated circuit layout.
BACKGROUND OF THE INVENTION
Many electronic circuits, particularly digital circuits, are required to function over a very broad frequency range from DC to very high frequencies. A load resistor is typically used to convert switching currents to output voltages. In order to extend the operating frequency of such gates, it is well known to place an inductor in series with the load resistor (see, e.g., “Transmission Lines for Digital and Communication Networks,” R. E. Matick, IEEE Press, New York, 1995). Such a load inductor raises the impedance for high frequency components of signals, while retaining the resistive load to generate a stable output voltage at low frequencies.
The load inductor can be realized as a lumped element at lower frequencies, but as operating frequencies rise above 10 GHz, the inductor must be treated as a transmission line having distributed inductance and distributed capacitance. These distributed effects are often detrimental, and may limit maximum operating frequencies.
Conventional digital loads are simply connections to load resistors (see, e.g., “39.5 GHz Static Frequency Divider Implemented in AlInAs/GaInAs HBT Technology” by J. F. Jensen et al., Proceedings of the 1992 GaAs IC Symposium, pp. 101-104). An inductive transmission line has been used as a load to enhance the operating frequency of digital circuits, as described by M. Wurzer et al. in “A 42 GHz Static Frequency Divider in a Si/SiGe Bipolar Technology” (Proceedings ISSCC, 1997, pp 122-123). This inductive transmission line approach, however, provides only about 5%-10% improvement in operating frequency over conventional load resistor connection techniques.
Current integrated circuit technology, using devices such as InP heterojunction bipolar transistors (HBTs), have the potential to operate at millimeter-wave frequencies exceeding 100 GHz. However, standard circuit design techniques limit operation to about half of that frequency. Previous efforts to increase operating frequency by modifying circuit loads with inductors or inductive transmission lines have resulted in only slight increases in circuit speeds. Thus, particularly to take advantage of the high frequency capabilities of present semiconductor devices, a need exists to improve circuit design techniques to support higher frequency operation of electronic circuits.
BRIEF DESCRIPTION OF THE INVENTION
The present invention addresses the need for better inductive loads for digital circuits. It can increase operating frequencies of digital differential stages by 20% to 30% without sacrificing low frequency operation.
The basis of the invention is to use the distributed impedance or transmission line characteristics of inductive digital loads to advantage, enhancing rather than hampering the performance of digital circuits. By adding a section of low impedance, or “capacitive,” transmission line to a high impedance, or inductive transmission line, the present invention causes the load to become much more inductive at high frequencies. The capacitive section tends to shunt the real load resistance at very high frequencies, such that the remaining impedance is more inductively reactive, and of higher impedance. The resulting load appears mostly resistive at low frequency, and mostly inductive at high frequency. The effect of reducing the real resistance components is to reduce time constants which, otherwise, will limit minimum circuit response times and thus limit maximum operating frequencies.
The preferred embodiment of the present invention modifies the collector loads for a digital switching circuit having a differential output, with each collector load connection forming one side of a transmission line. The differential output voltage is taken from output voltage nodes near each collector of the differential pair of current-controlling transistors. Between these nodes and the supply line (or other circuit common point), an inductive transmission line load is created by forming long, relatively thin connection lines which are well separated from each other. Then, between the inductive transmission line so formed and a resistive load attached to the circuit common, a section of highly capacitive transmission line is established by employing very wide traces which face each other closely. The net effect at very high frequencies is to cause the differential load to appear almost entirely inductive.


REFERENCES:
patent: 5039891 (1991-08-01), Wen et al.
patent: 5920224 (1999-07-01), Preslar
patent: 6094084 (2000-07-01), Abou-Allam et al.
patent: 6121809 (2000-09-01), Ma et al.
patent: 6121940 (2000-09-01), Skahill et al.
Jensen, J.F., et al., “39.5 GHz Static Frequency Divider Implemented in A1InAs/GaInAs HBT Technology,” Proceedings of the 1992 GaAs IC Symposium (1992) pp. 101-104.
Wurzer, M., et al., “A 42GHz Static Frequency Divider in a Si/SiGe Bipolar Technology,” Proceedings ISSCC (1997) pp 122-123.

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