Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Amplitude control
Reexamination Certificate
1999-08-31
2001-02-20
Cunningham, Terry D. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Amplitude control
C326S075000, C326S126000
Reexamination Certificate
active
06191635
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates to an electronic circuit, and more specifically to a level shifting circuit, and more specifically to a circuit which achieves analog signal level shifting, for example, level shifting a current mode logic signal from the positive to the negative rail in a circuit.
BACKGROUND OF THE INVENTION
In current mode logic (CML) circuits, voltages are typically referenced from the most positive supply voltage. However, an analog circuit connected to such a digital CML circuit may need to have its threshold voltages referenced to the negative supply. As a result, at the digital/analog interface, there is a need for a circuit which shifts the signal level between the two voltage supply rails.
JP-A-5-315936 discloses a level shifting circuit in which an input signal is applied to the base of a first NPN transistor. The emitter of this first transistor is connected to the collector terminal of a second NPN transistor and also, through a resistor, to the base of the second NPN transistor the base of this second transistor is connected to ground through a first current source circuit, while the emitter terminal of this second transistor is connected to ground through a second current source circuit, and the emitter terminal of the second transistor also acts as the circuit output.
JP-A-6-260925 discloses a further level shift circuit, in which first and second input terminals, for receiving a differential input voltage, are connected to the bases of first and second NPN transistors. The collector terminals of these transistors are connected together, and the emitter terminals are connected to respective output terminals which receive a differential output voltage. The emitters of these transistors are further connected through respective resistors to respective halves of a current mirror circuit.
An object of the present invention is to provide a level shifting circuit which, in preferred embodiments, can maintain a differential signal gain of unity, and, again in preferred embodiments, can provide a fixed output common mode level which is independent of the supply voltage and the input common mode voltage.
SUMMARY OF THE INVENTION
In accordance with a preferred aspect of the invention, first and second input terminals are connected across first and fourth resistors respectively to first and second output terminals. The first and-second input terminals are also connected respectively across second and third resistors to a reference node at which there appears a reference voltage, defined with reference to a different supply rail from that to which the input terminals are referred. The third and fourth resistors have equal resistance values. Means are provided for drawing currents through the first and fourth resistors, and from the reference node, to that supply rail. These current drawing means, and the resistance values of the four resistors, are chosen such that the voltage drops across the first and fourth resistors are equal. This has the advantage that the output voltage is referenced to the second voltage supply rail.
Preferably, these components are chosen such that, in the balanced condition, in which the input voltages are equal, the voltage drops across all four resistors are equal.
This has the advantage that the circuit then has unity gain from input to output.
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Filippi Raymond
Thompson John
Cunningham Terry D.
Jenkens & Gilchrist P.C.
Telefonaktiebolaget LM Ericsson
Tra Quan
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