Electronic digital logic circuitry – Function of and – or – nand – nor – or not – Bipolar transistor
Reexamination Certificate
2000-06-13
2001-07-03
Tokar, Michael (Department: 2819)
Electronic digital logic circuitry
Function of and, or, nand, nor, or not
Bipolar transistor
C326S075000, C326S124000
Reexamination Certificate
active
06255857
ABSTRACT:
This invention relates to signal level shifting circuits, and is particularly concerned with a signal level shifting circuit which can be used for coupling a signal between BCMOS (bipolar transistor and complementary metaloxide-semiconductor field effect transistor) circuits operating with different supply voltages.
BACKGROUND
In order to couple a signal between different devices (e.g. integrated circuits) or circuits which operate with different supply voltages, it is known to perform a level shifting of the signal, this being achieved by a level shifting circuit or interface circuit between the devices. The level shifting circuit can be regarded as an output circuit of the device supplying the signal, as an input circuit or buffer of the device receiving the signal, or as a separate interface circuit between the two devices. Typically, the extent of the level shifting will depend upon the differences between the supply voltages of the devices, and the nominal signal levels which typically also depend on the supply voltages.
For example, especially for handling signals at very high frequencies (e.g. 5 GHz), integrated circuit devices using BCMOS technology may be used which operate with nominal supply voltages of either +1.8 volts and −5 volts or 0 (zero) volts and −5 volts, in each case typically with a tolerance of ±10%. In the former case, a signal may have high and low voltage levels of 1.8 and 1.4 volts respectively, with a signal voltage swing of 0.4 volts; under worst case conditions, this signal voltage swing may be as little as 0.2 volts. In the latter case, a signal may have high and low voltage levels of 0 and −0.8 volts respectively, with a signal voltage swing of 0.8 volts; under worst case conditions, this signal voltage swing may be as much as 1.0 volts.
From these figures, it can be seen that an integrated circuit device with a differential supply voltage of about 7.5 volts (1.8 +5 volts, plus 10%) may produce a signal voltage swing of only about 0.2 volts, and a device with a differential supply voltage of 4.5 volts (5 volts minus 10%) may produce a signal voltage swing of 1 volt. Thus there is considerable variability of both the signal voltage swing and the differential supply voltages which may be present.
A considerable problem is presented in providing an appropriate interface circuit, for example as an input buffer to a device operating with the nominal 0 and −5 volt supply voltages, which can properly handle signals from a preceding device which may correspond to either of the two cases discussed above. Such an interface or signal level shifting circuit is desired to handle, relatively arbitrarily, a signal with either set of supply voltages and signal voltage swings, because a preceding device producing the signal may fit either of these two cases.
This problem is exacerbated by constraints which are imposed by the BCMOS technology and the desired functioning of the devices. For example, BCMOS technology may impose a constraint that a maximum voltage difference between any two terminals of any transistor or other active device be no more than 3.6 volts for devices operating at signal frequencies, or about 4.5 volts for devices used for biassing. In addition, the devices are desired to have substantially constant biassing conditions under the different possible signal handling conditions, to ensure that performance (e.g. bandwidth) is not dependent upon the nature of the supplied signal. In addition, low power dissipation is desired.
A need exists to provide a signal level shifting circuit which enables these desires to be satisfied.
SUMMARY OF THE INVENTION
According to one aspect of this invention there is provided a signal level shifting circuit comprising: an input transistor having a base coupled to an input terminal for an input signal, a collector coupled to a terminal for a supply voltage, and an emitter; at least one diode coupling the emitter of the input transistor to a junction point for providing a level shifted signal; a bias circuit coupled to the junction point for conducting a current which flows via a collector-emitter path of the input transistor and via said at least one diode to provide a forward voltage drop of said at least one diode providing a level shift of the input signal; at least one transistor switch having a controlled path in parallel with said at least one diode; and a control circuit responsive to said supply voltage for controlling said at least one transistor switch to be non-conductive when said supply voltage has a first, relatively higher, value and to be conductive when said supply voltage has a second, relatively lower, value, the conductive state of said at least one transistor switch providing a reduced voltage level shift of the input signal when said supply voltage has the second value.
Another aspect of the invention provides a differential signal level shifting circuit comprising: two bipolar input transistors each with an emitter-follower configuration having bases coupled to input terminals for a differential input signal, collectors coupled to a supply voltage, and emitters coupled via respective level shifters and respective junction points to respective bias circuits; and an output stage comprising two differentially-connected bipolar transistors having their bases coupled to the two junction points and providing a differential output signal from their collectors; wherein each of the level shifters comprises at least one diode arranged to provide a forward voltage drop providing a signal level shift, and at least one transistor switch having a controlled path in parallel with said at least one diode; the differential signal level shifting circuit also comprising a control circuit responsive to said supply voltage for controlling each transistor switch to be non-conductive when said supply voltage has a first value and to be conductive when said supply voltage has a second value less than the first value.
The invention also provides a signal level shifting circuit comprising a bipolar transistor having an emitter-follower configuration with a base supplied with an input signal, a collector coupled to a supply voltage, and an emitter coupled via a level shifter to a bias circuit, whereby a level shifted signal is produced at a junction point between the level shifter and the bias circuit, wherein the level shifter comprises: at least one diode arranged for conducting current between the emitter of the bipolar transistor and the junction point to provide a forward voltage drop providing a signal level shift; a MOS field effect transistor having a gate and having a source-drain path coupled in parallel with said at least one diode; and a control circuit having an output coupled to said gate, the control circuit being responsive to said supply voltage for controlling said MOS field effect transistor so that its source-drain path is non-conductive when said supply voltage has a first value and is conductive when said supply voltage has a second value less than the first value.
In addition, the invention provides a method of providing either of two different level shifts for an input signal which is supplied to the base of a bipolar transistor in an emitter-follower configuration in dependence upon a supply voltage supplied to a collector of the transistor, comprising the steps of: when the supply voltage has a first value, providing a level shift by a forward voltage drop of at least one diode connected to an emitter of the transistor; and in response to the supply voltage having a second value less than the first value, closing a transistor switch in parallel with said at least one diode to reduce said level shift.
REFERENCES:
patent: 4713560 (1987-12-01), Herndon
patent: 4999519 (1991-03-01), Kitsukawa et al.
Le Don Phu
Nortel Networks Limited
Tokar Michael
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