Emitter-coupled logic (ECL) is the pre-eminent logic family where high-speed operation is the overriding requirement. For example, the fastest computers available utilize ECL in the computational portions thereof. In contrast, complementary metal-oxide-semiconductor (CMOS) logic has been traditionally thought of as low-speed and low-power logic family. However, recent advances in CMOS circuit design and processing technology is narrowing the speed gap between ECL and CMOS logic while retaining the low-power feature of CMOS logic. As CMOS logic circuits approach the speed of ECL circuits, new designs, and redesigns of existing systems, that would traditionally require ECL are now utilizing CMOS logic circuits with little or no sacrifice in performance--with the added benefit of lower power dissipation. The lower power dissipation of CMOS logic allows greater integration density, hence more compact systems are produced. In redesign applications where ECL is replaced with CMOS logic, the above benefits are still obtained, but frequently the new CMOS logic must still interface with existing "peripheral" or "glue" ECL circuits. This involves converting the ECL logic levels (approximately -0.9 volts for a logic "one" and -1.8 volts for a logic "zero" ) to CMOS logic levels (typically greater than one-half the power supply voltage volts for a logic "one" and less than one-half the power supply voltage for a logic "zero") by an input buffer and converting the CMOS logic levels back to ECL logic levels by an output buffer.
One design of a CMOS to ECL output buffer circuit is described in U.S. Pat. No. 4,656,372. In FIG. 1, the output buffer is four series-coupled field effect transistors (FETs), similar to a conventional CMOS inverter, with two of the FETs (22, 23) adapted to provide output voltage level shifting. The series coupled FETs increase the output impedance, thereby reducing the speed of the buffer. Further, the design is limited to -3 volts power supply instead of the standard -4.5 to -5.2 volt power supply, and requires over-sized FETs to provide sufficient current sourcing capacity.
An alternative CMOS to ECL output buffer design is disclosed in U.S. Pat. No. 4,645,951. In its simplest configuration (FIG. 1,) a CMOS logic signal from CMOS logic 20 drives field-effect transistor F5. Transistor F5 operates as a switch for selectively coupling current source Is to resistor R5 and to the base of output transistor Q5. When transistor F5 is conducting, the voltage drop across load resistor R5, due to the current from current source Is flowing therein, is buffered by transistor Q5 to provide the ECL "zero" logic level at the emitter of transistor Q5. When transistor F5 is not conducting, load resistor R5 pulls up the base of transistor Q5 such that the emitter of transistor Q5 has the ECL logical "one" thereon. By relying on the load resistor R5 to pull-up the base of transistor Q5, the speed of the output buffer suffers due to the time required for charging the parasitic capacitances of transistors F5, Q 5 and the load resistor R5 itself.
Still another CMOS to ECL output buffer is disclosed in FIG. 4 of U.S. Pat. No. 4,437,171. This complex circuit relies on the voltage drop across series resistors 70 and 71 to establish the output voltage for an ECL logical "zero." As such, the ECL logical "zero" output voltage is dependent upon the power supply input voltage (Vee), requiring tight regulation of the Vee supply.
The published speciications for the ECL logic voltage levels are very tight and to optimize the speed of the interface between CMOS and ECL circuits, the ECL voltage specifications must be closely adhered to. In particular, the voltage difference between the ECL logical "one" and logical "zero" is very tightly specified, and is substantially invariant with temperature. However, the output voltage from prior art output buffers varies with temperature and processing variations during manufacturing. For example, the output buffer disclosed in U.S. Pat. No. 4,645,951, described above, suffers from this problem. Due to the manufacturing processes alone, the resistance of the load resistor R5 can have wide variations, typically thirty to forty percent. This, with current source Is providing a predetermined constant current, causes corresponding variations in the ECL logical "zero" output voltage which could exceed the specifications for the ECL logical "zero" voltage and the voltage difference between logical levels.
In FIG. 2 of U.S. Pat. No. 4,533,842, a current source which tracks variations in resistance of a load resistor is disclosed. Here, transistor 36, resistor 47 and voltage source Vcs form a constant current source, the sourced current through the collector transistor 36 being established by the voltage Vcs and the resistor 47. However, transistor 36 is a bipolar device, and to insure high-speed operation, the current through the collector of transistor 36 must not be allowed to fall to zero; to do so would force transistor 36 into saturation, requiring substantial recovery time should it be desired to have transistor 36 resupply current.Hence, a differential stage of transistors 32 and 33 switch the current flowing from transistor 36 between them through corresponding load resistors 45, 46, keeping a constant current flowing through transistor 36. Since a large current is flowing in transistor 36 at all times, this type of current source suffers from high power dissipation. With a large number of output stages on a chip, a very substantial amount of power must be dissipated.
Another approach to stabilization of the output voltage is addressed in U.S. Pat. No. 4,656,375. Here an unused ECL gate is utilized to generate the supply voltages (Va, Vb) to a conventional CMOS inverter acting as the CMOS to ECL output buffer. However, this design suffers from the same drawbacks the design in U.S. Pat. No. 4,464,372, described above, does with the additional drawback of the need for several power supply voltages: Vcc, Vss and Vee, as well as Va and Vb. The multiplicity of power supplies adversely impacts the number of pins available for data signals, etc., on a package containing such circuitry. Further, additional external power supply filtering is required (the one microfarad capacitors.)
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