Output buffer with programmable voltage swing

Details Output buffer with programmable voltage swing Output buffer with programmable voltage swing

1999-02-19

2001-10-09

Tokar, Michael (Department: 2819)

Electronic digital logic circuitry

Function of and, or, nand, nor, or not

Bipolar transistor

C326S068000, C326S078000

Reexamination Certificate

active

06300802

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to emitter coupled output circuits. More particularly, the invention concerns an integrated circuit having a pair of emitter coupled output transistors and an electrical connection area that is connectable to an external circuit to program the magnitude of the voltage swings of the outputs.
2. Description of the Related Art
Output buffers are common components of high speed logic circuits.
FIG. 1
illustrates a typical prior art current mode logic (CML) output buffer circuit
100
that has emitter coupled outputs. In the buffer circuit
100
, the emitters of output transistors Ql and Q
2
are connected together, and are thus “emitter coupled.” Resistor R
1
is connected to the collector of transistor Q
1
, and resistor R
2
is connected to the collector of transistor Q
2
. The collector of output transistor Q
2
is connected to output Y, and the collector of output transistor Q
1
is connected to complementary output YN. The output current
1101
for these emitter coupled outputs is set by a constant current source consisting of transistor Q
3
and resistor R
3
. The amount of current through the constant current source determines the magnitudes of the voltage swings of the outputs Y and YN.
Once an integrated circuit that includes a circuit such as circuit
100
is fabricated, the current
1101
through the constant current source becomes substantially fixed. Additionally, the voltage swings of the outputs are fixed. Generally, the voltage swings of the outputs will be substantially equal. In circuits such as circuit
100
, the bias voltage applied to the base of the current source transistor Q
3
is substantially fixed, although some sort of temperature compensation may be included. This bias voltage sets the amount of current through the current source, and the resulting voltage swings of the outputs, at fixed values.
The fixed magnitude of the voltage swings of the outputs in emitter coupled outputs, such as the outputs of the circuit
100
in
FIG. 1
, is typically set large enough to ensure satisfactory performance for a wide range of applications under worse case conditions, also taking into account imprecision in the magnitude of the voltage swings that may result due to limitations of the manufacturing process. Even though the magnitude of the output voltage swings is set large enough for most applications, some applications may require larger output current swings than is provided.
A significant shortcoming of prior art emitter coupled output circuits such as circuit
100
is that many applications do not require output voltage swings that are as large as the fixed output voltage swings provided, resulting in unnecessary power consumption and heat generation. For very fast and low jitter applications, the collector resistors that determine the output voltage swings must be set to match the impedance of the signal trace on the board, which will typically be 50 ohms. Thus, to achieve a 500 mV (single ended) swing, a minimum of 10 mA of current (500 mV divided by 50 ohms) would be required. Thus, the outputs of the circuit
100
may provide 500 mV voltage swings to an application requiring only 300 mV voltage swings, resulting in 4 mA of current being wasted for each of the outputs Y and YN. In an application with a 100 ohm line to line termination, the effective impedance is 25 ohms, which requires 20 mA of current (500 mV divided by 25 ohms) to achieve 500 mV voltage swings. In this case, if the outputs of the circuit
100
provide 500 mV swings to an application requiring only 300 mV voltage swings, 8 mA of current will be wasted for each of the outputs Y and YN. Conversely, another shortcoming of prior art emitter coupled output circuits such as circuit 100 is that some applications require output voltage swings that are larger than the fixed output voltage swings provided by these circuits.
Although prior art circuits, such as those disclosed in U.S. Pat. No. 3,760,200 of Taniguchi et al., provide for reduction of fluctuations of the amplitude of circuit outputs, these circuits do not have a default operating mode where connection of an external resistor or current source is not required, and they consequently require connection of an external resistor or current source to the integrated circuit containing the output circuitry at all times.
SUMMARY OF THE INVENTION
Broadly, the invention concerns an integrated circuit having a pair of emitter coupled output transistors and an electrical connection area. The electrical connection area is connectable to an external circuit to program the magnitude of the voltage swings of the outputs. The emitter coupled outputs may be current mode logic (CML) outputs, emitter coupled logic (ECL) outputs, or outputs in other types of logic families such as CMOS and NMOS.
An illustrative embodiment of the invention is shown in FIG.
3
. Like the prior art circuit
100
of
FIG. 1
, the integrated circuit device
300
of
FIG. 3
includes a pair of emitter coupled output transistors and a current source. In that regard, the device
300
includes emitter coupled output section
302
which includes emitter coupled output transistors Q
4
and Q
5
, current source transistor Q
8
, and current source resistor R
8
. In the prior art circuit
100
of
FIG. 1
the bias voltage applied to the base of the current source transistor Q
3
is fixed (except for temperature compensation), thereby causing the output drive current I
101
and the magnitude of the output voltage swings to be fixed. In contrast, in the device
300
of
FIG. 3
the bias voltage applied to the base of the current source transistor Q
8
is variable, thereby providing for adjustment of the output drive current I
301
and the resulting magnitude of the output voltage swings of outputs Y
1
and YN
1
. Within the operational range of the circuit, the magnitude of the output voltage swings can be programmed to be as low or high as required for a particular application. Programming the magnitude of the output voltage swings also allows for the elimination of output voltage swing variation that can be introduced due to manufacturing tolerances. To save power, the magnitude of the output voltage swings can be set as small as is possible without adversely affecting circuit performance.
In order to program the magnitude of the output voltage swings, the integrated circuit device
300
of
FIG. 3
includes a variable bias generator
315
that produces a bias voltage V
BIAS
that is connected to the base of the current source transistor Q
8
. The output drive current I
301
, and consequently the magnitude of the output voltage swings, are determined by the bias voltage. The variable bias generator is connected to an electrical connection area
225
of the integrated circuit device
300
of FIG.
3
. An external programming circuit can be connected to the electrical connection area in order to adjust (also referred to as setting) the bias voltage, to thereby program the desired magnitude of the output voltage swings. The external programming circuit, which is shown in
FIG. 3
as a resistance
307
, can be a resistance or an external voltage source (and could be other types of components in other embodiments). The variable bias generator can be any of a number of circuits that produce a bias voltage that is a function of the external programming circuit connected to the electrical connection area. If adjustment of the magnitude of the emitter coupled output voltage swings is not needed, then an external programming circuit is not connected to the electrical connection area, and the electrical connection area is left floating. When the electrical connection area is left floating, the variable bias generator produces a default bias voltage that results in a default magnitude of the output voltage swings.
The magnitude of the output voltage swings in more than one emitter coupled output section may be controlled by the bias voltage produced by the variable bias generator
315
. To illustrate this optional aspect of

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