Push-pull driver with reduced noise generation resulting from driver switching. A further transistor is arranged between the driver output transistor (which becomes conductive at the low output level) and the chip ground line. Its base is connected to a reference voltage source the other pole of which is connected to the ground plane of the circuit card to which the corresponding semiconductor chip is attached. If a noise voltage is generated on the chip ground line, the emitter potential of the further transistor is pulled up. As its base potential is maintained at a fixed value by the applied reference potential, this transistor becomes less conductive. As a result, the rate of current change in the output stage is reduced. The slowed down current rise, leads to a reduced noise voltage developing on the common chip ground line. According to another embodiment of the invention the output transistor and said further transistor are combined to form one transistor whose base is maintained at a fixed voltage by means of two series-connected Schottky diodes.

BACKGROUND OF THE INVENTION 
The invention relates to a monolithically integrated push-pull driver. More 
particularly, the invention refers to a push-pull driver circuit which 
minimizes the noise generated by the driver switching on a commonly shared 
voltage supply line, e.g., ground line. 
As a result of the progress made in the production of monolithically 
integrated semiconductor circuits, the circuits necessary for a data 
processing system can be increasingly miniaturized on the semiconductor 
chip. This permits producing a greater number of circuits in the same 
area, e.g., on a semiconductor chip. The higher integration density, 
however, leads to an increased number of chip output lines and thus to a 
greater number of driver circuits required. In the interest of fast data 
processing, the data handling has to be executed in parallel. This means 
that more and more driver circuits have to be switched simultaneously. 
However, the inductance of the package or conductor, by means of which the 
semiconductor chip is connected to the ground (potential) plane of the 
respective circuit card carrying the semiconductor chip, causes an 
increasing problem of noise generation on the chip ground line. As this 
noise could potentially affect adjacent storage elements serving to store 
the results of logical operations performed, the noise voltage must be 
kept below the switching threshold of the storage elements to prevent 
their state from being undesirably changed. As the magnitude of the noise 
voltage is, amongst others, dependent upon the number of simultaneously 
switching driver stages, this number must be limited to confine the noise 
voltage to a given minimum value. For this purpose it has been necessary 
to subdivide the total number of driver stages necessary for the 
particular data transfer situation into several groups of driver stages 
switching simultaneously in the respective group and to control the 
switching of the several groups in a time staggered fashion. However, this 
time-staggered switching results in reduced data transfer speed. 
Accordingly, it is an object of the invention to provide an improved driver 
circuit of the above-mentioned type. 
More specifically, it is an object of the invention to provide a push-pull 
driver circuit which allows to minimize the noise generated by the driver 
switching action. 
It is a further object of the invention to provide a new push-pull driver 
with a reduced noise generation on a shared voltage supply line which is 
suitably adapted for use in applications where several drivers are 
switched simultaneously. 
SUMMARY OF THE INVENTION 
To reduce the noise voltage on a chip ground line resulting from driver 
switching according to one embodiment of the invention, a further 
transistor is arranged between the driver output transistor (which becomes 
conductive at the low output level) and the chip ground line. The base of 
the further transistor is connected to one pole of a d.c. reference 
voltage source the second pole of which is connected to the ground plane 
of the circuit card accommodating the semiconductor chip. The reference 
voltage source maintains the base potential of the further transistor. Its 
voltage is chosen so that after the output transistor has become 
conductive the further transistor becomes also fully conductive. If a 
noise voltage is generated on the chip ground line in response to the 
current rise occurring when the driver output transistor becomes 
conductive, the emitter potential of the further transistor is pulled up. 
As its base potential is maintained by the applied reference potential, 
this transistor becomes less conductive. As a result, the rate of current 
change in the output stage is reduced. The slowed down current rise, leads 
to a reduced noise voltage developing on the common chip ground line. 
According to another embodiment of the invention the output transistor and 
said further transistor are combined to form one transistor whose base is 
connected to said reference potential. 
The advantages achieved by the invention are mainly that since the noise is 
reduced it is no longer necessary to limit the number of simultaneously 
switching driver stages and thus to time-stagger the switching of several 
groups of driver stages switching simultaneously within the respective 
group. As a consequence the data transfer speed is improved.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 shows a circuit diagram of the output stage of a conventional 
monolithically integrated push-pull driver circuit according to the prior 
art. L denotes the inductance of the connecting line by means of which the 
chip ground line ML is connected, via a conductive line on the chip 
substrate and a metallic connecting pin, to the ground plane ME of a 
circuit card accomodating the module made up of the particular 
semiconductor chip, its substrate and a cap. The operation of this known 
push-pull driver circuit is as follows: 
If a low voltage level is applied to input terminal 1, transistor T1 is 
non-conductive. As a result, the base current flow from transistor T1 to 
transistor T3 ceases, so that the latter becomes non-conductive, too. Now, 
current flows into the base of transistor T2 via resistor R1. This 
transistor thus becomes conductive and a high potential is available at 
output 2 of the push-pull driver circuit. 
If a high potential is applied to input terminal 1, transistor T1 becomes 
conductive, thus depriving transistor T2 of its base current and reducing 
it non-conductive. Instead, base current is applied through transistor T1 
to transistor T3, so that the latter becomes conductive and transistor T2 
non-conductive. The conductive transistor T3 leads to a low potential at 
the output 2 of the push-pull driver circuit. The current rise occurring 
in transistor T3 in response to its becoming conductive produces a noise 
voltage Un in inductance L at the chip ground line ML. The inductance L is 
essentially made up of a connector pin by means of which the ground line 
ML of chip and chip substrate is connected to the ground plane ME in the 
circuit card. This noise voltage increases with the current change per 
unit of time. If in the case of a plurality of push-pull driver circuits 
on the semi-conductor chip several, or in an extreme case all, transistors 
T3 are turned on simultaneously a correspondingly high switching current 
flows through inductance L common to all driver circuits, so that a 
correspondingly high noise voltage Un occurs at the ML ground line. The 
value of this noise voltage Un is computed in accordance with the 
relation: 
##EQU1## 
Depending upon the number of transistors T3 becoming simultaneously 
conductive, the noise voltage can reach such a value that, for instance, 
storage elements close to the push-pull driver circuits are caused to 
change their stored state. 
FIG. 2 shows a circuit diagram of the output stage of an integrated 
push-pull driver circuit according to the invention, in which the noise 
voltage produced when switching on transistor(s) T3 is considerably 
reduced. This circuit differs from that according to FIG. 1 by a further 
transistor T4 which is inserted between the chip ground line ML and the 
emitter of transistor T3. The base of transistor T4 is--in case of NPN 
transistors--connected to the positive pole of a d.c. reference voltage 
source URo having an internal resistance Ri. The negative pole is 
connected to the ground line in an integration package level higher than 
the chip level, e.g., to the ground plane ME of the circuit card. The 
voltage URo of the reference d.c. operation voltage source is chosen so 
that transistor T4 is fully conductive in the d.c. operation mode. 
The circuit of FIG. 2 operates as follows: Assume there is a positive noise 
voltage to ground, in response to a change in the current I flowing to 
ground plane ME. This occurs when several transistors T3 become 
simultaneously conductive. Then, the emitter potential of transistor T4 
will rise accordingly, while its base potential is maintained at the 
potential determined by the reference source. As a result, the 
base-emitter voltage Ube of transistor T4 is reduced, so that this 
transistor becomes less conductive, i.e., its resistance increases, 
counteracting the rise in the current supplied by transistor T3. Thus the 
rise of this current is slowed down. Consequently, the magnitude of the 
noise voltage, which is proportional to the rate of the current change 
supplied by transistor T3, is reduced. For the maximum acceptable noise 
voltage Unmax in relation to the reference voltage URo the following 
expression applies: 
EQU Unmax.ltoreq.URo-Ube.sub.(T4) (2) 
For a satisfactory operation of the circuit the internal resistance Ri of 
the reference voltage source should be sufficiently high to limit the base 
current of transistor T4. 
FIG. 4A shows a typical time diagram of the noise voltage Un developing at 
the chip ground line ML in a conventional push-pull driver circuit when 
transistor T3 becomes conductive. FIG. 4B shows the time diagram of the 
corresponding current I producing said noise voltage of FIG. 4A. 
In comparison thereto, FIGS. 5A and 5B show the corresponding noise voltage 
and current time diagrams of a push-pull driver circuit according to the 
invention. A comparison of FIGS. 4A, 5A and 4B, 5B show that, as a result 
of the slowed down rise in the current of transistor T3, the noise voltage 
of the push-pull driver in accordance with the invention would be only 
about half as high as that occurring in the circuit of FIG. 1. 
FIG. 3 shows a simplified version of the above-described push-pull driver 
circuit. Here, transistors T3 and T4 of the circuit of FIG. 2 are combined 
to form transistor T3', whose base is also connected to the positive pole 
of the reference voltage URo. In this embodiment the d.c. reference 
voltage source is realized by two series-connected Schottky diodes D1 and 
D2. The operation of this push-pull driver circuit is the same as 
described in connection with FIG. 2. 
For a given value of the maximum permissible noise voltage, the reduced 
noise voltage generation of the push-pull driver circuit in accordance 
with the invention permits a greater number of driver stages than 
previously possible to be simultaneously switched to the low output level. 
This also eliminates the need for time-staggered switching operation of 
the push-pull driver circuits grouped together and switched simultaneously 
within the respective group. As an overall result a significantly 
increased data transfer speed is achievable. To further reduce a potential 
influence of the push-pull driver circuits on adjacent storage elements 
providing a separate ground line ML serving the driver circuits as well as 
a separate connection of said line to the ground plane ME of the circuit 
card is an additional improvement. 
While the invention has been particularly shown and described with 
reference to the preferred embodiments thereof, it will be understood by 
those skilled in the art that various changes in form and detail may be 
made therein without departing from the spirit and scope of the invention.