Switching matrix for telecommunication exchanges

A switch matrix for telecommunication including change-over switches which can be used for both analog and digital signal transmission. Each change-over switch includes a selector circuit, an input and an output stage. The output stage generally includes two transistors coupled in a push-pull arrangement which are connected directly to the supply voltage allowing the capacitive load of the switch to be charged or discharged rapidly. Also, the Miller effect of the internal base-collector capacitance is absent. Consequently, a large frequency range (to approximately 3 GHz) is realized for analog use, and a switch rate of approximately 2 Gb/s is attainable for digital use.

BACKGROUND OF THE INVENTION 
The invention relates to a switching matrix for telecommunication 
exchanges, in which an input conductor is optionally connectable to an 
output conductor by means of a change-over switch comprising a selecting 
circuit for actuating the change-over switch in response to a junction 
selection signal to be applied to the selecting circuit. 
A change-over switch for such a switching matrix is known from the 
periodical "IEEE Journal of Solid State Circuits", Vol. SC-13, No. 2April 
1978, pp. 258-261, especially FIG. 1. 
The change-over switch described in the above correspondence comprises an 
amplifier section for analog signal transmission, and a selecting circuit. 
The amplifier section is constituted by a base-driven input transistor in 
whose collector line the input to a current mirror is realized. The output 
of this current mirror is serially connected to a diode-switched 
transistor whose emitter is connected to the emitter of the input 
transistor. The anode of the diode formed thus constitutes the signal 
output of the change-over switch. 
The selecting circuit of this known change-over switch is formed by a 
current source which is serially interrruptably connected to the two 
emitters. 
This known change-over switch is designed for switching through analog 
signals. The frequency band of this change-over switch for these signals 
stretches out to approximately 800 MHz; it further appears from the said 
pulication that when this change-over switch is actuated, there is a delay 
of approximately 5 ns and a rise time/fall time of approximately 10 ns. 
Furthermore, the current which can be supplied to the load impedance by 
this change-over switch is limited to a maximum of the current flowing 
through the serially connected current source. Consequently, only a 
limited voltage swing over the output impedance is possible. 
SUMMARY OF THE INVENTION 
The invention has for its object to provide a switching matrix which is 
suitable for transmitting both analog and digital signals with the higher 
frequency than is possible in the aforementioned state of the art, and 
this with a sufficiently large output voltage. 
For this purpose, the invention is characterized 
in that the change-over switch comprises an input stage and an output stage 
connected thereto; 
in that the input stage is designed with one input constituting the input 
to the change-over switch and two outputs, the input stage being designed 
for transmitting at a higher or lower d.c. level the input signal to these 
outputs; 
in that the output stage comprises two transistors of a mutually 
complementary conductor type which are connected in a push-pull 
configuration with interconnected emitters, constituting the signal output 
of the change-over switch; 
and in that the outputs of the input stage are connected to always a base 
of the transistors of the output stage. 
Because the signal output of the change-over switch is formed by the 
interconnected emitters of the push-pull stage, the ever present 
capacitive load of that output (that is to say, the parasitic capacitance 
of the output conductor plus the output capacitances of the non-conductive 
remaining change-over switches connected to that output conductor) can be 
recharged rapidly by one of the two transistors and discharged rapidly by 
the other. This is enabled by the low output impedance of the two input 
transistors switched as emitter followers and together forming the 
push-pull stage. 
Furthermore, the internal base-emitter capacitance of the push-pull 
transistors does not virtually affect the frequency behaviour of the 
change-over switch because the emitter assumes the same voltage as the 
base. Also the so-called Miller effect of the internal base-collector 
capacitance is absent because the collectors are directly connected to the 
supply voltage, so that no voltage gain occurs from the base to the 
collector. 
Since one base of the push-pull stage is adjusted to one diode voltage 
above the output voltage, and the other base to one diode voltage below 
the output voltage, an input stage is required shifting the d.c. level of 
the signal to be applied by the said values. 
An embodiment of the switching matrix in which the advantageous 
high-frequency properties of the change-over switch are maintained, is 
characterized 
in that the input stage is formed by a series circuit of a first current 
source, a first diode, a second diode and a second current source, the 
cathode of the first diode being connected to the anode of the second 
diode and this junction forming the signal input to the change-over 
switch, 
in that one of the outputs is formed by the anode of the first diode, 
and in that the other output is formed by the cathode of the second diode. 
A further embodiment of the switching matrix according to the invention is 
characterized 
in that the selecting circuit is constituted by two groups each including k 
(k.gtoreq.1) transistors having the same type of conductor per group, 
whose emitters are interconnected per group and whose collectors are 
interconnected per group, these groups being of a mutually complementary 
type of conductor, 
in that the emitters of one or the other group, respectively, are connected 
to the base of one or the other transistor, respectively, in the push-pull 
stage, 
and in that the bases of the transistors of both groups form the selection 
inputs to the selecting circuit. 
If, as is customary, there is a bundle of conductors in the switching 
matrix for supplying this selection signal, and there is also a bundle for 
the inverse selection signal, the selection inputs of the first group can 
be connected to the conductors of the two bundles showing a first logic 
value on selection of the relevant junction, and the selection inputs of 
the other group to the remaining conductors then showing the complementary 
logic value. The address of the change-over switch is thus determined by 
the connection mode of each of the bundles of conductors, so that all 
change-over switches from the switching matrix can have an identical 
structure.

DETAILED DESCRIPTION 
The switching matrix as shown in FIG. 1 consists of N input conductors 2-1 
to 2-N and M output conductors 4-1 to 4-M. For the interconnection of 
these conductors there are present M.times.N change-over switches 6.sub.ij 
(i+1,2, . . . ,N;j+1, 2, . . . ,M), of which the signal input 8 is 
connected to the input conductor associated to the junction and of which 
the signal output 10 is connected to the output conductor associated to 
that junction. The switching matrix further includes conductor bundles 
12-1 to 12-M through which the junction selection signal is transmitted to 
each of the junctions. Each conductor bundle 12 is branched off into a 
bundle of selection inputs 14 of a selecting circuit to be described 
hereinafter forming part of the change-over switch. The selection 
information is transmitted through a collective bundle 16 from the central 
control 18 (not shown) of the switching system in which the switching 
matrix is incorporated. 
When in operation, a number of change-over switches will be conductive 
whereas the other change-over switches will be non-conductive. It will be 
appreciated that a single input is connected to no more than a single 
output, the vice versa. (In a telephone system this implies that no more 
than two subscribers communicate with each other per connection). Thus, if 
one of the change-over switches connected to a specific output conductor 
is conductive, the other change-over switches will be non-conductive, for 
no more than one input signal at a time is allowed to be transmitted to 
that output. 
The selection of the conductive change-over switches is effected pe output 
conductor. If the number of change-over switches connected to that 
conductor is 2.sup.k (thus N+2.sup.k), k bits will be required for the 
selection of a junction. For reasons of simplicity of the circuit the 
conductor bundle can also transmit the inverse selection signal; in that 
case twice the amount of bits is required. 
FIG. 2 shows an embodiment of a change-over switch to be used in a 
switching matrix as shown in FIG. 1. This change-over switch 6 comprises a 
selecting circuit 20; an input stage 22 and an output. 
Output stage 24 comprises an NPN transistor 26 and a PNP transistor 28 
whose emitters are interconnected so that a push-pull configuration is 
realized. These interconnected emitters also form the signal output 10 of 
the change-over switch. The collectors of the transistors 26 and 28 are 
directly connected to the respective positive or negative supply voltage. 
Input stage 22 is formed by a series arrangement of a current source 34, an 
NPN transistor 36 switched as a diode, a PNP transistor 38 switched as a 
diode and a current source 40, in that order. The emitters of the two 
transistors are interconnected, that is, the cathode of the first 
transistor 36 switched as a diode is connected to the anode of the second 
transistor 38 switched as a diode and form the signal input 8 of the 
change-over switch. The bases of the two transistors, that is, the anode 
of the first transistor 36 switched as a diode is connected to the cathode 
of the second transistor 38 switched as a diode form the outputs 30 and 32 
of the input stage which outputs are connected to the bases of the 
respective transistors 26 and 28. The current sources 34 and 40 are 
connected to the respective positive and negative supply voltage. 
Selecting circuit 20 is shown in FIG. 2 in the form of a PNP transistor 42 
and an NPN transistor 44. The emitters of the respective transistors 42 
and 44 are connected to the interconnected bases of the respective 
transistors 26 and 36, 28 and 38. The collectors of the respective 
transistors 42 and 44 are connected to the respective negative and 
positive supply voltage. The two bases of these transistors form the 
selection inputs of the change-over switch. These two selection inputs are 
each connected to a conductor of the conductor bundle 12 which transmits 
the junction selection signal. 
The selecting circuit represented in FIG. 2 is only suitable for selecting 
a junction from a group of two. For selecting a junction from a larger 
group, a more extensive selecting circuit is required, as will presently 
be shown with reference to FIG. 3. 
The functioning of the change-over switch 6 is as follows. It is assumed 
that the base of transistor 42 is maintained at a high voltage, for 
example the positive supply voltage, and that the base of transistor 44 is 
maintained at a low voltage, for example the negative supply voltage. With 
this adjustment of the selection signal, the change-over switch is 
adjusted to the conductive state. Because of the adjustment of transistors 
42 and 44, the transistors 36, 38, 26 and 28 are conductive so that the 
transistors 36 and 38 conduct a specific current. Assuming that the 
transistors 26 and 36 are completely equal and transistors 28 and 38 are 
equal too, an equally large amount of current will pass through the series 
arrangement of the base-emitter diodes of the transistors 26 and 28 as 
through the series arrangement of the transistors 36 and 38 in the absence 
of a signal of input 8. The main current through the transistors 26 and 28 
will now be the current gain factor of these transistors times as large, 
for example, a factor 100. 
If the voltage at input 8 is varied, for example increased, the base 
voltage of the transistors 36 and 38 will be varied likewise because the 
diode voltage across these transistors remains substantially unchanged. 
The voltage across the base-emitter diode of the transistors 26 and 28 
does not substantially change either, so that output 10 continues to carry 
the same voltage as input 8. 
The current gain from input 8 to output 10 is equal to the current gain 
factor of the transistors, thus 100 in this numerical example. The high 
signal rate of the change-over switch, also with a capacitive load on 
output 10, is caused by the fact that this capacitor is charged and 
discharged from the emitters of the transistors 26 and 28, thus from a 
point of low impedance. This impedance is inversely proportional to the 
supplied current, so that with a large required current there is only a 
slight impedance which considerably adds to the signal rate of the 
change-over switch. 
FIG. 3 shows an embodiment of selecting circuit 20, suitable for use in a 
change-over switch in a matrix comprising more than two junctions per 
output conductor. The selecting circuit is composed of two groups of 
transistors 42-1 to 42-k and 44-1 to 44-k. In each group the emitters are 
interconnected. These emitters form the outputs of the selecting circuit 
which are connected to the bases of transistors 36 and 38 (FIG. 2). In 
each group the collectors are connected to the negative or positive supply 
voltage, respectively. The bases of each group form the selection inputs 
14 of the selecting circuit. The selection inputs 14 are divided into two 
groups 14-1 and 14-2. Group 14-1 is associated to the transistor group 42 
and group 14-2 to the transistor group44. From group 14-1 all selection 
inputs are to be brought to a high voltage level (for example the positive 
supply voltage) so as to conduct the relevant output current, for group 
14-2 a low voltage (for example the negative supply voltage) is required 
for this purpose. If not all selection inputs have reached the 
above-described voltage, change-over switch 6 will not be conductive. 
The address of the change-over switch 6 is obtained by distributing the 
selection inputs of the groups 14-1 and 14-2 over the conductors of the 
conductor bundle 12, composed of two sub-bundles 12A and 12B. These 
sub-bundles show a mutually inverse bit pattern, allowing at a specific 
bit pattern each selection input to be optionally provided with a high or 
a low voltage level. IF a distribution of high voltages and low voltages 
occurs over the conductors of bundle 12 such that all the selection inputs 
14-1 are high and all the selection inputs 14-2 are low, the junction to 
which this change-over switch is connected is selected to be the 
conductive switch. With all further distributions of high and low voltages 
over the conductors of bundle 12, always at least a single transistor from 
group 42 and also at least a single transistor from group 44 will be 
conductive, causing the bases of transistors 26 and 36 or 28 and 38, 
respectively, to be pulled to the negative or positive supply voltage, 
respectively, causing these transistors to be cut off. Consequently, the 
change-over switch will be non-conductive with any other high-low voltage 
combination (bit combination).