Voltage controlled current sources for active hybrid circuit

DETAILED DESCRIPTION In FIG.
1, a high impedance active hybrid circuit 1 is illustratively shown as part of a central office trunk circuit 2.
A bidirectional 2-wire path 3 connects hybrid 1 via other conventional trunk circuit apparatus (not shown) to the central office switching network.
A 4-wire long haul operator trunk 4 comprising a transmit path 5 and receive path 6 interconnects hybrid 1 and an operator console 7.
A balancing network 8 illustratively is terminated in hybrid 1 for circuit impedance balancing.
No negative impedance converter or its balancing network is required in the circuitry of FIG.
1.
A typical telephone system application of the circuitry of FIG.
1 is to bridge the 4-wire operator trunk 4 to one or more centrex customer trunks, such as path 3, with a minimal amount of impedance loading.
FIG.
2 shows the bidirectional line 3 having tip and ring conductors T and R coupled to conductors 9 and 10 by means of a transformer 11.
A pair of zener diodes 12 and 13 are connected cathode-to-cathode and in an overvoltage protection array referenced to a voltage 14 which lies between the differential voltage operational amplifier supply voltages and which is illustratively +12 volts above ground and is connected to conductor 10.
Transmit and receive paths 5 and 6 of the 4-wire trunk are shown at the upper and lower right-hand portion of FIG.
2.
Paths 5 and 6 each comprise respective tip and ring conductors TT, TR and RT, RR.
Signals from the bidirectional path 3 are transmitted to the transmit path 5 from conductor 9 via a bridge resistor 15, a variable gain transmit amplifier circuit 16 and a transformer 17.
Circuit 16 comprises a differential voltage operational amplifier 18 having noninverting and inverting inputs + and - and a single output 19.
The inverting input - is connected to a voltage divider network comprising a potentiometer 20 and resistors 21, 22 and 23.
Voltage 14 is connected to resistors 21 and 22 and together with potentiometer 20 set the bias operating voltage for amplifier 18