Patent GB-954488-A (GB)
Title: Improvements in or relating to negative impedance converters
IPC: H04B3/18
Filed: 1960-05-03
Abstract: 954,488. Transistor amplifying circuits; telephone transmission systems. WESTERN ELECTRIC CO. Inc. May 3, 1960 [Aug. 5, 1959], No. 15495/60. Headings H3T and H4R. In a negative-impedance converter of the type comprising an amplifying device having a terminating impedance connected thereto as a load and provided with positive feed-back around the amplifying device whereby there appears at suitably disposed input terminals a negative impedance related in value to that of the terminating impedance, the terminating impedance comprises a pure resistance and a phase-shifting network is arranged in the feedback path whereby the negative impedance at the input terminals has a magnitude which may be varied by varying the terminating resistance but a phase angle which depends only upon the phase angle of the phase-shifting network. The impedance converter may consequently be matched to a transmission line to which it is connected to reduce losses in the line by a suitable choice of the phase-shifting network while allowing adjustment to be made to the terminating resistance to control the reduction in losses. In the circuit of Fig. 1, the voltage V developed across the input terminals is applied with a reversal of sign through transformer 12 to the base of transistor to cause a current i 2  having a value dependent upon the value of the terminating resistor 11 to flow through the resistance in a direction opposed to the voltage V. The feed-back path includes the phaseshifting network 13, 14 whereby the current at the input terminals differs from that through the transistor and the negative input impedance, whose magnitude may be varied by adjustment of resistor 11, comprises a reactive component dependent upon the phase-shifting network. Fig. 2 shows a push-pull modification of the circuit of Fig. 1 in which the phase reversal corresponding to that due to the transformer 12 is obtained by a cross-coupling, involving the phase-shifting network, between the collectors and bases of the two transistors. The circuits of Figs. 1 and 2 constitute shunt negative impedances and in Fig. 3 such an impedance comprising transistors 60, 61, 62 and 63 is shown connected between the midpoints of transformer primary windings 32, 33 connected in series in transmission line 30. The circuit corresponds to that of Fig. 2, the pairs of transistors 60, 62 and 61, 63 behaving as the single transistors 20, 21 in the manner described in Specification 737,506. Feedback between the common collector circuits of the transistor pairs 60, 62 and 61, 63 and the base electrodes of transistors 61 and 60, respectively, is effected through transformers 74, 75, the desired phase-shift being effected by means of capacitors 77, 84 and resistors 80, 81. Fig. 3 also shows a series negative impedance converter comprising transistors 35, 36, 37, 38 coupled to the line 30 through a secondary winding 34 associated with primary windings 32, 33. The circuit also corresponds to that of Fig. 2 with the single transistors replaced by transistor pairs; however in this case the terminating resistance 45 is connected across the input terminals of Fig. 2 and the line 30 replaces the terminating resistance of Fig. 2. Specifications 743,450 and 849,883 also are referred to.