Patent GB-1405827-A (GB)
Title: Transmission-line amplifier
IPC: H03K19/018
Filed: 1973-08-13
Abstract: 1405827 Pulse shaping circuits MULLARD Ltd 13 Aug 1973 38205/73 Heading H3T In a transmission-line amplifier for producing trapezoidal complementary two-state output waveforms at 01, 02 including a single capacitor CEXT and means T1-T4 responsive to a first input condition to discharge the capacitor from a first direction of charge and charge it in an opposite direction to produce a first output condition and responsive to a second input condition to discharge it again and recharge it in the first direction to produce a second output condition, the central portion of the discharge- and-charge characteristic of the capacitor is selected to produce the positive-going and negative-going transitions of the trapezoidal waveform by current balancing means R10, R12, T12 and T20 which ensures that the output transitions are at their mid-point when the capacitor is half-way through its dischargecharge cycle so as to produce at output terminals 01, 02 two output waveforms of which the sum is substantially constant. With one or more of the inputs to T1 in a low (0 volt) state T1 is on, T2 is off as is T4 and T3 is on. The outputs from T3 and T4 are coupled by emitter followers T8 and T9 to turn T13, T14, T15 on and T17 off and T23, T26, T27 off and T28 on so as to provide complementary outputs at 01 and  02. If now all the inputs to T1 become positive T3 switches off and T4 switches on and positive and negative going ramps (Fig. 2a, not shown) occur on the bases of T8 and T9. The current in resistor R10 decreases and the current in T12 increases due to current increases in T11 of the current mirror circuit T10, T11, T12. After a certain time (#1) the current flowing into A through R10 will balance the currents in R12, the collector current in T12 and the collector current of T20 flowing in lead Z. At this time (#1) the base drive to T13, T14 and T14 will be reduced and the collector of T15 tends to rise towards the positive supply which in turn tends to reduce the current in R12 since T13-T17 form a negative feedback amplifier. The output voltage at the collector of T15 then follows the same shape as the ramp voltage on the base of transistor T8. Transistor T17 therefore turns on and its emitter voltage rises to VCC-IVBE and stabilises at this voltage at a time (#2). The point B performs the same function as the point A but in a reverse manner so that T27 is turned on and T28 is turned off to provide a complementary output. A current flow is then obtained from +VCC via T17 and T27 to earth. In an ideal circuit when the two plates of an external capacitor C EXT  are at the same potential the outputs 01 and 02 should be at a voltage equal to half way through the voltage swing. This is achieved by D4, R16, R17 and T18-T21. At the centre point the net current at A available to drive the base of T13 should be substantially zero, that is should be just enough to keep T13 on without sufficient drive to cause it to conduct further. When the voltage at both plates of the external capacitor C EXT  are equal, the currents in R10 and T12 are equal and opposite. The current into A through R12 is made to equal the current out of A into the collector of T20 by making R16 + R17 = 2R12. Thus for all temperature and supply voltages the condition will be satisfied since the change in R16 and R17 and the Vbes of the transistors will be matched in an integrated circuit. The line driver produces complementary output waveforms which are substantially trapezoidal in shape in order to minimize the interference between the transmission lines from one line driver and the transmission lines from an associated line driver. Therefore complementary waveforms (b and c, Fig. 2, not shown) which may represent the currents in each wire of a twisted wire pair will substantially cancel each other out during the transition instants (#1- #2 and #3-#4).