Patent GB-1035322-A (GB)
Title: Comparator circuit
Inventor: YOUNG FRINK MANSFIELD
IPC: H03K5/24
Filed: 1963-01-10
Abstract: 1,035,322. Transistor pulse circuits. ADAGE Inc. Jan. 10, 1963, No. 1254/63. Heading H3T. [Also in Division G4] A comparator suitable for use in an analogue to digital converter comprises two " Selector circuits" 112, 114 (Fig. 5, not shown) to respective ones of which is applied a reference voltage and the input signal and to both of which a ramp signal is applied from generator 110. When the ramp voltage reaches the voltage at the nearer of the two inputs the corresponding selector operates to produce an output at 98 or 100 and which operates a corresponding " Level sensing switch " 122, 124 to inhibit operation of the other selector. Accordingly, an output pulse occurs at 98 or 100 depending on whether the input signal is larger or smaller than the reference signal. In the embodiment a feedback path is provided through a diode 126 to balance out any unwanted feedback to the input signal source which might cause false operation. The Specification describes a basic complementary type multivibrator which is the subject of Specification 1,035,323 and how this may be modified to form a self-oscillating version of the present invention. In the basic multivibrator (Fig. 1) a capacitor 34 charges through resistor 40, resistor 36 (if provided) and a conducting clamping diode 46. The rising voltage on the upper plate of the capacitor causes transistor T 1  to become more conducting and thus, through a connection to the collector electrode, the base of transistor T 2  to become more negative. At the end of the timing period transistor T 2  becomes conducting, effectively clamping the potential at the collector of T 1 . The current through T 2  rises but the collector remains at a constant potential until its current exceeds the current through clamping diode 46 whereupon the potential rapidly rises. This causes the potential at the upper electrode of the capacitor to rise correspodingly until it is clamped by diode 44. This rise is communicated through the emitter-base path of T 1  to point 18. The potential at the lower plate of 34 continues to rise, though more slowly until the capacitor is discharged when the potential rises above that at the emitter of T 1  allowing diode 37 to conduct so that the emitter potential is carried positive until transistor T 1  is cut off. This is aided by regeneration from the collector of T 1  to the base of T 2 , the latter transistor being driven to cut-off during the process, allowing a new cycle to commence. Thus transistor T 1  remains conducting for most of the time, T 2  for only a short time and a very short dwell occurs between the time at which the collector of T 1  reaches its minimum value the collector voltage pulse starts on T 2 . A further transistor T 3  is provided to inhibit oscillation when desired. Fig. 3 (not shown) illustrates the modification of Fig. 1 to form the comparator of Fig. 5. T 3  is omitted and T 2  is duplicated (T 5 , T 6 ) the bases being paralleled through diodes 230 and 232 and collectors through diodes 240 and 242, the additional drive necessary being obtained by replacing T 1  by cascaded stages T 4 , T 7 . The emitter of T 5  is connected to the reference voltage source 246 and the emitter of T 6  to the input signal source 94 via a level changer 250. Accordingly, when the rising voltage present at the upper plate of the capacitor (now numbered 218) is communicated through T 4 , T, to the bases of transistors T 5  and T 6  one or other eventually becomes conducting (to serve the function of T 2 ) depending on whether the reference voltage is larger than the input voltage and an output pulse which occurs at the corresponding collector is applied to output terminals 98, 100 prior to the next cycle of oscillation. The output pulse is also fed through a corresponding one of common base transistors 260, 262 to the base of the other one of transistors T 5 , T 6  to inhibit its operation. In addition, during the short period (previously referred to) prior to the production of a pulse at the collector of T 5  or T 6 , the potential at the base of the transistor is fed through a diode 126 to neutralize any stray coupling back to the input circuit. The circuit of Figs. 5 and 6 may be used in an analogue to digital converter (Fig. 4, not shown). The analogue signal is compared with a feedback signal in 64 and the output is amplified and used to control a reversing switch indicating which signal is the larger. This is applied as the input signal to a comparator 96 of the type shown in Fig. 6, the pulses at one output stepping up the feedback signal until it exceeds the analogue signal whereupon a pulse appears at the other output to stop operation.