Patent GB-978171-A (GB)
IPC: D06P3/24
Filed: 1962-01-17
Abstract: 978,171. Seismic signal recording; transistor amplifying circuits. JERSEY PRODUCTION RESEARCH CO. Jan. 17, 1962 [March 31, 1961], No. 1704/62. Headings H3T and H4D. Sesmic signals from a plurality of geophones and supplied via individual amplifiers, the gains of which may be varied by fixed steps, are periodically and sequentially sampled, converted to digital form and the digitalized signals recorded together with signals indicative of the actual step-gain of the amplifier supplying the particular sample. Fig. 1 shows the general arrangement in which signals from individual geophones (or individual groups of geophones) 12A . . . 12n are supplied via respective amplifiers 14A . . . 14n to a known form of sampling and digital conversion means 16 the outputs from which are supplied via amplifier 22A . . . 22N to a tape recorder 24. To compensate for the falling off in amplitude with time of the received signals, the gain of each amplifier 14A . . . 14n is increased in fixed steps under the control of a circuit 20 controlled by the sampling commutator 18 of conversion means 16 and the particular value of each gain step in the form of signals of a three-unit binary code (Fig. 7, not shown), which allows a maximum of seven step gains, is supplied via amplifiers 30A, 30B, 30C to tape recorder 24 and recorded concurrently with the recording of the related digitalized sample from the appropriate geophone. In order to monitor the operation of the apparatus the recorded tape is played-back and, with switches 15, 26, 28 and 32 in positions opposite to those shown, conversion means 16 operates as a digital-to-analog converter and supplies signals to a display 42, comprising an oscillographic camera, via amplifiers 36A . . . 36n the gains of which are varied in similar manner to that of amplifiers 14A . . . 14n under the control of a circuit 40 to which step gain signals from the tape are supplied. Fig. 2 shows details of each of the amplifiers 14A . . . 14n of Fig. 1. As shown, each amplifier comprises a cascade arrangement of identical amplifier units (see Fig. 3 for circuit details) 41A . . . 41n having a filter 43 passing frequencies in the band 2 to 200 c.p.s. between the first pair of units and a plurality of switches 44A . . . 44m each of which, when closed, connects the output of a respective amplifier unit to an output amplifier stage 48 which supplies its signals to commutator 18 (Fig. 1). The switches are closed and opened sequentially by means of pulses from a shift register 60 which is set in operation by the output of an " AND " gate 56 on coincidence of a pulse from commutator 18 (Fig. 1) and a signal of a predetermined (low) level derived from output stage 48 in rectifier 50, low-pass filter 52 and an amplitude detector 54, which may comprise Schmitt trigger. Initially, the shift register 60 is set to a binary zero, one or two given code step and an artificial code number 7 is induced in the coding matrix 62 by means of a flip-flop 47. When a signal is sensed (i.e. "first-kick") at the output of unit 41n by amplitude detector 45 the flip-flop changes state thereby removing the artificial code and allowing matrix 42 to revert to the actual step-gain condition of the shift register 60. This condition is then signalled by the matrix as a three-unit binary code and recorded on the tape as described in connection with Fig. 1. If it is desired to commence amplification of the signals at a stage other than the first and to stop the stepgain at a particular stage the shift register may be modified as described in connection with Fig. 2A and 2B (not shown). Fig. 3 shows circuit details of one of the amplifier units such as 41A (Fig. 2). This consists of a non- inverting amplifier composed of two complementary transistor stages the first stage including a NPN transistor 64 and a PNP transistor 66 and the second stage a PNP transistor 68 and a NPN transistor 70 connected in a basic bridge configuration and supplied with equal operating potentials +V 1  and -V 2  (with respect to earth). The two halves of the amplifier are symmetrical and opposite pairs of resistors have equal valves. Resistors 71, 73 and 72, 74 provide a feed-back path which stabilizes the bias of transistors 64 and 66 respectively. Degenerative feed-back in transistors 64, 66, is introduced by resistor 80 and in transistors 68 and 70 by resistors 77, 78 and a further feed-back path, which sets the overall gain, is provided by resistors 79 and 80 which feed a fraction of the output voltage (at point 79A) back to the common emitter line 65 of the first stage. Instead of varying the overall gain of the amplifier 14 Qf Fig. 1 by switching in or out different amplifying units, such as shown in Fig. 3, and as described in connection with Fig. 2, a single amplifying unit following the filter 43 of Fig. 2 may be employed and the overall gain controlled by switching various resistors in parallel in a feedback path across the entire amplifying unit by means of switches similar to the switches 44 of Fig. 2 (Figs. 4A and 4B, not shown). Details of the unit 40 of Fig. 1 which controls the gain of the amplifiers 36A . . . 36n feeding the display unit 42 are described with reference to a block-circuit diagram (Fig. 6, not shown) and details of the amplifiers (36A . . . 36n) are given with reference to a block-circuit diagram (Fig. 8, not shown).