Patent GB-1467091-A (GB)
Title: Film circuit for introducing a large time constant
IPC: H03H7/06
Filed: 1975-02-21
Abstract: 1467091 Printed circuit assemblies PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 21 Feb 1975 [26 Feb 1974] 7353/75 Heading H1R [Also in Division H3] In a film circuit comprising an integrated circuit (Fig. 1) first and second NPN transistors T 1 , T 2  are connected in a common emitter circuit over resistor 8 to negative supply 13 while input voltage Vi at 1 is connected to the base of T 1  and output voltage Vu at 2 to the base of T 2 . The collector of T 1  is connected to positive supply 12 while the collector of T 2  is connected to the supply over heating resistance layer 11 thermally contacting NTC resistor 10 between the negative supply 13 and the base of T 2 ; also connected to positive supply 12 over resistor 9. The circuit is film fabricated on substrate 17 (Figs. la, 1b) on which a resistance layer 11 is deposited and isolated by thin insulant layer 20 overlain by NTC 10; the transistors T 1 , T 2  and resistors 8, 9 being formed in block 16 connected to 10, 11 and 12, 13 over conductive tracks 18, 19, 21, 22. In operation, a negative voltage transient at time t 1  decreases the emitter current of T 1  to produce a positive transient in the collector current of T 2  (Fig. 2) and the current in resistance 11 to increase its temperature and that of the NTC 10, with a delay due to heat capacity. The resistance of NTC 10 decreases, as does the base emitter voltage and collector current of T 2  and the heat dissipation in resistance 11, until the latter just compensates for cooling losses at Vu=Vi, after a time interval. The elapsed time γ between time t 1  and the instant at which the output voltage has performed fraction of its total variation represents the RC time constant of the circuit, and determines the input frequency 1/γ up to which the output amplitude follows the input amplitude. It is adjustable by the heat resistance between 10 and 11 acting by analogy with a long transmission line. At instant t 2  a positive transient at the input induces a reverse sequence, heat loss of the NTC resistance 10 arising through conduction to the environment so that the time constant γ 2  of the positive transient response differs from that of the negative response. The resistance may be thermally insulated to make γ 1 =γ 2 . The arrangement avoids the use of large time constant capacitors. In a modification (Fig. 3, not shown) resistor 9 is replaced by a NTC resistance heated by a resistance between the collector of T 1  and positive supply. In a further modification (Fig. 4, not shown) the input voltage Vi supplied over a long line is remotely adjustable and the output voltage Vu is fed to a control circuit; the circuit of Fig. 1 being inserted between input and output to reduce the adverse effect of capacitative and inductive interference and to produce a D.C. control voltage Vu#the D.C. level of V L . In a further modification (Fig. 5) an amplifier comprises three successive transistors T 3 , T 4 , T 5  the collector circuits of which include feed resistors 16, 17, 18 and Zener or series diode D.C. elements D 1 , D 2 ; D 3 , D 4  between the collector of T 3  and the base of T 4  and between the collector of T 4  and the base of T 5 . The collector of T 5  is connected to output 25 and provides feedback to the input 24 over the transistor circuit of Fig. 1; modified by the replacement of resistor 9 by a NTC resistance heated by a resistance between the collector of T 1  and positive supply (Fig. 3, not shown). The feedback voltage in series with the input signal appears at the base of T 3 . In operation each amplifier phase shifts by # so that increase of base voltage for each transistor produces decrease of collector voltage and the input voltage receives full negative feedback at low frequency for stabilization, with high frequency feedback presented by the circuit of T 1 , T 2  without heat resistance material between the heating resistance and temperature sensitive resistance layers. In a further modification (Fig. 6) transistors T 1 , T 2  are connected with a current source 2I in the common emitter circuit, input voltage Vi in the base circuit of T 1 , a potential divider comprising NTC resistances 10, 14 in the base circuit of T 2 , and heater resistances 11, 15 in the collector circuits of T 1  and T 2  respectively; output voltage Vu being derived from the collector of T 1 . In operation, the D.C. collector currents of T 1 , T 2 =I and the A.C. collector currents are phase opposed proportionally to the A.C. component of the input. Then the voltage at terminal 25 comprises a D.C. component independent of input signal, and an A.C. component proportioned to and phase opposed to the input signal A.C. component; so that the circuit operates as a peak detector. The NTC resistance layers may be replaced by PTC resistance layers with interchange of such layers or their heating resistance layers in the circuits.