GB2284956A - An active capacitor network - Google Patents

Abstract

An active capacitor network includes a PNP and an NPN transistor the respective base and emitter electrodes of which are connected to each other, a first diode the an ode electrode of which is connected to the collector electrode of the PNP transistor, a second diode the cathode electrode of which is connected to the collector electrode of the NPN transistor, the cathode electrode of the first diode being connected to the anode electrode of the second diode, and a capacitor connected betwen the base electrodes of the transistors and the point of connection of the diodes to each other. The active capacitor network is a capacitance multiplier. <IMAGE>

Description

An active capacitor network The present invention relates to an active capacitor network.

The invention provides an active capacitor network including a PNP transistor and an NPN transistor, the base and emitter electrodes of one transistor being respectively connected to the base and emitter electrodes of the other transistor, a first diode the anode electrode of which is connected to the collector electrode of the PNP transistor, a second diode the cathode electrode of which is connected to the collector electrode of the NPN transistor, the cathode electrode of the first diode being connected to the anode electrode of the second diode, and a capacitor connected between the base electrodes of the two transistors and the common connection of the diodes to each other, the terminals of the network being, respectively, the common connection of the emitter electrodes of the transistors and the common connection of the diodes with the capacitor.

If the two transistors have identical current gains, the active capacitor network appears to a circuit connected to the network as a capacitor of capacitance of (l+ss)C, where B is the common-emitter current gain of each of the transistors, and C is the actual value of the capacitor of the network. Since B is likely to be of the order of 100, a capacitor of value lOOnF would appear to have a value of lOpF to a circuit connected to the network. The apparent capacitance multiplication is the result of the impedance transformation of the base circuit impedance of each transistor "seen" at its emitter electrode.

The active capacitor network is bidirectional because each transistor conducts current in only one sense and the transistors are complementary types connected in shunt with each other with opposite relative polarities.

No external d.c. supply is required for the two transistors; signal voltages applied to the network cause the operation of the transistors provided that they exceed the forward breakdown voltage of the base-emitter junctions of the transistors.

If the transistors do not have substantially the same current gain ss, the capacitance presented by the network is dependent on the polarity of the signal applied to the network. The transistors might be intentionally so fabricated as to have very different current gains in order to provide a capacitance that varied with the signal polarity.

The active capacitor network may be used as a replacement for electrolytic capacitors in order to avoid the leakage and unreliability to which electrolytic capacitors are prone. Also, the active capacitor network may, because it can be fabricated as a monolithic integrated circuit, be arranged to be lighter and less expensive than most electrolytic capacitors which it could replace, so that it has an especial advantage in applications where small size and lightness are important.

The present invention also provides an active capacitor and switch network including a PNP transistor and an NPN transistor, the base and emitter electrodes of one transistor being respectively connected to the base and emitter electrodes of the other transistor, a first diode the anode electrode of which is connected to the collector electrode of the PNP transistor, a second diode the cathode electrode of which is connected to the collector electrode of the NPN transistor, the cathode electrode of the first diode being connected to the anode electrode of the second diode, a capacitor connected between the base electrodes of the two transistors and the point of connection of the diodes to each other, a resistive circuit so connected to the two transistors as to provide resistances in series with the emitter electrodes and in shunt with the collector-base circuit of the two transistors, and a semiconductor switching device connected in shunt with the resistive circuit, the terminals of the network being the terminals of the semiconductor switching device.

The active capacitor and switch network is especially useful as a maintenance termination unit (MTU) in telephone systems. An MTU is a circuit that is installed permanently in a telephone line and can be switched on and off from a remote location by means of the semiconductor switching device, in order to connect or open the telephone line, say, for detecting faults in the line.

Two forms of network in accordance with the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 shows an active capacitor network and; Fig. 2 shows an active capacitor and switch network.

Referring to Fig. 1 of the accompanying drawings, the active capacitor network includes a PNP transistor 1, an NPN transistor 2, a capacitor 3, and two diodes 4 and 5, respectively. The active capacitor network has connection terminals 6 and 7.

The base and the emitter electrodes of the transistor 1 are respectively connected to the base and emitter electrodes of the transistor 2, the anode electrode of the diode 4 is connected to the collector electrode of the PNP transistor 1 and its cathode electrode is connected to one terminal of the capacitor 3. The other terminal of the capacitor 3 is connected to the commoned base electrodes of the transistors 1 and 2.

The cathode electrode of the diode 5 is connected to the collector electrode of the NPN transistor 2 and the anode electrode of the diode 5 is connected to the cathode electrode of the diode 4 and to the capacitor 3.

The connection terminal 6 is connected to the emitter electrodes of the transistors 1 and 2 and the connection terminal 7 is connected to the point of connection of the diodes 4 and 5 to each other.

In the operation of the active capacitor network, the PNP transistor 1 conducts current only in the direction shown by the arrow A and the NPN transistor 2 conducts current only in the direction shown by the arrow B, but current flow through the active capacitor network is bidirectional to an external circuit connected between the terminals 6 and 7. Current flow through the capacitor 3 is also bidirectional, being in the direction of the arrow A when the PNP transistor 1 is conductive and in the direction of the arrow B when the NPN transistor 2 is conductive.

The diodes 4 and 5 serve to block reverse current flow through the transistors 1 and 2, respectively, and the absence of any base biassing resistor results in the network blocking a direct current in either direction.

The network is, of course, responsive to a.c. signals provided that those signals exceed the base-emitter forward conduction voltage of the transistors, that is, about 700 millivolts.

Assuming that the network is excited by an a.c.

signal of complex frequency s and that the capacitor 3 has a capacitance C, then the capacitor 3 will provide a conductance of sC in the base circuit of the transistors.

If the voltage developed across the network is v, a current of vsC flows in the capacitor 3 and a current of (l+ss)vsC flows in the emitter of the conductive transistor, where B is the common emitter forward current gain of the transistors. The network appears as a conductance (1+B)sC to the source of the signal, which means that the source of the signal "sees" a capacitance of (l+B)C. If the two transistors 1 and 2 do not have have identical gains the actual capacitances "seen" would be (l+ssl)C in one direction and (1+B2)C in the opposite direction, where B1 and B2 are the respective commonemitter forward current gains of the transistors.

Although the difference in value between Bl and B2 may be expected to be small for normal production runs, the transistors may be designed deliberately to provide widely different values of B1 and B2 in order to achieve a network with a current-direction-dependent or signalpolarity-dependent capacitance.

Referring to Fig. 2 of the accompanying drawings, the active capacitor and switch network includes the network shown in Fig. 1, a first resistor 20 connected between a first anode electrode of a bidirectional thyristor 22, known in the art as a triac, and the terminal 6 of the active capacitor network, a second resistor 21 connected between the terminals 6 and 7 of the active capacitor network, and a bidirectional voltage-sensitive trigger device 23, known in the art as a diac, connected between the first anode of the bidirectional thyristor 22 and its gate electrode, a second anode electrode of the bidirectional thyristor 22 being connected to the terminal 7 of the active capacitor network. The anodes of the bidirectional thyristor 22 are connected to respective input and output terminals 26 and 27 of the active capacitor network and switch.

The bidirectional thyristor 22 and the bidirectional voltage sensitive trigger device 23 constitute a voltagesensitive switch which is controlled by the direct voltage applied between the terminals 26 and 27. When the voltage-sensitive switch is conductive, it acts as a short-circuit to the active capacitor network and the resistors 20 and 21, but when the voltage-sensitive switch is not conductive, the active capacitor network and the resistors 20 and 21 serve as a capacitanceresistance network for the external circuit connected to the terminals 26 and 27.

One advantageous use of the active capacitor network and switch shown in Fig. 2 is as a maintenance termination unit (MTU) in a telephone system. In that application, the active capacitor network and switch is connected permanently in a telephone line and the direct voltage applied to the line switches on the bidirectional thyristor while the telephone line is in service. The telephone line may be taken out of service by removing the direct voltage and the a.c. characteristics of the telephone line, say, may be determined by driving an a.c.

signal through the active capacitor network and the resistors. Faults on the telephone line are detectable in that way.

The transistors 1 and 2, and the diodes 4 and 5, may be of conventional planar construction in a monolithic semiconductor integrated circuit form of the active capacitor network shown in Fig. 1. That form of construction would provide diodes 4 and 5 which have reverse breakdown voltages of the order of 60 volts and circuits suitable for operation with maximum total reverse voltages of the order of 50 volts, say. Telephone systems use voltages of the order of 50 volts.

The active devices belonging to the network shown i Fig. 2 are likely to be of bulk semiconductor material construction in a monolithic semiconductor integrated circuit form of that network, the principal component of which is the bidirectional semiconductor switching device 22 which is a PNPN device. In that construction, the reverse breakdown voltage of the diodes 4 and 5 may be expected to be higher than the 60 volts expected for planar diodes.

Claims (7)

CLAIMS:

1. An active capacitor network including a PNP transistor and an NPN transistor, the base and emitter electrodes of one transistor being respectively connected to the base and emitter electrodes of the other transistor, a first diode the anode electrode of which is connected to the collector electrode of the PNP transistor, a second diode the cathode electrode of which is connected to the collector electrode of the NPN transistor, the cathode electrode of the first diode being connected to the anode electrode of the second diode, and a capacitor connected between the base electrodes of the two transistors and the common connection of the diodes to each other, the terminals of the network being, respectively, the common connection of the emitter electrodes of the transistors and the common connection of the diodes with the capacitor.

2. An active capacitor and switch network including a PNP transistor and an NPN transistor, the base and emitter electrodes of one transistor being respectively connected to the base and emitter electrodes of the other transistor, a first diode the anode electrode of which is connected to the collector electrode of the PNP transistor, a second diode the cathode electrode of which is connected to the collector electrode of the NPN transistor, the cathode electrode of the first diode being connected to the anode electrode of the second diode, a capacitor connected between the base electrodes of the two transistors and the common connection of the diodes to each other, a resistive circuit so connected to the two transistors as to provide resistances in series with the emitter electrodes and in shunt with the basecollector circuits of the two transistors, and a semiconductor switching device connected in shunt with the resistive circuit, the terminals of the network being the terminals of the semiconductor switching device.

3. A network as claimed in claim 1 or claim 2, wherein the PNP transistor and the NPN transistor have substantially equal common-emitter current gains.

4. A network as claimed in claim 1 or claim 2, wherein the common-emitter current gain of the PNP transistor is substantially different from that of the NPN transistor.

5. A network as claimed in any one of claims 1 to 4, fabricated as a monolithic integrated circuit.

6. An active capacitor network substantially as described herein with reference to, and as shown in, Fig 1 of the accompanying drawings.

7. An active capacitor and switch network substantially as described herein with reference to, and as shown in, Fig. 2 of the accompanying drawings.