GB2163320A - Telephone circuit - Google Patents

Abstract

A telephone tone ringer chip, usable, inter alia, in the "chip set" of our Application No. 8317706 including tone generation circuitry (33-34), which can produce several different tone combinations under control of selection inputs (17, 18). The tone generation circuitry is powered by DC derived from a ringing signal on the telephone line via a diode bridge (32) and a DC-DC converter and anti-tinkle circuit (31). The chip includes an amplifier (30) which doubles as a speech amplifier when off-hook and when a loudspeaking facility is being used. When on-hook the amplifier (30) acts as a tone amplifier. To do this, its input stage includes two differential amplifiers, one for tone and one for speech, only one of which is enabled at any one time. The outputs from these two amplifiers go via a common amplifier to an output amplifier. The diode bridge is a complex arrangement in which each arm of the bridge includes a coupled pnp transistor and npn transistor which together act as a diode. <IMAGE>

Description

SPECIFICATION Telephone circuit This invention relates to an electrical integrated circuit unit for use in a telephone subscriber's apparatus for the provision of a tone ringing function when the apparatus is in the on-hook position and also for providing a speech amplification function when the apparatus is in the offhook condition.

According to the invention there is provided an integrated circuit for use in telephone subscriber's apparatus, which includes a rectifier bridge to which an alternating voltage ringing signal may be applied when the circuit unit is in use, tone generation means in the circuit unit which responds to an output from the rectifier bridge to generate ringing tone, a plural stage amplifier whose first stage has a tone amplification portion and a speech amplification portion, first connections from the tone generation means to the tone amplification portion of the first stage, second connections from a speech input to the circuit unit to the speech amplification portion of the first stage, and control connections whereby when the circuit is in use and is responding to a ringing signal said first connections are effective to enable the tone amplification portion of the first stage of the amplifier, said control connections also being effective to enable the speech amplification portion of the amplifier when the circuit is in use and the subscriber's apparatus is in the "off-hook" condition.

Such a circuit unit can be one of a set of chips which together provide the functions and facilities needed for a high quality telephone subscriber's apparatus which provides, inter alia, loudspeaking telephone and handsfree facilities. One example of such an arrangement is described and claimed in out Application No. 8317706 (P.F.Blomley 35 al 11-7-3-1), which claims, inter alia; "An integrated circuit chip set for a telephone instrument, the chip set including a primary chip having first and second channels for received and transmitted signals, each said channel including a preamplifier and an amplifier, a first ancillary chips having a tone generator coupled to a loudspeaker amplifier and provided with means whereby the output of the receiver channel amplifier may be coupled selectively to the input of the loudspeaker amplifier, and a second ancillary chip having first and second attenuation channels respectively for the received and transmitted signals and control means whereby the attenuator loss of each said channel is controllable, wherein the primary chip has means whereby the attenuator channels of the second ancillary chip may be coupled between the amplifiers and the preamplifiers so as to provide a variable attenuation path for the received and transmitted signals and wherein said control means is adapted to attenuate either the receiver or the transmitter channel so as to prevent acoustic coupling therebetween." It will be seen that the circuit unit described herein is suitable for use as the first ancillary chip of the chip set of the above application.

An embodiment of the invention will now be described with reference to the accompanying drawings, in which Figure 1 is a block diagram of an integrated circuit unit embodying the invention.

Figure 2 shows in detail the diode bridge used in the circuit unit of Fig. 1.

Figure 3 shows in some detail the main features of the LS Amplifier block of Fig. 1.

General Introduction The integrated circuit unit whose block diagram is shown in Fig. 1 is a monolithic bipolar integrated circuit, encapsulated in an eighteen-pin copper lead plastics package. The circuit unit has two modes of operation, an on hook tone ringer mode and an off-hook loudspeaker mode.

The loudspeaking amplifier 30 is used in both modes as will be described below. The chip has an "on-chip" sinusoidal DC-DC converter, in the block 31, which allows direct drive of a low impedance loudspeaker, and avoids the need for a transformer. There is also an internal rectifier bridge 32, which will also be described in some detail below, and which reduces the number of external components needed.

The tone generation is by the oscillators 33 and 34, which oscillators are fed via a current ratio selection block 35, and there are four logic selectable tone spectrum options; (a) Normal tones, in which the generator is switched between 740Hz and 570Hz at 5.5Hz.

(b) Normal tones with reduced volume.

(c) Quickshift, in which the generator is switched between the same frequencies as above, but at 12Hz.

(d) Low tones, in which the generator is switched between 435Hz and 335Hz at 5.5 Hz.

The HF Oscillator 33 generates the four tones, while the low frequency oscillator 34 generates the 5.5Hz or 12Hz whichever is needed.

This integrated circuit is a complex circuit, which includes several hundred transistors and a comparable number of resistors. In the detailed circuit portion of this text, only as much of these components as is felt necessary are described.

Pin Description Pin 1 An oscillator reference resistor is connected between pins 1 and 14 to set up an internal reference current which determines the triangular charge/discharge rate of capacitors connected between pins 2 and 14 and pins 3 and 14 and which determines the frequencies generated by the oscillators. This reference resistor also sets up internal "pull down" currents for the tone input select pins 1 7 and 18.

Pins 2 and 3 are oscillator input pins, and capacitors connected between them and pin 14 determine the oscillator frequencies.

Pins 4 and 5 are negative and positive amplifier inputs respectively. Pin 4, the loudspeaker speech input, is only used when in the "loudspeaker" (off-hook) mode.

Pin 5, the positive amplifer input, is internally held at half the supply rail voltage V(9-14) by two 51 Kohm resistors and is decoupled via a small external capacitor to pin 9. It is not used for signal input.

Pin 6 is a bias input which is only used in the loudspeaker amplifier (off-hook mode), and it is energised by means not in the circuit unit of Fig. 1 when the associated instrument is off-hook.

The input voltage between it and pin 9 is 2.6 volts + 0.1 volt, and it accurately sets the current bias levels in the loudspeaker amplifier 30. This voltage also acts as an absolute amplifier on/off switch, over-riding the amplifier earphone select input pin 7.

Pin 7 is the loudspeaker/earphone select input; in the logic "1" state it selects amplifier active or "loudspeaker" mode and is defined as V(7, 14) is > or = 1.5V. If it is at logic O it selects amplifier non-active (low current) "earphone" mode and is defined as an unconnected or floating pin, or an input voltage in the range 0 to 300 mV.

Pin 8 is a so-called bootstrap input pin and is always coupled to pin 10 via a large electrolytic capacitor, which ensures that the amplifier output stage can saturate on positive or negative cycles, thus providing maximum output power to the loudspeaker.

The loudspeaker is connected directly between pins 8 and 9.

Pin 9 is a low voltage supply input, typically 2.5 to 5 volts and has two modes of operation.

In the "on-hook" mode, the internal converter 31 generates a low voltage supply from the high voltage output of the bridge 32 to drive the tone ringer, and when "off-hook" the externally derived supply between pins 9 and 14 drives the amplifier 30.

Pin 10 is the loudspeaker amplifier signal, tone or speech dependent on the operational mode, output.

Pins 11, 13 are AC diode bridge inputs on which the AC ringing signal is received. The bridge 32 rectifies the AC line voltage to produce the high voltage supply, at pins 12-14 which is used by the converter 31. The AC line signal is coupled into the bridge via a capacitor and a resistor in series (not shown).

Pins 12, 14 provide the high voltage rectified output from the bridge. Pin 1 2 is the positive rectified output and pin 14 is the chip 0 volts. A decoupling capacitor, 10 F, and a protective zener diode, 30 volts, are always connected between these pins when the chip is in use.

Pins 15, 16 An L-C combination is connected between these pins and pins 12 to form a series resonant circuit which is made to oscillate by a drive signal from pin 1 6 when a ringing signal is applied to the bridge. The capacitor couples the high voltage oscillator into pin 15, where internal diodes couple the rising voltage cycles to charge up the low voltage supply and the falling voltage cycles are returned to 0 volts, pin 14. This LC combination with the converter circuitry in the block 31 forms an oscillatory DC-DC converter.

Pins 1 7 and 1 8 are the tone select inputs, which generate the tone spectra referred to above, as follows: Pin 17 Pin 18 Tone Spectra O O Quickshift O 1 Normal (Reduced. Values) 1 0 Normal 1 1 Low tones On-Hook Condition of the Circuit The incoming ringing signal is coupled in via a 1yF capacitor and 1 Kohm series resistor to the bridge 32. Rectification and smoothing via an external capacitor takes place and when the resulting direct voltage rises to approximately 9 volts the anti-tinkle part of block 31 starts to turn on. The anti-tinkle circuit and smoothing capacitor sets up an input threshold at approximately 24 volts, which must be overcome before the converter 31 starts to operate and normal tone ringer operation commences. This ensures that low voltage AC on the line will not produce an annoying output from the loudspeaker which could simulate ringing. An external zener diode limits the high voltage supply from the bridge 32 to 30 volts to prevent damage to the internal circuitry.

When the anti-tinkle threshold is overcome, the converter 31 starts to oscillate at a frequency, e.g. 25 KHz, defined by an external L-C circuit. The converter switches off, due to an internal hysteresis effect, at approximately 1 8 volts. The impedance of the tone ringer is adjustable by suitable choice of L-C, and is nominally set to 10 Kohm for maximum power transfer from the line to permit parallel operation with other tone ringers or bells.

On each rising edge of the resonat L-C oscillator, the converter capacitor couples a charging pulse via an internal diode to the low voltage supply (pin 9) where it is smoothed by a largevalue capacitor. On falling edges, charging pulses are returned to the zero volt line. Thus the current maximum at resonance is utilized to generate high current, low voltage supply from a high voltage low current supply.

The tone spectras are generated by the oscillator 33, which is a two-tone high frequency oscillator which alternates with a shift frequency generated by the low frequency oscillator 34.

As already indicated, these oscillators' frequencies are set by external components. Four tone output options are selectable by the logic states of the tone select inputs, pins 17, 18, this selection being effected via block 35, which sets current ratios to levels suitable for the oscillation.

The switch HF signal thus emitted by the HF Oscillator 33 is applied via a limiting amplifier 37, which determines the time, in any one period of oscillation, that the power amplifier 32 is actually driving the loudspeaker. This impedance transformation, combined with the converter action, enables the low current, high voltage requirement at the bridge 32 to match the high current, low voltage requirement at the loudspeaker.

Off-Hook Condition of the Circuit In this condition, only the loudspeaker amplifier 30 on the chip is active, and it then works in an AC coupled "bootstrapped" output configuration, where the output swings around the positive low voltage supply. Reference currents are set up in the amplifier by the bias input 6, held at 2.6V + 0.1 volts above the low voltage supply. The amplifier 30 into a 50 ohm loudspeaker and is internally switched off for voltages below + 2 volts, so that on long lines the power available to drive the earphone amplifier (which is on a network chip, which is another one of the chips of the three chip set mentioned above) is not compromised. The amplifier is held in loudspeaker mode by the application of a suitable voltage, in this case greater than 1.5 volts, between pins 7 and 14.The amplifier is disabled, to go into the earphone mode, by disconnecting pin 7.

Diode Bridge Arrangement As can be seen from Fig. 2, which shows the diode bridge 32, each diode is made up from the combination of a pnp transistor such as T1 and an npn transistor such as T2. Thus the two AC inputs are: (a) the junction of the collector of a pnp transistor (T1) and the emitter and base of an npn transistor (T3).

(b) the junction of the collector of a pnp transistor (T4) and the emitter and base of an npn transistor (T5).

The two DC outputs from the bridge are: (i) The junction of the collector of a pnp transistor (T6) and the collector and base of an npn transistor (T7).

(ii) The junction of the emitter and base of an npn transistor (T2) and the emitter and base of an npn transistor (T8).

These diodes thus each consist of a combined non/pnp device which are so layed out as to ensure the minimum amount of substrate and sideways injection of current carriers.

The diodes consist of inverted npn transistors configured with bases connected to emitters, making use of the collector-base junction and its high voltage breakdown capability. Thus the flow of "conventional" current is given by: (a) During positive half-cycles of the input Ac, current flows from terminal 1 3 via T6-T3 to terminal 12, though the load (in this case mainly the converter) to terminal 1 4 and then through T4-T8 to terminal 11.

(b) During negative half cycles of the input AC, current flows from terminal 11 via T7-T5 to terminal 12, through the load to terminal 14 and through T1 -T2 to terminal 1 3.

In this configuration the base is at a higher potential than the epi n-type land during diode conduction, and injection of holes into the epi land cannot be avoided. However, by connecting a p-type base surround to the epi land a pnp diode is formed across the collector-base junction of the npn device with the base of the npn device acting as the emitter of the pnp device. Both devices thus formed are active in handling diode current.

For the negative diodes, T1-T2 and T4-T8, some of the available npn emitter area is used to control the substrate. Thus if T1-T2 is conductive, pin 13 is at - Vbe relative to the 0 volt rail pin 14, since the npn device operates in the inverted mode, the emitter controlling the substrate acts as a collector and saturates so that the substrate is held at Vsat above the negative voltage then present on pin 1 3.

In most integrated circuits the substrate is connected to the most negative supply on the chip.

Since p-type substrates are used, this prevents the substrate from forward biassing and injecting holes into the circuit devices, which would cause failure of the circuit. However, in the present arrangement, the internal bridge has inputs which can go negative with respect to the bridge 0 volt output. To overcome this problem, the substate is "floated", and is so controlled that when the bridge input goes negative to 0 volts, the substrate is made to follow.

In the tone ringer mode the main substrate control is from the bridge diodes, where the npn transistors operating in inverted mode are used. During the dead zone of the bridge diode, when no diode conducts, the substrate loses its active control. Hence under these conditions the control from the bridge diodes is supplemented by a transistor T631, which switches on only when the converter fed by the bridge is running, i.e. when all low voltage circuitry is running, which causes maximum injection into the substrate. Then the turn on of T631 maintains the control of the substrate condition which is needed. Diode T630, which is connected across T63 1 and also controlled from the converter provides protection against current spikes, possibly causing circuit latch-up.

To further improve substrate control, lateral pnp transistors are used in various parts of the circuit where normally substrate pnp transistors would be used, so that the injection of holes into the substrate, which would drive the substrate positive relative to 0 volts, is avoided.

In the loudspeaking amplifier mode, under peak voltage swing, feedback current mirror pnp transistors (see below) can inject holes into the substrate which would overcome the ability of the bridge diodes to maintain the substrate near 0 volts. Hence another transistor is added with its collector-emitter path between substrate and the zero voltage rail to keep the substrate at (O volt + Vsat) during amplifier operation.

Loudspeaker Amplifier This consists of a preamplifier portion, which occupies the left-hand part of Fig. 3 and the output stage, which occupies the right-hand part of Fig. 3. Basically the preamplifier includes two differential amplifier pairs T118-T119 and T11 7-T120, of which T118-T119 is the amplfier for tone and T11 7-T120 for speech.

During the tone ringer mode, the tone is applied via a resistor to the base of T1 1 9, which is part of the first-mentioned pair, whose emitters are suitable biased by the connection marked TR bias when the chip is in the tone ringer mode. The collector output of T1 1 8 goes to transistor T1 22 of a current mirror T121-T122, from which it is amplifier by a Darlington pair T125-T126. The output from T1 25-T126 goes to the base of T134 via R 110 in the output stage, the output being taken from the junction between T1 34 and T137. Note the feedback connection from this point via R115 to the base of T119.

In the speech amplification condition, i.e. the loudspeaking mode, the speech is applied to the base of T120, the T1 1 7-T1 20 then being effective, the output of which goes to T122, as before.

In the high gain Darlington circuit T125-T126, a capacitor C1 provides compensation to ensure amplifier stability.

The input to the output stage also goes to the base of a transistor T124, which is part of a positive feedback loop formed by T1 23 and T1 24. Another positive feedback loop includes Tri 33 and T136, of which T133 is the current mirror. These loops effectively drive the output transistors Ti 34-T1 37, in conjunction with the input to the base of T1 34 via the resistor R1 10.

The upper one of these feedback loops, i.e. Ti 23-T1 24, is controlled by transistor T126, which is the last transistor of the input stage. The lower feedback loop, i.e. T1 33-T136,is controlled by the "upper stage", transistor T128 via the current mirror T1 27-T132. The resistor R107 connected between the base of T123 and the amplifier output connection, provides start-up current for the upper feedback loop.

The lower feedback loop gets its start-up current via another transistor, which is part of a biassing network (not shown), and sets the quiescent current of the amplifier. This biassing network is controlled from the DC rail of the chip, and the details thereof are omitted from this description to avoid unnecessarily complicating the circuit.

We will now discuss the operation of these feedback loops. As the input signal varies, so the amount of current drawn from the upper feedback loop (the one including T1 23-T124) by T1 26 also varies, controlling the amount of current which flows in the output transistor T134.

The emitter area of T124 is much smaller than that of T134, and since the resistor Tri 08 in the emitter of T124 is equal to Tri 10 in the base of T134 the current "mirrored" back round into T1 34 by T1 23 via T124 is greater than or equal to the base current of T1 34. This holds as long as the beta of the npn transistors exceeds 50.The excess current for beta greater than 50 is sunk by T126. Thus the combination of T124-R110--R108 senses the base current of T134, and feeds this current back via the mirror transistor T123 into the base of T134. The resistor R 107 powers up this upper loop, and provides more current as the DC bias output increases. Hence Tri 34 will saturate well on positive cycles, even for low beta of the pnp transistor.

The lower feedback loop, which includes Tri 36, T133, which is another current mirror transistor, with a resistor R116 in the base of T137 operates on the same principle as does the upper feedback loop. However, this time, the base current reflected into T137, the other output transistor, is controlled at T133 by T128 via the mirror T127-T132. The resistor Tri 21, connected to the emitter of T128 is smaller than R110. Hence the upper stage does not have to switch on too hard, which would cause a loss of power efficiency during negative going output stage cycles in order to control the lower stages. The start-up bias current is applied to the lower stage by another transistor in the biassing network referred to above via T133 due to the connection to its base.This determines the quiescent current of the amplifier since to keep the output level at half the supply rail level the upper stage has to turn on enough so that T132 subtracts excess current from the lower feedback loop.

Control as to whether the amplifier arrangement is effective for tone ringer mode or loudspeaker amplifier mode (speech) uses some additional circuit elements. Thus the loudspeaker tone ringer select mode pin when in use has a voltage which depends on which mode is to be used. If it is above a first threshold and below a second threshold, it sets a latch (not shown) as a result of which the circuit is in the tone ringer mode. When the voltage falls, this latch is controlled in a hysteresis manner so that the control voltage can fall a bit further below the tone ringer select level before the latch switches off. If the arrangement is in the loudspeaker amplifier mode, the other side of the latch is effective to bias different parts of the amplifier, as can be seen from T117-T120.

DC-DC Converter (Fig. 4) The converter circuitry includes a voltage sensing circuit 60, a current latch 61, and a driver 62 controlled by the latch. The driver, although shown as a switch is in fact an electronic device. The circuit uses a choke L and a capacitor C, which are external components.

The sensing circuit 60 is connected across the DC input and also to a point between the choke and the capacitor. When the voltage it is sensing reaches a preset level, the latch 61 is operated to cause the driver 62 to switch on, see Fig. 5. This operation of the driver applies a pulse to the LC circuit to start it oscillating or to maintain it in oscillation. Output is coupled into the low voltage output via a "positive" diode D1 and a reservoir capacitor C2. During negative going parts of the waveform, the "negative" diode D2 returns current to the smoothing (reservoir) capacitor C2 via the 0 volts rail.

The use of a dc-dc converter in the tone ringer i.c. enables the transformer, that more conventional tone ringers require, needed to drive a low impedance loudspeaker to be replaced with very cheap converter L/C components.

Claims (14)

1. An integrated circuit for use in telephone subscriber's apparatus, which includes a rectifier bridge to which an alternating voltage ringing signal may be applied when the circuit unit is in use, tone generation means in the circuit unit which responds to an output from the rectifier bridge to generate ringing tone, a plural stage amplifier whose first stage has a tone amplification portion and a speech amplification portion, first connections from the tone generation means to the tone amplification portion of the first stage, second connections from a speech input to the circuit unit to the speech amplification portion of the first stage, and control connections whereby when the circuit is in use and is responding to a ringing signal said first connections are effective to enable the tone amplification portion of the first stage of the amplifier, said control connections also being effective to enable the speech amplification portion of the amplifier when the circuit is in use and the subscriber's apparatus is in the "off-hook" condition.

2. An integrated circuit as claimed in claim 1, in which the DC output from the rectifier bridge is applied to a DC-DC converter whose output provides DC supply for part at least of the integrated circuit, and in which threshold means ensures that the converter does not respond to DC from the bridge which is below a preset threshold, which threshold corresponds to the lowest acceptable ringing signal amplitude.

3. An integrated circuit as claimed in claim 2, and in which an externally-connected zener diode is used to ensure that the voltage generated by the rectifier bridge cannot rise above an upper, safety, threshold.

4.An integrated circuit as claimed in claim 2 or 3, and in which the DC-DC converter is of the oscillatory type, the frequency of the oscillations being fixed above the audio range ("'25KHz) by an externally-connected LC combination.

5. An integrated circuit as claimed in claim 2, 3 or 4, in which the DC-DC converter is switched off when the DC applied to it from the rectifier bridge falls below a further threshold less than the first-mentioned threshold.

6. An integrated circuit as claimed in claim 1, 2, 3, 4 or 5, in which the tone generation means includes a high frequency oscillator which can oscillate at either one or two tone frequencies, a low frequency oscilllator which can oscillate at a frequency which is low compared with either one of said two tone frequencies, and connections from the low frequency oscillator to the high frequency oscillator via which the last-named is caused to switch between its two tone frequencies at a rate defined by the frequency at which the low frequency oscillates.

7. An integrated circuit as claimed in claim 6, in which the low frequency oscillator can run at either one of two frequencies each of which is low compared with either of the tone frequencies, and in which control as to the frequency at which the low frequency oscillator runs is by input signal to the integrated circuit.

8. An integrated circuit as claimed in claim 6 or 7, and in which the output from the oscillator is applied via a limiting amplifier to the tone amplification portion of the first stage of said amplifier, so as to determine the time in any one period of oscillation for which the amplifier drives the loudspeaker.

9. An integrated circuit as claimed in claim 1, 2, 3, 4, 5, 6, 7 or 8, in which the amplifier, when in the speech mode, operates as an AC coupled amplifier, the control therefor being such that when the circuit is in use in a telephone instrument which is off-hook only the amplifier in its speech amplification mode is effective.

10. An integrated circuit as claimed in any one of claims 1 to 9, in which each arm of the rectifier bridge includes a diode arrangement formed by an npn transistor and a pnp transistor, the base of the npn transistor being connected to the emitter of the pnp transistor and to its own emitter and the collector of the npn transistor being connected to the base of the pnp transistor, and in which the collector of the pnp transistor is connected to its own base.

11. An integrated circuit as claimed in claim 1, in which two of the npn transistors, whose emitters are connected together and to one of the DC outputs from the bridge, each has a second emitter, and in which said second emitters are connected together and to the substrate of the integrated circuit to exercise a control on the DC level of that substrate.

1 2. An integrated circuit as claimed in any preceding claim, and in which each of the two portions of the first stage of said amplifier is a differential amplifier pair of transistors, in which the bases of one transistor of each said pair is connected to a common bias terminal, in which the base of the other transistor of the tone portion pair is connected to an input from the tone generation means, in which the base of the other transistor of the speech portion paid is connected to a speech input to the circuit, in which the emitters of the two portions are separately biassed, and in which the output of whichever of said portions is in use is taken from the collector of its said one transistor.

1 3. An integrated circuit as claimed in claim 12, in which the collector outputs from the first stage are connected together and to one transistor of a current mirror the output of which is connected via a further transistor to the base of one of a pair of series connected transistors in the second stage of the amplifier.

14. A dual purpose transistor amplifier, which includes a first sgage which has a first amplification portion and a second amplification portion, and a second stage which is fed from whichever of the two first stage portions is in use, in which each of said two amplification portions is a differential amplifier pair of transistors, in which the base of one transistor of each said pair is connected to a common bias terminal, in which the base of the other transistor of said first portion is connected to a first AC input, in which the base of the other transistor of said second portion is connected to a second AC portion, in which the emitters of the transistors of the two portions are separately biassed with one pair only enabled at a time, dependent on which AC input is to amplified, and in which the output of whichever of said portions is in use is taken from the collector of its said one transistor.

1 5. A rectifier bridge implemented in an integrated circuit which includes four bridge arms each of which includes a diode arrangement formed by an npn transistor and a pnp transistor, in which the base of the npn transistor is connected to the emitter of the pnp transistor and to its own emitter, in which the collector of the npn transistor is connected to the base of the pnp transistor, in which the collector of the pnp transistor is connected to its own base, in which one AC input to the bridge is connected to the junction of the collector of a pnp transistor and of the emitter and base of an npn transistor, in which the other Ac input to the bridge is connected to the junction of the collector of another pnp transistor and of the emitter and base of another npn transistor, in which one of the DC outputs from the bridge is connected to the junction of the collector of a pnp transistor and the collector and base of an npn transistor, and in which the other DC output from the bridge is connected to the junction of the emitter and base of an npn transistor and the emitter and base of another npn transistor.

1 6. A rectifier bridge implemented in an integrated circuit which includes four bridge arms each of which includes a diode arrangement formed by a pnp transistor and an npn transistor, in which the base of the pnp transistor is connected to the collector of the npn transistor and to its own collector, in which the emitter of the pnp transistor is connected to the base of the npn transistor, in which the emitter of the npn transistor is connected to its own base, in which one AC input to the bridge is connected to the junction of the collector of a pnp transistor and of the emitter and base of an npn transistor, in which the other AC input to the bridge is connected to the junction of the collector of another pnp transistor and of the emitter and base of another npn transistor, in which one of the DC outputs from the bridge is connected to the junction of the collector of a npn transistor and the collector and base of an pnp transistor, and in which the other DC output from the bridge is connected to the junction of the emitter and base of an npn transistor and the emitter and base of another npn transistor.

1 7. An integrated circuit for use in a telephone subscribers instrument, substantially as described with reference to the accompanying drawings.