CA1064632A - Switching circuit - Google Patents

Abstract

ABSTRACT

A switching circuit comprises an operational amplifier with two resistive negative feedback paths including respective unilateral-ly conducting devices (e.g. diodes) connected in opposite polarity to the output of the amplifier. A first terminal is connected to the electrode of the unilaterally conducting device in one path remote from the output of the amplifier, and a second terminal is connected to the electrode of the unilaterally conducting device in the other path remote from the output of the amplifier. Signals of one polarity only applied to the first terminal appear at the second terminal and signals of the opposite polarity only applied to the second terminal appear at the first terminal. The circuit may include three or more such feedback paths. The values of the resistors in the feedback paths may be adjusted to charge the transfer gain of the circuit. The circuit is of particular value in a telephone conference amplifier using unilateralised signals for connecting each channel to a point common to the channels.

Description

~ 6~63Z ~, This in~ntion relates to a switching circuit which is of particular value in telephone conference systems.
; In telephone conference systems it has been proposed to convert a speech signal to be distributed to telephone lines 5 joined together for ~e purpose of the conference to a uni-lateralised signal. A unilateralised or unidirectional polarity signal is a signal which normally alternates about the zero ""
voltage level, but which has been offset relative to the zero voltage level so that the signal is always of the same polarity, lO,with the limitation that a signal of zero voltage produces a ,~-, unilateralised signal of zero ~voltage. A method of uni-; lateralising a signal is to derive from it an envelope signal ~,; and then add this envelope signal to the original with such '~
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amplitude that the resulting signal does not cross the zero 15 voltage level. A unilateralised signal is used in a conference telephone circuit in the following way.
;~ Each channel taking p~rt in the conference is connected to a common point by a diode circuit such that signals originating ~'~
" in that channel a~ter unilateralisation can be applied through ~`

`, ~ 20 the diode to the common point. The channels are also connected ~,~

-~ to respond to the presence of signals at the common point of such polarity as would be produced by a unilateralised signal ~' ,,i originating in one of the channels so that that signal'can be received by all of the other channels. ~-,~ 25 In Canadian patent specification no. 965,890 ~hich issued April 8, 1975 to,Jones et al ~a telephone conference system '~-,~ of the above,type is described. However,-the '~

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circuits described in the above specification are relatively complex.
It is an object of the present invention to provide a - switching circuit which is suita~le for use in a telephone confe~ence amplifier of simpler construction than that described in the above patent specification.
Accordlng to the present invention there is provided a switching circuit including an inverting d.c. amplifier with first and seco~d resistive negative feedback paths having first and second resistors connected in series respectively with first and second unilaterally conducting devices connected in opposite polarity to the output of the amplifier, a first signal source and a second signal utilisation circuit connected to the electrode of the first unilaterally conducting device remote from the output of ~e amplifier and a second signal source and a ` first signal utilisation circuit connected to the electrode of the second unilaterally conducting device remote from the output ., .
of the amplifier, whereby only an input signal of polarity such :, , as to cause the first unilaterally conducting device not to conduct can be applied from the first signal source to the first .,j ~ .
signal utilisation circuit and only an input signal of poiarity such as to ca~se the second unilaterally conducting device not to conduct can be applied from the second signal source to `
the second signal utilisation circuit.
According~to a second aspect of the invention there is provided a telephone conference amplifier havingaplurality of channels, each channel including means for receiving an input ; signal and producing therefrom a unidirectional polarity signal, switch means including an inverting d.c. amplifier with first ;.
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~06463;2 and second resistive negative feedback paths having first and second resistors connected in series respectively with first and second unilaterally conducting devices connected in opposite polarity to the output of the amplifier, means for applying the unidirectional polarity signal from the producing means to the electrode of the first unilaterally conducting device remote from the output of the ~mplifier, the polarity of the first unilaterally conducting device being such that the unidirectional polarity signal does not cause it to conduct, means connecting the electrode of the second unilaterally condueting device remote from the output of the amplifier to a point common to all of the channels and means connected to a point on the first resistive negative - feedback path to derive therefrom, in response to a unidirectional polarity signal from the common point, a unldirectional polarity signal of the same polarity as that applied tothe electrode of the first unilaterally conducting device and to produce therefrom a symmetrically polarised output signal.
In one embodiment of the invention the first unilaterally conducting device includes an emitter-base diode of a transistor.
~ 20 A switching circuit according to the invention may include .
a third resistive negative feedback path having an third uni-.: .
laterally conducting device connected to the output of the amplifier which unilaterally conducting device is connected in the same polarlty as the first unilaterally conducting device.
If such a switch~ng circuit is~connected as p~rt of a telephone conference amplifier the symmetrically polarised output signal may be derived from a unidireetional polarity signal established at the electrode of the third unilaterally conducting device . . , , ...... , - , .

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remote from the output o~ the ampli-fier. The ~witching circuit ,~
; may include more than three resistive negative ~eedback paths each having a unilaterally conducting devlce.
A telephone con*erence ampli~ier according to the invention may include a connectlon for d.c. si~nals ~rom the means connected ` to the output o~ the inverting d.c~ ampli~ier to derive there~rom a u~idirectional po~arity signal in response to a unidirectional ; ~ , ~ignal applied to the common point by another channel ~o the means ~or receiving an input signal. This is for the purpose o~ reducing i~
the sensitivity o~ the means for receiving the input signal when a unidirectional polarity ~ignal i~ being applied ~o the common point by another cha~nel.
A telephone con~erence amplifi~r according to the invention may also include a positive feedback connection for d.c. signals '~;
l$ ~rom the output of the inverting d.c. ampli~ier to the means for - reoeiving an input signal. ~his is ~or the purpose o~ mainta~l1ng~
the unidirectional nature and the switching o~ signals at low levels at wh~ch unulateralisation by the means ~or recei~ing and - ~ switchin~ by the switch means can be imper~ect.
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`~ 20 In order that the inYention may be ~ully understood and readily carried into e~ect lt will now be described with reference to the accompanying drawings, o~ which: '~
; Figure 1 ~s a diagram of one example o~ the switching circuit according to the invention;
~igure lA 3hows a modification o~ Figure l;
- Figuxe 2 i~ a diagram o~ one e~ample o~ a channel of a -:. ~; :

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belephone conference ampli~ier according to the invention and using a switching circuit according to the invention; and ure 3 is a diagram o~ one channel of a telephone conference ampli~ier according to a second example of the invention and including the second example o~ a switching circuit '~ according to the inve~tion.
~' Re~erring now to Figure l, the switching circuit shown includes an inverting d~c. ampli~ier l to the input o~ which are connected resistors 2 and 3, which may be o~ ~qual value but are not necessarily so. ~he output o~ the ampli~ier l is connected to the resistor 2 through a diode 4 and to the resistor 3 through a diode'5. ~he anode o~ the d~od~-4 and the cathode o~ the diode ,' ~ are connected to the output o~ the ampli~ier l. A ~irst , term~nal 6 is connected to the resistor 2 and the cathode of the ~, 15 diode 4. A second terminal 7 is connected to the resistor 3 ''' _ and the anode of the diode 5.
~, In the operation o~ the oircuit ~hown in Figure 1 a ~` posit~ve-going input signal app].ied to the terminal 6 is rout'ed ~ ' - through the resistor 2 to the ~nput oi the amplifier l because ,',', 20 the diode 4 doe~ not co~duct. ~he positive-going input oi the ;
,' ampli~ier 1 produces a negative-going signal at i-ts output - .
'' ~ which is able to pa~s through the diode 5 and re~istor 3 o that' ~; a conventional see-saw ampli~ier circuit is pr~ uced with the ~ result that a negative-going output signal appears at the ,' 25 terminal 7 being related to the magnitude o~ the input signal - ' applied to the terminal 6 in Ohe same proportion as the value ~ ' ' , . . . . , . , ~ ~ ~; .
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of the re~istor 3 to the value o~ the resistor 2. I~ a negati~e-going input signal were applied to the terminal ~, any part of the signal which reaches the input o~ the ampli~ier 1 would appear at the output o~ the ampli~ier as a positive-going signal so that the terminal 6 is held at zero voltage by the conduction - o~ the diode 4 and con3equently no output signal appearæ at the terminal 7. ~y symmetr~ a negative-going signal applied as an i~put to the terminal 7 will appear as a positive-going ou~put ~?
: signal at the terminal 6 related to the magnitude of the input ~ignal by the ratio of the value of the resistor 2 to the valu.e -~ - :
o~ the r0sistor 3. ~urther, a positive-going input applied to ;~
the terminal 7 produces no output at the terminal 6 becau~e `.of the conduction of the diode 5. -;
~his circuit is o~ partioular value in telephone conference ampli~iers in which it is required to apply to a point :; ¢ommon t~hl o~ the channels o~ the ampli~ier a unid~re¢tional polarity signal ~rom the channel which is active at the time, -~
; that is to 5a~ the channel which has a speech input. The uni-- directional polarity signal applied ~o the common point is requ~red to be derivea from the common point by all o~ the cha~nels other than the active channel. A switching circuit .. . .- , acoording to the i~vention would be used in such an amplifierj ~:
- one to each channel with the common poinb connected to, for example~ the second terminal 7 and the unidirectional polarity ; 25 signal derived ~rom the input signal to the channel being ~ applied to the ~irst terminal 6, ~; ~
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The switching circuit may include a third resisti~e negative feedback path having a further re~istor connected to tha input o~ the ampli~ier 1 and a further diode connected to the output o~ the amplifier 1, the further diode must of course - 5 be connec-ted in the same polarity as one or other o~ the diodes 4 and 59 and in the example to be described below it is in the ¦-same polarity as the diode 4 ~or application to a telephone conference ampli~ier. I -In Figure 1, it will be apparent that the relation .~ 10 between ~he magnitudes of the resistances 2 and 3 determi~es the ; . . relation between the amplitudes of the input and output signals.
~lgure 1~ shows a modification o~ the circuit of ~igure 1 in :: .
which the feedback resistors 2-and 3 are replaced by a potentio .meter 8 having a wiper 8A which is connected to the inverting input o~ the ampli~ier 1. Adjustment o~ the po~ition o~ the :, 1:
wiper 8A changes the relationship between the amplitude~ o~ the input and output signals of the c:ircuit. ~he non-inverting put of the ampli~ier 1 may be c:onnected to earth directly or through a resistor 9 a~ shown in Figure 1~
. ~ 20 Figure 2 shows one channel o~ a telephone con~erence ~ ~ :
`. ampli~ier i~n whlch the two-wire telephone line is connected ~o :
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the pr~mary ~inding 10 o~ a tra~s~ormer 11. A secondary winding ~:
., 12 o~ the transformer ll~is connected to an ideal diode circuit `~ io~med by a d.c. ~mpli~ier Al ha~ing two resistive negat1ve ~eed- :
back paths, the ~irst having a resistor R4 in series with a :
diode Dl and the second having a resistor R5 ln series with diodes D2 and D~. ~he cathode o~ the diode Dl and the anode . . . .

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; 1~64632 o~ the diode D2 are connected to the output o~ th~ ampli~ier ~1.
: At a point 1~ connected to the cathode o~ the diode D3 a signal representing the envelope of any input speech wa~reform applied to the winding 10 is set up, being smoothed by the capacitor Cl - 5 and resistors R6 and R7 in parallel. ~he point A is co~nected through a second secondary win.ding 13 of the transformer 11 and . a resistor R8 to a point ~ at which is established the sum o~
: an incoming speech wave*orm and its envelope, which together iorm a unidirectional polarity signal. In the pressrt ' :
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10~;4~i32 example this signal i~ po~itive-goi ng. The point B is connected via resistor R12 to the invert~ng input o~ an ampliYier A2 which has a ~irst re~istive ~egative ~eedback path formed by a resistor Rll in series with a diode D4 and a second resisti~e negative feedback path ~ormed by a '~
re~istor R12 in serie~ with a diode D5 and the base emitter ' diode o~ a tran~istor Q. AB Can be æeen, the point B is ,' sonnected to the ba~e o~ the transistor Q. A negati~e-going unidirectional polarity ~ignal is ~et up by the amplifier,2 ~ 10 at the anode o~ the diode D4, which electrode is connec~ed :~ to the commo~ point, in this in~tance shown as a conductor 14..-" The common point is common ~o all channel~ o~,the conference '~
.~ ' , .ampliiier. The collector.of the transistor Q is connected to:earth through resistors R14,R15 and R16 in ~erie~, the junction ~" 15 o~ resi~tors R14 and R15 being connected~to the lower end o~ . , ' the winding 12, and the ~unction o~ the resistors R15 a~d R16 .' being con~ected to the output o~ the ampliiier A2 through'a :~
,.~ negative temperature coe~icient resistor R13~
'- j ~he poin~ i8 connected through a capacitor C3 and re~istor R9 to the non-inver~ing i~put of the ampli~ier A2, ~hlch is co~nected to ground ~hrough a resi~tor RlO, the~: .
. ~unction o~ the ~ignal ~ed through the capacitor C3 being to `~-' , correct some 8ignal aistortion~ which occur beoau~e capacitor ~ ' , ~ a2 i~ connected in shu~t with the diode D3 and not to earth a~
.. 25 .shown $n the above patent ~peci~ication. The operation o~ thi '~
'' part o~ the circult i8 di~cu~sed more ~ully in the 10 ~
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6~63Z
specification of co-pending Canadian patent application serial no. 265,632 filed Novem~er 15. 1976 in the name of Jones and will not be considered further in this specification.
In the operation of the circuit of Figure 2, the positive going unidirectional polarity signal appearing at point B is applied through the resistor R12 to the non-inverting input of the amplifier A2 because the base emitter diode of the transistor Q and the diode D5 are turned off by the positive-going signal.
- A negative-going outp~t signal appears at the output of the amplifier A2 and is therefore applied to the common point 14 through the diode D4. Because the transistor Q is turned off by the positive-going signal at the point B no positive-voltage is -applied to the lower end of the winding 12 because no collector current is flowing through the transistor Q. Thus in an active ; 15 channel the sensitivity of the amplifier Al is maintained because a positive-going signal is not applied to its non~
` inverting input tending to prevent the production of an envelope `-~
signal at the point A. Moreover, when at the end of an , utterance the voltage at point B falls towards zero, the voltage .
at the output of amplifier 2 falls towards lV (the sum of the kn~e voltages of diode D5 and the emitter-base of transistor Q) so that the positive feedback path from the output of amplifier A2 to the inverting input of amplifier Al via resistors R13 ;~
- and R15 becomes effective. This maintains the output - 25 at point A at a few tens of millivolts. The effect is known as retention because it causes retention of control of ~`
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~6~63~' the common point by an active channel during natural breaks in the speech in that channel. One advantage of retention is that distortion of very low level signals during the unilateralisation and switching processes is avoided. Temperature compensation is discussed in the specification of the aforesaid co-pending ~ Canadian Patent Application Serial No. 265,632.

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In a non-active channel the negative-going signal on the common point 14 is applied to the input of the amplifier A2 - through the resistor Rll because the diode D4 does not conduct, 10 so that a positive output signal appears at the output of the ~ -amplifier A2 and the diode D5 and transistor Q become conducting.
A positive-going unidirectional polarity signal appears at the point B and induces a corresponding symmetrically polarised - output signal in the winding 10 because the speech components are ~:
able to pass through the winding 13 and capacitors C2 and Cl to ; earth. Because the transistor Q is conducting a positive voltage ~ is applied to the lower end of the winding 12 which biases into .~
non-conduction the idealdiode formed by the amplifier A1 and its , associated componen~s. This prevents the formation of a d.c.
` 20 voltage at point A which would oppose the conduction of diode DS
and the switching on of t~ansistor Q causing malfunctlon of the circuit. Furthermore spurious low level signals appearing in the winding 10 of a non-active channel do not appear as corresponding signals at the common point 14 because the non-, . . .
conduction of diodes D2 and D3 prevents the formation of an ! envelope component. They may however appear in a mutilated ~-, form which it is thought contributes to the '~break-inl!`facility.

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, 64~2 l~e negative temperature coefficient of resistor R13 is provided to compensate for the variation with tem~erature of the forward voltage drop across the diode D4 under retention conditions. It has been found that the compensation with 5 only the resistor R13 is not quite adequate and to increase it a resistor R of high value may be connected in parallel with the diode D5. If the transistor Q is a silicon one a resistor may also be connected from its collector to its emitter.
In order to match the circuit to the telephone line connected to the winding 10, it is desirable that the impedance of the circuit looking into the winding 10 should ~, be as near to 600 ohms as possible. With regard to the circuit itself the input impedance is naturally high in ~
15 the receive state and low in the send state. ~`
If these impedances are arranged to be 1200 ohms and 300 ohms respectively the return loss to a 600 ohm line is ~" the same in both cases and the overall insertion loss is a i~ minimum when the two return losses are the same. ~he resistor Rl which is effectively in parallel with the winding 12 reduces the impedance in the receive state and the resistor R8 connected in series with the winding 13 increases the impedance in the send state. It is not possible ~. .
to achieve the desirable result of the circuit presenting ~ `~
;~ 25 600 ohms impedance for both receive and send without ~;~
incurring excessive insertion loss due to the losses ; resulting from the impedance adjustment. The values -~
shown are by way of a compromise which produces a 2 dB

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: , :: . .. : : . , 1~64632 insertion loss and 9~ dB return loss.
Figure 3 showq an alternative form of telephone conference amplifier with connections for a four-wire telephone circuit. The incoming signals are received via a transformer lOl and the signals are returned to the telephone line via a transformer 102. An ideal diode circuit 103 is provided to produce at the point 104 the envelope of the signal incoming via transformer 101. This envelope signal is combined with the incoming signal in a summing ampliier 105 so that at the point 106 a uni-directional polarity signal is produced. The point 106 is connected through a resistor 107 to one terminàl of a switching circuit according to an example of the invention having three resistive negativ~ feedback loops. As the unidirectional polarity signal is positive-going at the point 106 a negative-going signal is applied to the common point represented by the line 108 through a diode 109.
Negative-going signals from the common point 108 are applied to the input of amplifier 110 through a resistor 111 so that a positive-going signal appears at the points 112 :, and 113 in the third and first negative feedba~k loops respectively. The signal at the point 113 cannot proceed ;-further back to the circuit because it is stopped by the ;~;
; summing amplifier 105. The signal at the point 112, however,~
'~ 25 is applied via a capacitor 114 to the input of an amplifier 115 having a negative feedback path. At the output of the ~, ~mplifier 115 appears a symmetrically polarised signal derived from the umidirectional polarity signal at the ',`; ;:
1 ~ 14 -.,,,,,~ - ' ..

common point 108, the symmetrically polarised signal being applied via the transformer 102 back to the telephone circuit.
The point 112 i3 also connected to the non-inverting 5 input of the amplifier in the ideal diode circuit 103 which `
serves to establish a threshold for the input signal which is effective for all channels except the active one. A
capacitor 116 and resistor 117 in series are connected from the point 104 to the non-inverting input of the amplifier 10 105 for the purpose of reducing distortion in the envelope signal due to the fact that it is smoothed by a capacitor 118 only (discharge of capacitor llB is through the two resistors connected to it). A negative temperature coefficient ,~ resistor 119 is connected in series with a fixed resistor 15 120 in a path between the output of the amplifier 110 and earth; the junction of the resistors 119 and 120 is connected . ;, , , ~ to the point 112 through a resistor 121. The purpose of ~;1 :~.
the negative temperature coefficient of resistor 119 is to -~

compensate for temperature variation in the forward voltage 20 of diode 109 so as to render the retention effect less dependent on temperature.

From a consideration of Figure 3 relative to Figure 2 ` it will be apparent that the transistor of Figure 2 has been ' ` omitted in Figure 3, thie being made possible by the use of 25 the switching circuit having three negative feedback paths.

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Although the invention has been described with reference to a specific example and its application to telephone conference amplifiers, it will be appreciated . .
.~ that alternative constructions for the invention are ~ 5 possible and that they may be employed in other applications.
-~ One possible alternative application of the invention is in analogue computing.
Advantages of the switch described above are that it is simpler than circuits previously proposed for the same function, it can give-a lower aggregate insertion loss for signal transmission in both directions through a telephone ;~:
. conference amplifier using it when compared with prior :~ proposals, and it can be fabricated using lower tolerance . .
resistors than earlier circuits providing the same .~.
15 performance. :
In Figure 3 the switch has a 6 dB loss which is `; `:
~ compensated by the amplifier 115. .The circuit can enable :
,.~ a telephone conference amplifier to be constructed without ~ germanium transistors.

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Claims (11)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A switching circuit including an inverting d.c.
amplifier with first and second resistive negative feed-back paths having first and second resistors connected in series respectively with first and second unilaterally conducting devices connected in opposite polarity to the output of the amplifier, a first signal source and a second signal utilisation circuit connected to the electrode of the first unilaterally conducting device remote from the output of the amplifier and a second signal source and a first signal utilisation circuit connected to the electrode of the second unilaterally conducting device remote from the output of the amplifier, whereby only an input signal of polarity such as to cause the first unilaterally conducting device not to conduct can be applied from the first signal source to the first signal utilisation circuit and only an input signal of polarity such as to cause the second unilaterally conducting device not to conduct can be applied from the second signal source to the second signal utilisation circuit.

2. A circuit according to claim 1 wherein one of the unilaterally conducting devices includes the emitter base diode of a transistor.

3. A circuit according to claim 1 including more than three resistive negative feedback paths each including a resistor connected in series with a respective unilaterally conducting device connected to the output of the amplifier, at least one of the unilaterally conducting devices being connected in the opposite polarity to the other unilaterally conducting devices.

4. A circuit according to claim 1 wherein the resistive components of the first and second negative feedback paths are formed by a potentiometer the wiper of which is connected to the input of the amplifier, whereby the gain of the circuit can be varied by adjustment of the position of the wiper of the potentiometer.

5. A telephone conference amplifier having a plurality of channels, each channel including means for receiving an input signal and producing therefrom a unidirectional polarity signal, switch means including an inverting d.c. amplifier with first and second resistive negative feedback paths having first and second resistors connected in series respectively with first and second unilaterally conducting devices connected in opposite polarity to the output of the amplifier, means for applying the unidirectional polarity signal from the producing means to the electrode of the first unilaterally conducting device remote from the output of the amplifier, the polarity of the first unilaterally conducting device being such that the unidirectional polarity signal does not cause it to conduct, means connecting the electrode of the second unilaterally conducting device remote from the output of the amplifier to a point common to all of the channels and means connected to a point on the first resistive negative feedback path to derive therefrom, in response to a unidirectional polarity signal from the common point, a unidirectional polarity signal of the same polarity as that applied to the electrode of the first unilaterally conducting device and to produce therefrom a symmetrically polarised output signal.

6. An amplifier according to claim 5 in which the first unilaterally conducting device includes the emitter base diode of a transistor.

7. An amplifier according to claim 5 including a connection for d.c. signals from the means connected to the point on the first resistive negative feedback path to derive therefrom a unidirectional polarity signal in response to a uni-directional signal at the common point, the connection being connected to the means for receiving an input signal, whereby the sensitivity of the means for receiving an input signal is reduced when a unidirectional polarity signal is being applied to the common point by another channel.

8. An amplifier according to any of claims 5, 6 or 7 in which the input signal is received from, and the output signal applied to, the same transmission line and the output and input impedances of the amplifier are arranged relative to the impedance of the line so as to produce substantially the same return loss on an input signal received from the line as it produces on an output signal applied to the line.

9. A switching circuit including an inverting d.c.
amplifier with first, second and third resistive negative feedback paths having first, second and third resistors connected in series respectively with first, second and third unilaterally conducting devices, the second device being connected in opposite polarity to the other two devices to the output of the amplifier, a first signal source connected to the electrode of the first unilaterally conducting device remote from the output of the amplifier, a first signal utilisation circuit and a second signal source connected to the electrode of the second unilaterally conducting device remote from the output of the amplifier, and a second signal utilisation circuit connected to the electrode of the third unilaterally conducting device remote from the output of the amplifier, whereby only an input signal of polarity such as to cause the first unilaterally conducting device not to conduct can be applied from the first signal source to the first signal utilisation circuit, and only an input signal of polarity such as to cause the second unilaterally conducting device not to conduct can be applied from the second signal source to the second,signal utilisation circuit.

10. A telephone conference amplifier having a plurality of channels, each channel including means for receiving an input signal and producing therefrom a unidirectional polarity signal, switch means including an inverting d.c. amplifier with first, second and third resistive negative feedback paths having first, second and third resistors connected in series respectively with first, second and third unilaterally conducting devices, the second unilaterally conducting device being connected in opposite polarity to the first and third devices to the output of the amplifier, means for applying the unidirectional polarity signal from the producing means to the electrode of the first unilaterally conducting device remote from the output of the amplifier, the polarity of the unidirectional polarity signal being such as not to cause the first unilaterally conducting device to conduct, means connecting the electrode of the second uni-laterally conducting device remote from the output of the amplifier to a point common to all of the channels, and means connected to a point on the third resistive negative feedback path to derive therefrom, in response to a unidirectional polarity signal from the common point, a unidirectional polarity signal of the same polarity as that applied to the electrode of the first unilaterally conducting device and to produce therefrom a symmetrically polarised output signal.

11. An amplifier according to claim 10 including a connection for d.c. signals from the means connected to the point on the third resistive negative feedback path to derive therefrom a unidirectional polarity signal in response to a unidirectional signal at the common point, the connection being connected to the means for receiving an input signal, whereby the sensitivity of the means for receiving an input signal is reduced when a unidirectional polarity signal is being applied to the common point by another channel.