OUTPUT STAGE WITH AUTOMATIC LEVEL CONTROL FOR POWER LINE SIGNALLING

Abstract: An output stage with automatic level control for power line signalling, including a controlled amplifier (F₁) and a feedback circuit for sensing the output voltage (V O +v O ) of the output stage and for supplying a control signal (v⁴) to a control input of the controlled amplifier (F₁). The feedback circuit includes a comparator (K) for comparing an alternating voltage (v₁) which is responsive to the alternating voltage component (v O ) at the output of the output stage and which is furthermore superposed on a first direct voltage (V₁) with a second direct voltage (V₂). As control signal to the controlled amplifier (F₁) there is utilised the voltage (v₄) across the capacitor (C₄), the voltage of which increases or decreases in response to the comparison result of the comparator (K). The amplitude of the alternating voltage component at the output of the output stage can thus be kept to a substantially constant value determined by the first and second direct voltages (V₁ and V₂).

Description: TECHNICAL FIELD The invention relates to an output stage with automatic level control for power line signalling, including a controlled amplifier and a feedback circuit for sensing the output voltage of the output stage and feeding a control signal to a control input of the controlled amplifier. BACKGROUND ART An apparatus of the kind given above is described in the US patent 4451801. The voltage amplitude of the signal fed to the power line is responsive to the size of the supply voltage of the apparatus, and the voltage amplitude is limited automatically if it tends to be too high in relation to the supply voltage, which can depend on high power line impedance. DISCLOSURE OF INVENTION The object of the present invention is to provide an output stage of the kind mentioned in the introduction, where the voltage amplitude of the output signal can be selected in a simple way and simultaneously independent of the power line impedance, the input signal amplitude and the magnitude of the supply voltage, at least within certain limits. In addition, variation from high to very low values for the output signal shall be enabled. This is achieved by an alternating voltage proportional to the alternating voltage component on the output of the output stage and furthermore superposed on a direct voltage being compared with a second direct voltage, the voltage across a capacitor connected to a control input of the controlled amplifier being increased or decreased in response to the comparison result present. The amplitude of the alternating voltage component at the output of the output stage can thus be kept at a substantially constant value which is determined by both direct voltages. The distinguishing features of the invention are apparent from the claims. BRIEF DESCRIPTION OF DRAWINGS The invention will now be described in detail with reference to the drawings, on which Figure 1 illustrates an embodiment of an output stage in accordance with the invention, and Figure 2 is a timing chart illustrating the voltages in certain points of the output stage in Figure 1. BEST MODES FOR CARRYING OUT THE INVENTION An embodiment of an output stage in accordance with the invention is illustrated in Figure 1. The output stage input comprises a differential input of a voltage controlled amplifier F₁ with a gain of up to about 25 times. The controlled amplifier is connected to a power amplifier F₂, which is suitably a class-A stage, the task of which is to give the output stage a sufficient current drive capacity. The power amplifier output is connected conventionally to the power line, e.g. a 220 volt mains, on which signalling shall take place via a tuned circuit C₁ - L₁, a transformer T and a capacitor C₂.V O denoting the supply voltage of the output stage. During operation, an output signal comprising an alternating voltage v O which is superposed on the supply voltage V O occurs at the output of the power amplifier F₂. This output signal is supplied not only to the transformer T, but also to a feedback circuit between the power amplifier output and a control input of the controlled amplifier F. The supplied voltage V O is blocked in the capacitor C₃ and the alternating voltage v O is supplied to a voltage divider R₃, R₁.A comparator denoted by K is adapted to compare the sum of a first direct voltage V₁ and the alternating voltage v₁ which is obtained at the output of the voltage divider with a second direct voltage V₂. The direct voltages V₁ and V₂ are obtained across their respective resistors R₁ and R₂ with the aid of two current generators I₁ and I₂. The gain of the controlled amplifier F₁ is determined by the voltage v₄ across a capacitor C₄. The capacitor is charged by a current generator I₃ during the times when the voltage V₁+v₁ is less then the voltage V₂ and is discharged via the comparator K during the times when the voltage V₁+v₁ is greater than the voltage V₂. In the following it is assumed that the resistor R₁ has the resistance R₁ ohms, the current generator I₁ generates the current I₁ amps and so on. In the following, Figure 2 is also referred to, this being a timing chart of the voltages at certain points in the output stage. More specifically, the direct voltages V O , V₁ and V₂ as well as the alternating voltages V O +v O , V₁+v₁ and v₄ are shown. Since the supply voltage V O is blocked in the capacitor C₃, the voltage fed back and thus also the voltage amplitude of the alternating voltage component v O in the output signal from the output stage will be independent of the supply voltage. The condition for the comparator K to function with the desired result in the feedback circuit is that its input voltages are kept within a certain amount of margin between zero volts and the supply voltage V O . Downward division of the alternating voltage component v O from the output of the power amplifier F₂ to the alternating voltage v₁ in the voltage divider R₃, R₁ and the superposition of this on the direct voltage V₁ enables the feed-back circuit to function for relatively high output signal levels from the power amplifier F₂. The graphs illustrated in Figure 2 correspond to a down division up to one fifth. The voltage V₁ should suitably be equal to about half the supply voltage, which gives the greatest possible margin to the zero voltage and to the supply voltage V O . The feedback circuit functions also for very low output signal levels. For low output signal levels no down division is necessary, however, i.e. the resistor R₃ can then be excluded. However, the resistor R₁ is required for both high and low output signal levels, since the alternating voltage v₁ must be superposed on a direct voltage for the input voltage of the comparator K always to exceed zero volts. The value of the capacitance C₄, the charging current I₃ and the current with which the capacitor is discharged through the comparator K determines, inter alia, how rapidly the control takes place after a load change on the power line. The discharge current is suitably about 100 times greater than the charging current I₃. For equilibrium, the capacitor is here discharged during about 1% of the total time, which results in that the top value of the sum of voltages V₁+v₁ will only insignificantly exceed the direct voltage V₂. See Figure 2. The capacitance C₄ should be sufficiently large for the voltage across it not to be changed as much during a period that distortion occurs. The voltage v₄ across the capacitor C₄ will thus substantially be a direct voltage, the magnitude of which is responsive to the power line impedance and to the desired voltage amplitude of the output state output signal. If the currents I₁ and I₂ are equally as great and have the value I O , there is the approximate relationship (1) R₁ x I O + v₁₀ = R₂ x I O , where v₁₀ denotes the top value of the voltage v₁. If v OO denotes the top value of the alternating voltage component v O , there is also applicable the approximate relationship (2) v₁₀ = v OO x R₁/(R₁ + R₃). If (2) is inserted in (1) there is obtained that v OO = (R₂ - R₁) x I O x (R₁+R₃)/R₁, i.e. v OO is approximately proportional to R₂ - R₁. The desired voltage amplitude of the output signal can be obtained therefrom, e.g. via suitable selection of the resistor R₂. The amplifiers F₁ and F₂, comparator K, diode D and current generators I₁₋I₃ are suitably included in a single integrated circuit in practice, where the illustrated resistors, capacitors and coils are outside it, and thus can be adapted to the desired application. For example, it is essential that the resistor R₂ can be selected in dependence on the desired output signal level. Furthermore, the down division resistor R₃ should be able to be matched to the output signal level so that the alternating voltage component v₁ will be appropriately great. An overvoltage protector for protecting against heavy transients from the power line is suitably applied in practice to the illustrated output stage outside the mentioned integrated circuit. Such a protector may be, for example, a resistor in series with the output of the power amplifier F₂ together with a Zener diode between the same output and ground. For the current sources I₁ and I₂ to give just as great and well defined currents irrespective of the suppy voltage V O and the temperature, the integrated circuit is suitably provided with a voltage to current converter, which generates a current from the reference voltage, e.g. from a band gap reference. A reference resistor associated with the convertor is also suitably placed outside the integrated circuit so that the resistance of the resistor can be well defined and independent of temperature. A second, equally as great current can be conventionally generated by current mirroring. further balance circuit comprising a capacitor and a coil connected in series with the balanced circuit C₁-L₁. The coil shall then be inductively connected to the transformer T, and the voltage V O shall be connected between both balanced circuits. Here the capacitor C₃ is connected to the balanced circuit added in this way, instead of to the circuit C₁-L₁. The currents I₁ and I₂ are, for example, 200 x 10⁻⁶ A. For a supply voltage of abut 5 V the resistance R₁ is selected to be about 12.5 x 10³ ohm, whereby the voltage V₁ is given the value 2.5 V. The capacitors C₃ and C₄ may each have, for example, a capacitance of 50 nF. The current I₃ is, for example 2 x 10⁻⁶A. As mentioned above, there is obtained an output stage with the described embodiment for power line signalling with which the voltage amplitude of the output signal is independent of the input signal amplitude, feed voltage magnitude and the power line impedance. In addition, a desired amplitude can be set in a simple way by suitable selection of the resistance of the resistor r₂, for example, and can thus be caused to vary between high and very low values. The described embodiment can of course be altered within the scope of the patent claims. For example, the direct voltage V₁ and V₂ can be partially built up via diodes in series with the resistors R₁ and R₂. The number of diodes in series with each of the resistors is namely equal to three in such a case, which gives a voltage equal to just over two volts across each diode chain. With retained current strength from the current generators I₁ and I₂ the resistance R₁ may be selected to be about 2 x 10³ ohm. The resistances R₂ and R₃ must also be matched to this embodiment. In the case where certain components are included in a single integrated circuit, as proposed above, the series-connected diode should also be included in this circuit. It is also conceivable to replace the current generator I₂, resistor R₂ and optional diodes in series with it by a voltage source generating the voltage V₂. It is naturally also conceivable to alter the llustrated circuits in a way such that the voltage across the capacitor C₄ decreases instead of increases when V₁ + v₁ are less than V₂ and vice versa.

Claims: Output stage with automatic level control for power line signalling, including a controlled amplifier (F₁) and a feedback circuit for sensing the output voltage (V O + v O ) of the output stage and feeding a control signal (v₄) to a control input of the controlled amplifier (F₁), characterized in that the feedback circuit includes a first series circuit in which there is included a first capacitive element (C₃) for blocking the direct voltage component (V₀) in the voltage at the output of the output stage, and in that the feedback circuit also includes a comparator (K) for comparing the sum of a first direct voltage (V₁) and an alternating voltage (v₁) which is proportional to the alternating voltage component (v O ) in the output voltage of the output stage with a second direct voltage (V₂), a second capacitive element (C₄) connected between said control input and a reference voltage (GND), and means (I₃, D, K) for changing the voltage (v₄) across the second capacitive element (C₄) in one direction when said sum (V₁+v₁) is less than said second direct voltage (V₂) and for changing the voltage (v₄) across the second capacitive element (C₄) in the other direction when said sum (V₁+v₁) is greater than said second direct voltage (V₂), whereby the voltage amplitude of said alternating voltage component (v₀) can be kept to a substantially constant value determined by said first (V₁) and second (V₂) direct voltage. Output stage as claimed in claim 1, characterized in that the feedback circuit also includes a first resistor (R₁), and that a current generator (I₁) is connected to a first point between said first capacitive element (C₃) and said first resistor (R₁) to achieve said first direct voltage (V₁) in said first point. Output stage as claimed in 2, characterized in that the feedback circuit also includes a second series circuit, in which there is included a second resistor (R₂) and a second current generator (I₂) for achieving said second direct voltage (V₂) in a second point. Output stage as claimed in claim 3, characterized in that in the first series circuit there is also included a given number, e.g. three, of diodes situated on the same side of said first point as said first resistor (R₁), and in that in the second series circuit there is also included said given number, e.g. three, of diodes situated on the same side of said second point as said second resistor (R₂). Output stage as claimed in claims 2-4, characterized in that in the first series circuit there is included a third resistor (R₃), situated on the same side of said first point as said first capacitive element (C₃). Output stage as claimed in claims 1-5, characterized in that the voltage (v₄) over said second capacitive element (C₄) is altered in one direction by current supply from or to a current generator (I₃), and in that the voltage is altered in the other direction by a current to or from the output of the comparator (K).

Inventors: SOEDERSTROEM, LEIF EVERT, Söderström, Leif Evert
Applicants: TELEFONAKTIEBOLAGET L M ERICSSON
Application year: 1987
Main classification: H03G3
IPC codes: H04B3, H03G3