DE2262717A1 - CIRCUIT ARRANGEMENT FOR A DIFFERENTIAL AMPLIFIER - Google Patents

Description

Patent attorneys Dipl.-Ing. R ¥ eickman4,

Dipl.-Ing. H. Weickmanjn, Dipl.-Phys. Dr. K. Fincke Dipl.-Ing. ΕΑ-Ψειοκμανν, Dipl.-C he μ. Β. Huber

XBI

Technical Management ~ ™ 1 * Γ 2262717

Services, Ine, möhlstrasse 22, phone number 983921/22

P.O. Box 218

Westfield, New Jersey 07091

Y.St.A.

Circuit arrangement for a differential amplifier

The invention relates to a circuit arrangement for a differential amplifier whose output signal polarity can be switched is. The invention finds particular application in pulse length modulators, Pulse amplitude modulators and in multipliers, dividers and watt meters.

There are many uses for fast switching devices, especially for those arrangements that quickly switch the polarity of a signal. The problem lies with the invention based on creating such a circuit arrangement that is simple and relatively cheap to build, but very accurate works and can be used in precision equipment.

There is also a need for highly accurate, stable, and reliable multipliers, dividers, and wattmeters. In particular, the lack of power meters of sufficient accuracy leads to a very high cost burden on the power supply

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industry. Although there are many uses for electronic wattmeters, the use of such instruments has grown hitherto kept within narrow limits due to insufficient accuracy, high costs and other factors.

The invention is therefore based on the further object of providing a very precisely working and relatively cheap and reliable To create a multiplier circuit, divider circuit or wattmeter circuit, and also to specify methods for their use. In addition, it should be possible to build modulators that modulate the pulse length, the pulse amplitude and the frequency enable. Such devices should be constructed with a uniform circuit arrangement to be specified by the invention can be.

A circuit arrangement for a differential amplifier, the output signal polarity of which is to be switched and the above fulfilled, is characterized according to the invention by one between one of the amplifier inputs and one first signal input for signals of a first strength switched impedance and by a switch device for alternating Connection of the second amplifier input to the first signal input and a second signal input provided for signals of a lower strength than the first.

The switch provision is effected by switching between the first and the second signal input change the polarity of the difference between the input signals at the differential amplifier, so that the output signal of this amplifier is switched accordingly.

A modulator constructed with the circuit arrangement according to the invention contains a source (preferably a switch device of the type mentioned above) to generate an output

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signals that are generated when the input signals are switched between two Level values is changed. The output of the source becomes added to the modulating input signal, the signal sum is integrated by an integrator. A level detector detects the output signal of the integrator and generates a change in the output signal whenever the output signal of the integrator has one of two different predetermined level values achieved. The output signal of the level detector is fed back as an input signal to the source ~. This modulator generates a pulse length modulation and a frequency modulation. A pulse amplitude modulator works (preferably below Application of a further switch device of the type mentioned above) at the output of the pulse length modulator, whereby a Multiplier circuit is formed. If any of the input signals to the multiplier is proportional to a load current that is other input signal is proportional to a load voltage, this arrangement can be used as an electronic precision power meter be used. However, it can also be used as a divider by keeping one of the input signals constant, while the level of the signal source is changed along with one of the input signals. The same circuit arrangement can can be used as a further frequency modulator by applying the output signal of the pulse amplitude modulator to the level detector is fed back so that the level values at which the Output signals of the integrator are evaluated, a change are subject. ■

The invention is described below with reference to the exemplary embodiments shown in the figures and signal curves described. Show it:

1 shows a circuit arrangement according to the invention. FIGS. 2 and 3 are equivalent to the arrangement shown in FIG Circuits in different states,

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4 shows the circuit of a pulse length and frequency modulator, pulse amplitude modulator, multiplier _f divider and power meter, constructed with a circuit according to the invention,

5 shows signal profiles in the circuit arrangement shown in FIG and

6 shows the signal curves shown in FIG. 4 when certain input signal quantities change.

Fig. 1 shows a circuit arrangement 10 according to the invention is constructed. It contains an integrated differential amplifier 12, which is surrounded by a dashed line is. The differential amplifier 12 has a non-inverting input 16, an inverting input 14 and an output A negative feedback resistor 28 connects the output 18 to the inverting input 14, so that a differential amplifier is formed with a high gain.

A resistor 26 is connected to the inverting input 14, the other connection of which is at a signal input 30. Furthermore, a control circuit 20 is provided, which is formed from field effect transistors 22 and 24. The source-drain path of the field effect transistor 22 is between the signal input 30 and the non-inverting input 16 of the differential amplifier switched. The source-drain path of the field effect transistor 24 is between the non-inverting input 16 and the common circuit point, i.e. ground potential, switched. The gate electrodes 32 and 34 of the two field effect transistors are connected to one or more sources (in FIG. 1 not shown) connected, each of which opens one of the field effect transistors while the other is blocked.

The differential amplifier 12 is a common integrated amplifier. It contains a summing circuit 36, which the input

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signal of the non-inverting input 16 receives. It also receives the input signal of the inverting input 14 through an inverter 38. The output of the summing circuit is fed to an amplifier 40 whose output 'corresponds to the output 18 of the differential amplifier.

Fig. 2 shows the effective circuit connections of the circuit of Fig. 1 in the inverting mode, i.e. in the state in which the field effect transistor 24 is turned on and the field effect transistor 22 is blocked. The non-inverting input 16 is connected to ground potential, the inverting input 14 is connected to the signal input 30 via the resistor 26. The relationship between the input voltage Ee at the signal input 30 and the output voltage Ea at the output 18 results from the following equation:

(1) Ea = -Ee (R28 / R26)

R28 is the resistance value of resistor 28, R26 the resistance value of resistor 26.

It can be seen that if the resistance values R26 and R28 match, the circuit 10 only has a gain factor of 1 Has.

3 shows the circuit 10 in the non-inverting mode. Here, the field effect transistor 22 is turned on, the field effect transistor 24 is blocked. So now that is not inverting input 16 is not connected to ground potential but to signal input 30. The full input voltage will be fed to the non-inverting input 16, while the voltage drop across the resistor 26 (together with a negative feedback signal via the resistor 28) is fed to the inverting input 14 of the differential amplifier is supplied. The result is that

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initially a differential voltage appears between the inputs 14 and 14, which can be traced back to the control circuit 20. This differential voltage only remains during the very short switching time of the amplifier output signal. Upon reaching In the steady state, the signal at input 16 matches that at input 14 when the output signal at output 18 is equal to the signal at signal input 30 and no current flows through resistors 26 and 28. thats why the output signal of the circuit shown in Fig. 1 is not inverted. This means that if the polarity of the input signal is positive, the output signal is positive and equal, and vice versa. The polarity of the output signal has thus been reversed.

The circuit shown in Fig. 3 is only a follower circuit. The relationship between the output voltage and the Input voltage is therefore independent of resistors 26 and 28 and is expressed by the following equation:

(2) Ea = Ee

In the circuit shown in Fig. 3, the values of the resistors 26 and 28 can be changed within wide limits without noticeably changing the output signal, since the amplifier inputs 14 and 16 have a relatively high impedance value and no current flows through the resistors.

The circuit arrangement shown in Fig. 1 has numerous advantages, especially for electrical measuring instruments. It provides an output signal that can be switched as quickly as the Operational amplifier 12 allows, i.e. the switching time can be in the order of magnitude of a few nanoseconds. The switchover can also be done very precisely, since the circuit practically does not have the very good inherent accuracy of the operational amplifier

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worsened. For example, the accuracy is not due to the forward resistances of the field effect transistors 22 and 24 impaired because the blocking resistance is a factor larger than the forward resistance, which is on the order of millions. ■ ,.

The circuit arrangement 10 also has significant advantages in terms of manufacturing costs compared to such circuits, as previously used in measuring instruments for similar purposes. A typical circuit of the previous type is for example in the article "An Electronic Multiplier for Accurate Power Measurements" by Tomota, Sugiyama and Yamaguchi in "IEEE Transactions on Instrumentation and Measurement", Vol. IM-17, No. 4, December 1968. In such a circuit, two separate precision voltage sources are used used, the signals of each voltage source are alternately through series-connected field effect transistors scanned. A differential output amplifier is used as a buffer stage. Compared with the circuit after the Invention, such a circuit requires an additional precision voltage source. The circuit arrangement according to the invention so it is cheaper to build. Furthermore, the forward resistances of the field effect transistors and the special additional Voltage source to possible errors that nüit occur with the invention.

Other known circuits work with a single precision voltage source, Field effect transistors for switching, an inverting operational amplifier and an output operational amplifier as a buffer stage. Such circuits are also more expensive and more error-prone than the circuit according to of the invention because they require an additional operational amplifier. Furthermore, the series-connected field effect transistors provide and the additional operational amplifier, of course, additional sources of error.

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Fig. 4 shows several circuit arrangements in which a circuit 10 is used. The first circuit arrangement is a modulator 84, which is shown in the left part of FIG. Of the Modulator 84 provides an alternating output pulse signal, whose length and frequency is modulated by an input signal at an input Z or an input X, the input X corresponds to the signal input 30 shown in FIGS. 1, 2 and 3.

The modulator 84 contains the circuit arrangement 10, its output signal occurs via a resistor 42 at a node a. An input signal is sent via input Z to the switching point a fed through a resistor 44 and added to the output signal of the circuit. The sum of the two signals is fed to an integrator 46 via a line 50. The output of the integrator 46 is a comparator or Level detector 56 supplied * The output of the circuit is fed to the gate electrodes 32 and 34 of the Fekdeffekttransistors 22 and 24 of the circuit 10 via a line 68.

The operation of the modulator 84 will now be described with reference to FIGS. FIG. 5 shows the signal curves at the switching points a, b and c of the circuit shown in FIG. 4 for a signal variable with the value zero at the input Z. Furthermore, the signal at the input X is a continuous direct current signal. The square-wave signal a in FIG. 5 is integrated by the integrator 46, which generates an output signal whose waveform is triangular, as shown at b in FIG. 5. The reason for this is that when the output voltage of the integrator approaches a first value e ,, the comparator abruptly reverses the polarity of its output voltage c. As a result, the circuit 10 reverses the polarity of its output signal and initiates a fall in the output signal of the integrator 46 according to a linear curve. When the output of the integrator 46 reaches a second value e ₂ , which is equal to the value e ^, however

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has opposite polarity, the \ &? same reverses again reverses the polarity of its output voltage so that circuit 10 begins the cycle described again.

6 shows the change in the signal curves described above when a negative direct current signal is applied to the input Z. First, the entire signal curve a is shifted downward, which means that the positive parts are lower, but the negative parts are larger. The slope of the positive parts of the signal run b is therefore lower. Since the negative parts of the signal curve a are larger than before, the slope of the negative parts of the signal curve b is greater. Since the distance e between the comparator level values e ^ and e _{2 is} constant, the duration t. the positive running parts of the output signal of the integrator longer than the time duration tp of the negative running parts of this signal course. This is in contrast to the signal curve b in FIG. 5, in which the time t ^ is equal to the time tp. ■

The signal curve c in FIG. 6 shows the result of this change in the output signal of the modulator. The positive pulses have the duration t ^, the negative pulses the duration t ₂ · The length of the pulses is modulated by the signal at the Z input. In addition, as can be seen from the following equations, the frequency of the signal curve b has also changed as a result of the input signal. The modulator 84 is thus a pulse length modulator and at the same time a frequency modulator.

The following equations describe how the Modulators:

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(4) t ₂ = -e C / (I _Z - Ι _χ )

t ₁ - t ₂ (R28 / R26)

(6) I ₇ / I _Y = (if R28 = R26)

L A "C ₁ + ΐ ₂

t ₁

(7) ¹ ZZ ¹ X = (if R26 is infinitely large)

(8) f = 1 / (t ₁ + t ₂ ) = (I _x ² - I _z ² ) / 2I _x eC

In these equations, t ₁ , t ~ and e are the quantities shown in FIGS. 5 and 6, C is the integration capacitance of capacitor 52, I _{7 is} the current at input Z, I _{Y is} the current at input X, R26 and R28 are the resistance values of Resistors 26 and 28 and f the frequency of the output signal of the modulator, ie the frequency of the signal curve b in FIGS. 5 and 6.

The modulator 84 can also be viewed as a free-running oscillator, the output frequency f of which is reversed changes proportionally to the size of the signals at input X or Z. Equation (8) shows this change.

The above equations also show the effect of the input signal at input X to the pulse length modulation of the output signal. The signals at inputs X and Z can i.e. modulate the pulse length and the pulse frequency of the output signal.

The integrator 46 is a precision integrator constructed in the usual way. It includes a differential amplifier 48 and a negative feedback capacitor 52. The input signal is fed to the inverting input, the non-inverting input

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Input is grounded. The capacitance C of the condensaXOr'S '^ S 1st is set so that the frequency is within a predetermined range according to the above equation (8). For For DC input signals, the frequency f is not particularly critical. For AC or variable DC signals the frequency f should be so high that only a very small part of the input signal during each cycle of the modulator is sampled. For precision measurements, the frequency f should be more than 100 times greater than the frequency of the input signal, preferably at least 200 times greater.

The comparator circuit 56 contains a further differential amplifier 58, further is a resistor 64 on the non-inverting one Input 60 of amplifier 58 fed back. The inverting input receives the output of the integrator 46. Resistor 64 and resistor 62 connected to earth form a resistor network, the connection point Both resistors 62 and 64 are connected to the non-inverting input 60. This creates between exit and the non-inverting input of the comparator 58 is positive Feedback.

During operation of the circuit, the comparator circuit generates a high DC output signal (for example of 10 volts) of one polarity if the signals at the inputs are mutually different in one direction. Furthermore, a direct current signal of the same magnitude but opposite polarity is output if the input voltage difference is in the opposite direction. The ratio of the resistance values of the resistors 64 and 62 determines the value of the voltages e 1 and e ₂ (FIGS. 5 and 6) at which the signals at the inputs of the amplifier 58 are equal to one another can be regarded as a level detector, since it generates a change in the output signal when the input «

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signal reaches a predetermined, fixed level value. In this context, the circuit 56 can also be used as a level-controlled Flip-flop can be viewed.

In the connection of the output of the comparator circuit 56 with the field effect transistors 22 and 24, an inverter 92 is between the connections 32 and 34 are provided. It is used in view of the fact that the field effect transistors 22 and 24 match Conductivity type, i.e. both transistors are either η-channel or p-channel field effect transistors. Of the Inverter 92 ensures that the gate electrode of a field effect transistor always receives a signal with one polarity, that is opposite to that of the signal at the other gate electrode. When field effect transistors with opposite Conductivity type are used, the inverter can be omitted and replaced by a normal line.

The middle part 86 of the circuit shown in Fig. 4 is a pulse amplitude modulator. It contains a circuit arrangement 10, as shown in Fig. 1, 2 and 3, which has an input Y corresponding to the signal input 30 at which the modulating Input signal occurs. The pulses from the comparator 56 are applied to the gate terminals 32 and 34 in FIG V / iron fed. The waveform d in Figs. 5 and 6 shows how the height of the pulses is increased by the signal Y. From equations (1) and (2) it is easy to see how this Pulse amplitude modulation is achieved.

Another frequency modulator can be implemented by simply changing the circuitry of the modulators 84 and 86. In this embodiment of the invention, the connection between resistor 64 and the output of amplifier 58 is removed. Furthermore, the output line 70 of the pulse amplitude modulator 86 is connected to the right via a line 90 (shown in dashed lines) Terminal of the resistor 64 connected. In addition, there is no signal at input Z.

3 0 9 8 2 6/1 1 6 A

. ■ - 13 -

With this arrangement, the output signal of the pulse amplitude modulator 86 is changed so that the distance e between the voltage values e, j and e _{2 of} the comparator circuit 56 is changed. This results in a further possibility of changing the frequency f of the output signal at point c in FIG. 4. Equation (8) shows the effect of such a change in the amount e on the frequency f of the output signal. The following equation describes the simple relationship between the input signals at inputs X and Y in the circuit described here:

(9) f = I _X / I _Y ".

Here, Ι _{γ is} the input current at input Y, and Iy is the quantity already described.

It is easy to see that this circuit can be used very well as a voltage-frequency converter, in particular as a 'dividing circuit, the output signal of which is in the form of a frequency value is available. Furthermore, such a circuit can be used very well for analog-digital conversion.

The entire circuit shown in Fig. 4 forms a precision multiplier, a power meter and a divider. The circuit contains the modulators 84 and 86 and a smoothing circuit 88, which smooths the output signal of the pulse amplitude modulator 86 and a direct current signal at the output that is proportional to the product or division of the two input signals. The smoothing circuit 88 includes an input resistor 72, a differential amplifier 78 and a negative feedback network composed of a resistor 74 and a resistor 76 connected in parallel to this, the input signal is fed to the inverting input 80. Essentially, the smoothing circuit 88 operates as a low pass filter.

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22627Ί7

The mode of operation of the circuit shown in FIG. H depends on the particular method used. The general equation of the circuit is:

(10) Ia = I ₂ I _Y / I _X

Here Ia is the output current at terminal 82, the other variables have already been explained.

The circuit can be used as a multiplier by keeping the input signal at the input X constant and the Signals at the inputs Y and Z are supplied as variable signals. This can be taken from the equation (IO) that the output signal is the product of the input signals at the inputs Z and Y.

When the circuit is used as a power meter, it works as a multiplier. One of the input signals at input X or Z is made equal to the load current of the power to be measured, the other input signal is made equal to the load voltage. The output signal is then the product of the load current and the load voltage, i.e. it indicates the power consumed.

When used as a divider circuit, either the input signal at input Z or at input Y kept constant while the signals at input X and at input Z or Y changed will. From equation (10) it can be seen that the output signal is proportional to the division result of the signal Z or the signal Y by the signal X is.

The circuits described above have numerous advantages, especially .for · precision measuring instruments. Through the Invention, the construction of a multiplier, divider and wattmeter is possible, these circuits work in the

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In practice, much more accurate than conventional circuits. Furthermore, this greater accuracy becomes with a substantial reduction the cost of components, especially on expensive operational amplifiers. Furthermore, a circuit according to the invention work very quickly (depending on the cost allowed for the op-amps), besides, it is very reliable operational.

The invention is particularly suitable and works unexpectedly precisely with time-varying input signals, their signal course is relatively steep at low voltages and relatively flat at higher voltages. Typical such signal curves are sine waves that have their steepest slope at 0 volts and the lowest slope at their peaks. As already stated, the operating frequency f of the circuit is at least times, preferably 200 times as large as the frequency of the input signal at input Z. The reason for this requirement is that it is then guaranteed that every part of the Input signal approaches a direct current signal as closely as possible. Because the accuracy is decreased when approximating this is not carefully observed, a relatively high operating frequency should be used in such areas, in which the waveform has a steep slope and tends to deteriorate the approximation. A lower operating frequency is sufficient if the signal curve is relatively flat and is to be understood as a direct current signal anyway. The frequency f of the circuit is automatically highest in the lowest input signals. When the input signal at the input Z has the value zero, the highest frequency f results when the input signal is highest (Z must be lower than X), so the lowest frequency occurs. Thus, there is an unexpected advantage of the circuit, which is that it automatically adapts its operation very precisely to the signal characteristics.

2 6/116 /.

- Λ6 -

The following are values for the elements used in the circuit according to FIG. 4 for an exemplary embodiment which has been used successfully specified. It should be noted that the components for circuit 10 are included in this listing.

Elements operational amplifiers 12, 48, 53 and 78 and inverter 92

Field Effect Transistors 22 and Resistors 26 and Resistor 42 Resistors 62, 64 and Resistor 44 V / i of the stand and 74 capacitor

Model 44 capacitor from Analog Devices Corp .; a fast settling differential amplifier.

Switching timeι 100 nanoseconds amplitude accuracyϊ 2 · 10 " %

3N153 (N-channel depletion) 2000 ohms (precisely adapted) 1000 ohms 10 000 ohms 2000 ohms 5000 0hm 0.02 microfarads (exactly for voltage / frequency converter) 2 microfarads

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Claims (1)

Pat en.t to sprue h e Λ) Circuit arrangement for a differential amplifier, the output signal polarity of which is to be switched, characterized by an impedance (26) switched between one of the amplifier inputs (14, 16) and a first signal input (30) for signals of a first strength and by a switch device (20) for alternating Connection of the second amplifier input (16) to the first signal input (30) and a second signal input (earth) provided for signals of a lower strength than the first. 2. Circuit arrangement according to claim 1, characterized in that the switch device (20) has two field effect transistors (22, 24), of which one (22) between the second amplifier input (16) and the first signal input ('3O) " and the other (24) is connected between the second amplifier input (16) and the second signal input (earth). 3. Circuit arrangement according to claim 1 or 2, characterized in that that the differential amplifier (12) is an operational amplifier, the output (18) of which via a resistor (28) is connected to the first amplifier input (14), 4. Circuit arrangement according to claim 3 »characterized in that the first ■ amplifier input (14) is an inverting ■ Input, the second amplifier input (16) is a non-inverting input. 309826/1184 5. Circuit arrangement according to one of claims 2 to 4 »thereby characterized in that one of the field effect transistors (22, 24) is a p-channel transistor and the other is an n-channel transistor is, and that the gate electrodes (32, 34) of these field effect transistors (22, 24) together with a switching signal source are connected. 6. Circuit arrangement according to one of claims 2 to 4, characterized characterized in that the field effect transistors (22, 24) have the same conductivity type that the gate electrode (34) of a field effect transistor (24) is connected directly to a switching signal source, and that the The gate electrode (32) of the other field effect transistor (22) is connected to the switching signal source via an inverter (92) is. 7. Circuit arrangement according to one of the preceding claims, characterized in that the impedance (26) is an ohmic Resistance is. 8. Circuit arrangement according to one of the preceding claims, characterized in that the second signal input (earth) is assigned jointly to the input and the output of the circuit arrangement. 9. Circuit arrangement according to claim 8, characterized in that that the second signal input carries reference potential. 10. Circuit arrangement according to one of the preceding claims, characterized in that the differential amplifier (12) a summing circuit (36), one between one (14) of the amplifier inputs (14, 16) and the summing circuit (36) switched inverter (38) and one of the summing circuit (36) 309826/1 downstream amplifier (40) which supplies the output signal of the differential amplifier, and that the second Input of the summing circuit (36) with the non-inverting Amplifier input (16). connected is. 11. Modulator circuit for pulse length and pulse frequency modulation, Containing a circuit arrangement according to one of Claims 1 to 10, characterized by one of the circuit arrangements (10) downstream integrator (46), the one as a source for switching signals with the switch device (20) connected level detector (56) controls, and by one with the switch device (20 ") or with the Input of the integrator (46) connected signal input (X, Z) for a modulating signal. · 12. Modulator circuit according to claim 11, characterized in that the level detector (56) has two permanently defined level values evaluates the output signal of the integrator (46). 13. modulator circuit according to claim 11 or 12, characterized in that that the level detector (56) comprises an operational amplifier (58) whose inverting input is connected to the Output (54) of the integrator (46) is connected, while its non-inverting input (60) with a predetermined Potential (c) is connected, and its output via an ohmic resistor (64) with the non-inverting Input (6o) is connected. 1'4. Modulator circuit according to one of Claims 11 to 13, characterized characterized in that the frequency of the output signal of the level detector (56) is at least 100 times greater than the frequency of the modulating signal. 309826/1164 15. Modulator circuit according to one of claims 11 to 14, characterized in that two inputs (X, Z) are provided for a modulating signal. 16. Modulator circuit according to one of claims 13 to 15, characterized in that the predetermined potential (c) by one with the ohmic resistor (64) located at the output of the level detector (56) and with another Ohmic resistance (62) formed and connected to ground potential voltage divider is generated. 17. modulator circuit according to one of claims 11 to 16, characterized in that the level detector (56) operates as a comparator circuit. 18. Multiplier circuit comprising a circuit arrangement according to one of claims 1 to 10 and a modulator circuit according to one of claims 11 to 17, characterized in that one working as a pulse amplitude modulator (86) further circuit arrangement (10) designed according to one of claims 1 to 10 is provided, the switch device of which (22, 24) by the output of the level detector (56) is controlled. 19 · Multiplier circuit according to claim 18, characterized in that that as a pulse amplitude modulator (86) working circuit arrangement (10) at its output (70) with a Smoothing circuit (88) is connected. 20. Application of a multiplier circuit according to claim 18 or 19, characterized in that for performing mathematical Calculations of the signal input (X) of the pulse length and pulse frequency modulator (84) a first input signal, the Connection (Z) between the pulse length and pulse frequency 309826/1 1 modulator (84) and the integrator (46) a second input signal and the signal input (Y) of the pulse amplitude modulator (86) a third input signal is supplied. 21. Application according to claim 20, characterized in that the signal input (X) of the pulse length and pulse frequency modulator (84) supplied input signal is constant. 22. Application according to claim 20, characterized in that the second or the third input signal are constant. 23. Application according to claim 21, characterized in that the second or the third input signal is proportional to one the electrical voltage dropping to a consumer, the first input signal proportional to that of the consumer flowing electric current is. 309826/1164