Abstract:
In accordance with the present invention, two unsymmetrical differential amplifier stages are fed from a common signal source that supplies an AC voltage signal, but to which the two differential amplifier stages are connected with respect to the unsymmetry in opposite senses. The outputs of the two differential amplifier stages, however, are connected with respect to the unsymmetry in like sense and to an output circuit that provides the load resistances and whose output supplies the rectified a.c. voltage signal that also has an approximately quadratic characteristic.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a circuit arrangement for rectifying an AC voltage signal generated by a signal source where the circuit has a differential amplifier including at least a first and a second transistor and where the a.c. voltage signal is supplied to inputs of both transistors. 
     2. Background of the Invention 
     A circuit arrangement of this kind is known from &#34;Halbleiterschaltungstechnik&#34; (&#34;Semiconductor circuitry&#34;) by U. Tietze and Ch. Schenk, Springer-Verlag 1978, 4th edition, FIG. 25.13. In this, the AC voltage signal is fed to a transistor in the differential amplifier stage while the base electrode of the second transistor is at a reference potential. The collector potentials of these two differential amplifier transistors are fed to two emitter followers connected in parallel so that the positive collector potential is transferred in each case to the output. 
     Another circuit for the rectification of AC voltage signals is known from &#34;Analog IC design: the current-mode approach&#34; by C. Tamazou, F. J. Lidgey and D. G. Haigh, published by Peter Peregrinus Ltd, London 1990, FIG. 2.10, in which the rectified signal has an approximately quadratic characteristic. This circuit is shown in FIG. 5 and includes three npn transistors T7, T8 and T9. The emitter surface area of the middle transistor T8 is n times greater than the equivalent surface areas of the other two transistors T7 and T8. The emitter electrodes of these three transistors are fed from a single constant current source that supplies a current I1. A resistor R7 is connected between the bases of transistors T7 and T8 and another resistor R8 with identical resistance value is connected between the bases of transistors T8 and T9. Consequently, an a.c. voltage signal Ue supplied from a signal source 4 is fed directly to the base electrode of transistor T7, but only one-half of this signal is fed to transistor T8. Finally, the collector electrodes of transistors T7 and T9 are connected directly to an operating voltage source UB whereas that of the middle transistor T8 is connected to this operating voltage source via a load resistance R9. The function of this circuit will now be explained below with reference to the Ue - Ic diagram as shown in FIG. 6. With a surface area ratio n between the transistors T8 and T7 and also between T8 and T9, in the quiescent state a collector current Ic8 of transistor T8 flows with a magnitude of I1·n/(n+2) through the load resistor R9. In the diagram shown in FIG. 6, this is the intersection of the curve Ic8 with the ordinate axis. 
     With an input voltage Ue≠0, however, the transistor T7 or T9 takes an increasingly large proportion of the current as curves Ic7 and Ic8 in FIG. 6 show. At the same time, current Ic8 reduces and its curve in the region of zero of the input voltage Ue (see region d in FIG. 6) approximates a parabola of the second degree. Consequently the voltage at the collector of transistor T8 rises above the quiescent state value. This corresponds to a rectification of the a.c. voltage signal Ue with quadratic characteristic. 
     In this known circuit in accordance with FIG. 5, however, the power consumption in the two resistors R7 and R8 is disadvantageous. Moreover, for this circuit to operate reliably, a high input voltage Ue is required. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to specify a circuit arrangement of the type described at the outset that can be operated reliably with a low supply voltage and also draws only a small amount of current. 
     In a circuit arrangement for rectifying an a.c. voltage signal generated by a signal source, where the circuit has a differential amplifier stage comprising at least a first transistor and a second transistor and where the a.c. voltage signal is supplied to both their inputs, this object is accomplished by providing a second differential amplifier stage and with both stages being of unsymmetrical or symmetrical design, but made functionally unsymmetrical by means of a suitable offset voltage. These two differential amplifier stages are connected to the signal source in opposite senses with respect to their unsymmetry, whereas the outputs are connected in the same sense with respect to the unsymmetry. The outputs are connected to an output circuit that supplies the rectified a.c. voltage signal. 
     This circuit arrangement in accordance with the present invention has the advantage compared with the one shown in FIG. 5 that, for otherwise identical characteristic curves, only half the input voltage Ue is required for reliable operation because there is no voltage divider on the input side. Moreover, this circuit in accordance with the present invention operates much faster than conventional bridge circuits made with rectifier diodes. 
     A further advantage is provided by the output characteristic of the circuit arrangement in accordance with the present invention that has an approximately quadratic form in the region of the zero point because this represents a variable that is proportional to the power. 
     In an advantageous development of the circuit arrangement in accordance with the present invention, the unsymmetry of the two differential amplifier stages is created by an area or width ratio of the associated transistors. 
     In another advantageous development of the circuit arrangement in accordance with the present invention, this unsymmetry of the two differential amplifier stages can also be created by an auxiliary voltage that overlays the a.c. voltage signal which is to be rectified. Thus the differential amplifier stages are made unsymmetrical by external offset voltages instead of the surface area ratio of the emitters, where in accordance with an advantageous development of the present invention these offset voltages are created at pick-offs on load resistors in a differential amplifier stage on the input side. In another embodiment of the invention, the unsymmetry is created by one-sided emitter resistors as shown in FIG. 7 by resistors R9 and R10. The use of such symmetrically designed differential amplifier stages offers the advantage that transistors which are identical to one another can be used. 
     Furthermore, the circuit in accordance with the present invention can be developed further in such a way that the output circuit contains two ohmic load resistors each of which is connected with an output of the differential amplifier stages. So that the resulting output signal here assumes the value zero in the quiescent state of the circuit arrangement, the conductance values of these two load resistors can be designed to be proportional to the value that corresponds to the unsymmetry of the differential amplifier stages. 
     Furthermore, the two outputs can also be connected via a current balancing circuit, where in accordance with a further embodiment of the present invention the balancing factor is set in accordance with the value forming the unsymmetry in the differential amplifier stages so that here too the quiescent current assumes the value zero. At the same time, one obtains in an advantageous manner an addition of the quadratic components of the output current. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described and explained below on the basis of embodiment examples together with the drawings. The drawings show: 
     FIG. 1 is a circuit arrangement as embodiment example of the invention; 
     FIG. 2 is an alternative output circuit for the embodiment example shown in FIG. 1; 
     FIG. 3 is a circuit arrangement of a further embodiment example of the invention; 
     FIG. 4 is current/voltage characteristic curves to explain the function of the embodiment examples; and 
     FIGS. 5 and 6, referred to above, illustrate the state of the art. 
     FIG. 7 is a circuit arrangement of the present invention using one-sided emitter resistors to create an unsymmetry. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The circuit arrangement in FIG. 1 shows two differential amplifier stages made up from npn transistors T1 and T2 as well as T3 and T4. A signal source 4 generates an a.c. voltage signal Ue as an input voltage which is supplied to both differential amplifier stages at the same time. In the two differential amplifier stages , one transistor T1 and T3, respectively, has an emitter surface area that is n times greater than that of the partner transistor T2 and T4, respectively, and this is indicated in the Figure by double emitter arrows. The differential amplifier stage T1/T2 has two inputs E1 and E2 each being connected to one terminal of signal source 4. The two inputs E3 and E4 of the second differential amplifier stage T3/T4 , however, are not connected in the same sense to the signal source 4 with respect to the unsymmetry in comparison with the first differential amplifier stage T1/T2. The input E3 of the larger transistor T3 is therefore not connected to the same terminal of signal source 4 as the corresponding larger transistor T1 in the differential amplifier stage T1/T2, but to the terminal to which the input E2 of the smaller transistor T2 is connected. Equally, input E4 of the smaller transistor T4 in the second differential amplifier stage T3/T4 is connected to the same terminal of signal source 4 as input E1 of the larger transistor T1 in the first differential amplifier stage T1/T2. 
     The collector terminals Ac1 and Ac2 as well as Ac3 and Ac4 in the two differential amplifier stages are, however, connected in the same sense with respect to the unsymmetry, i.e., the collector terminal Ac1 of the larger transistor T1 in the first differential amplifier stage T1/T2 is connected to the same point as the collector terminal Ac3 of the larger transistor T3 in the second differential amplifier stage T3/T4, this point representing output A1, and similarly collector terminals Ac2 and Ac4 of the corresponding smaller transistors T2 and T4 are connected together to a second output A2. 
     At these two outputs A1 and A2, an output circuit 6a with a first and second ohmic load resistors R1 and R2 are each connected on one side to the first and second output A1 and A2 and on the other side to the potential of the positive operating voltage UB. 
     An output voltage UA, representing the difference between the two connected collector potentials in the circuit, can be picked off at the two outputs A1 and A2. 
     Finally, each of the two differential amplifier stages T1/T2 and T3/T4 is suppl led from a constant current source 1 and 2 respectively to which in turn the potential of the negative operating voltage UB is applied. Consequently, a constant current I1 flows into the first differential amplifier stage T1/T2 and a constant current I2 into the second differential amplifier stage T3/T4. 
     The functioning of the circuit in accordance with the present invention and depicted in FIG. 1 will now be explained in conjunction with the Ue - Ic characteristic curves as shown in FIGS. 4a to 4c. In the quiescent state, that, is with an a.c. voltage signal of Ue=0, the incoming current I1 and I2 respectively splits up in accordance with the surface area ratio n. A current Ic1 from the first differential amplifier stage T1/T2 and a current Ic3 from the second differential amplifier stage T3/T4 with the value ##EQU1## thus flows to output A1. However, the current flowing to output A2 is the sum of the two collector currents Ic2 and Ic4 of the respective smaller partner transistors T2 and T4 with the value ##EQU2## The corresponding values at Ue=0 are also shown in FIGS. 4a and 4b. 
     If the ratio of the conductance values of the two resistors R1 and R2 is chosen to correspond to the surface area ratio n of the transistors in the two differential amplifier stages, then a quiescent current with value zero settles in at output UA. 
     With positive modulation, the collector current Ic1 in the first differential amplifier stage T1/T2 continues to build up and the collector current Ic2 of the partner transistor T2 reduces further, whereas the collector current Ic3 of the larger transistor T3 in the second differential amplifier stage T3/T4 reduces on account of the connection in opposite sense to the signal source 4 and at the same time the collector current Ic4 of partner transistor T4 increases. The relevant characteristics are shown in FIG. 4a. The variation of the total currents IA1 and IA2 fed to outputs A1 and A2 can be seen in FIG. 4b. The two outputs A1 and A2 thus each supply a rectified a.c. voltage signal which has an approximately quadratic form in the region of Ue=0. The output voltage UA represents the difference between the electrical potential at the two outputs A1 and A2 and is shown in FIG. 4c; in the region of Ue=0 the approximately parabolic variation is retained. 
     The circuit arrangement given in FIG. 1 has a special feature in that, when the modulation is very high, the output currents IA1 and IA2 tend towards the same value irrespective of the polarity of the input voltage Ue, as shown in the diagram of FIG. 4b. 
     The circuit arrangement shown in FIG. 2 is an alternative output circuit 6b, but otherwise connected up in the same way as in FIG. 1. The output circuit 6b includes a current balancing arrangement with two pnp transistors T7 and T8 and the equivalent circuit of a sequence stage comprising a d.c. voltage source 5 and an output resistor Ra. Transistor T7, which acts as a diode, is connected to the first output A1 and the associated current source transistor T8 is connected to the second output A2. In addition, the series circuit consisting of d.c. voltage source 5 and output resistor Ra bridges the emitter-collector section of the current source transistor T8 in the current balancing circuit. Finally, the emitter electrodes of the transistors T7 and T8 making up the current balancing circuit are connected to the positive potential of operating voltage source UB. In this current balancing circuit, the balancing value corresponds to the surface area ratio n in differential amplifier stage T1/T2 and T3/T4, respectively, which results in the quiescent current being compensated to value zero, i.e., with an input voltage Ue=0, an output current IA with value zero flows through the output resistor Ra. The corresponding current variation diagrams are given in FIGS. 4d and 4e. The output current IA also, as shown in FIG. 4e, has an approximately parabolic form in the region of the input voltage with value Ue=0, the quadratic components of the two output currents IA1 and IA2 being added together. 
     FIG. 3 shows an embodiment of the invention in which the two differential amplifier stages T1/T2 and T3/T4 are each made up of identical transistors. The unsymmetry in these two differential amplifier stages is achieved by means of an auxiliary voltage UH1 and UH2 respectively which is provided by means of a voltage divider R3/R4 and R5/R6, respectively. These two voltage dividers each represent load resistors from a third differential amplifier stage made up of npn transistors T5 and T6. Here, the signal source 4 is connected directly to the two bases of transistors T5 and T6 and their emitter electrodes are fed from a third constant current source 3. Either output circuit 6a in accordance with FIG. 1 or output circuit 6b in accordance with FIG. 2 can be used as output circuit 6. 
     The embodiment examples of the invention as shown in FIGS. 1 to 3 can also be made up of bipolar transistors of the opposite type of conductivity. 
     Instead of using differential amplifier stages of the simple design shown by the embodiment examples in the Figures, it is also possible to use differential amplifier stages of complex design such as Darlington differential amplifier stages, complementary differential amplifier stages or Darlington/complementary differential amplifier stages.