Abstract:
An electronic amplifier having a transmission factor which can be varied by means of a control voltage, specifically an expander. A switch is provided for selective operation of the amplifier with a fixed control voltage or with a variable control voltage, with this switch being coupled with circuitry for reducing, during operation of the amplifier with a fixed control voltage, the output voltage of the control circuit generating the variable control voltage.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an electronic amplifier circuit having a transmission factor which can be varied by means of a control voltage and in particular an expander. It is known to make such amplifiers so that they can be switched on and off. For this purpose, the amplifier may be operated with a variable control voltage in one operating position and with a fixed control voltage in another operating position. 
     SUMMARY OF THE INVENTION 
     It is the object of the present invention to realize the switching between the two operating positions in such a manner that destruction of sensitive components, for example, as a result of excess voltages, is avoided. 
     The above object is achieved according to the present invention by an electronic amplifier circuit, particularly an expander circuit, which comprises: an amplifier having a transmission factor which can be varied by means of a control voltage, with the amplifier having a signal input, a signal output and a control voltage input; a control circuit means, responsive to the signal in the transmission path of the amplifier, for generating a variable control voltage; a source of fixed control voltage; a switching circuit means for applying the variable control voltage to the control input of the amplifier when in a first condition and for applying the fixed control voltage to the control input of the amplifier when in a second condition; and means, responsive to the condition of the switching circuit means, for reducing the output voltage of the control circuit means when the switching circuit means is in the second condition. 
     According to one embodiment of the invention the means for reducing is a variable gain amplifier circuit (or variable attenuator circuit) connected in series with the input of the control circuit means whose gain is reduced (or attenuation increased) when the switching circuit means is in the second condition, i.e. when the main amplifier is operating with the fixed control voltage. 
     According to a further embodiment of the invention, the means for reducing is a circuit arrangement which causes the output voltage of the control circuit means to assume a fixed reduced value. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a basic represenation of a circuit with a mechanical switch and a fixed control voltage used to explain the invention. 
     FIG. 2 shows a modification of the circuit shown in FIG. 1 with a transistor switch. 
     FIG. 3 is a circuit diagram of a modification of the circuit shown in FIG. 2 with coupled reduction of the control voltage according to the invention. 
     FIG. 4 is a circuit diagram with coupled reduction of the input voltage of the control circuit according to a further embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, the amplifier 1 receives a signal voltage at its input 2 and furnishes at its output 3 an amplified or changed, e.g. expanded, signal voltage. The degree of gain of the amplifier 1 is variable by means of the output signal from a control circuit 4. The control circuit 4 generates the control voltage for amplifier 1 in dependence on the signal voltages obtained at the input or at the output of the amplifier 1. In the amplifier circuit illustrated here, the signals for generating the control voltage are obtained from the input signals to the amplifier 1 because this results in a typical expander circuit (see Proceedings of the IEE, Vol. 111, No. 3, March 1964, pages 505 et seq.) The control voltage produced by control circuit 4 is fed to a control input of amplifier 1 via a switch 5 whose other switch position connects the control input of amplifier 1 with a fixed voltage source for the voltage U =1 . The range of variation of the control voltage produced by circuit 4 is assumed to range from 0.5 to 10 V. The fixed voltage U =1  is assumed to be 5 V. Although this can be realized in the classical technique as shown, it is unsuitable for monolithically integrated circuits because in that case the characteristics of mechanical switches, i.e. very high blocking resistance and very small forward resistance, cannot be realized. 
     FIG. 2 shows an integratable modification of the circuit of FIG. 1 including a transistor 8 whose collector-emitter path forms the switching path of switch 5 from the control input of amplifier 1 to the fixed voltage U =1  of FIG. 1. The base of transistor 8 is connected with a positive operating voltage via a base voltage divider 9, 10 and a switch 11. In the ON position of switch 11, as shown, the collector-emitter path of transistor 8 is blocked so that the amplifier is controlled by the output signal of control circuit 4. In the OFF position of switch 11, the collector-emitter path of transistor 8 has a low resistance and the control input of amplifier 1 lies essentially at voltage U =1 . 
     Control circuit 4 includes, in addition to the actual rectifier circuit 6, an output stage comprising a transistor 7 connected as an emitter follower, with the output electrode of transistor 7 likewise being connected with the control input of amplifier 1. In dependence on the input signal of amplifier 1, rectifier circuit 6 furnishes a control voltage between 0.5 and 10 V, independent of the position of switch 11. As long as this control voltage, with switch 11 in the OFF position and thus with the collector-emitter path of transistor 8 having a low resistance, is less than the voltage across transistor 8 plus the forward voltage of the base-emitter path of transistor 7, the base-emitter path of transistor 7 is polarized in the blocking direction. If, however, this control voltage increases further, the base-emitter path of transistor 7 is in danger. If the voltage U =1  across transistor 8 is about 5 V, as in FIG. 1, the base-emitter path of transistor 7 would be endangered by all control voltages lying between 5.6 V and 10 V, i.e. 5.6 V divided by 5 V+ average forward voltage of a silicon diode. 
     FIG. 3 shows an integratable circuit according to FIG. 2 in which the above-mentioned danger to transistor 7 caused by the switching is avoided. For this purpose, a transistor 12 is connected to rectifier 6 such that its collector-emitter path is connected between the output of rectifier 6 and a fixed direct voltage U =2 , and its base is connected with switch 11 via a base voltage divider 13, 14, so that transistor 8, which is connected with the fixed voltage source U =1 , and transistor 12, which is connected with the fixed voltage source U =2 , are switched simultaneously. With each switching of switch 11 to the positive operating voltage, i.e. the OFF position, the transistor 8 is thus switched to provide the fixed direct voltage U =1  at its collector and the transistor 12 is switched to provide a control voltage corresponding to the fixed voltage U =2  at the base of transistor 7. If voltage U =2  is dimensioned to be less than voltage U =1 , the base-emitter path of transistor 7 will always be polarized in the blocking direction when transistors 8 and 12 are switched through and will not be in danger. In FIG. 3, switch 11 is simultaneously utilized as the switch for transistors 8 and 12. However, two separate, coupled switches could also be used. 
     FIG. 4 shows another integratable modification of FIG. 2 with which the drawbacks of FIG. 2 can be avoided. As in FIG. 2, the input signals of amplifier 1 are fed to control circuit 4, but in this case via an operational amplifier 15 connected ahead of the actual rectifier circuit 6. The output electrode of this operational amplifier 15 is connected via a resistor 16 with its second input electrode (the negative input as shown) which is connected to ground via a resistor 19. The series connection of a resistor 17 and a switch 18 is connected in parallel with resistor 16. With switch 18 in the illustrated position so that only feedback resistor 16 is associated with the operational amplifier 15, rectifier 6 furnishes a control voltage as is desired for operation of amplifier 1 as an expander. If switch 18 is closed to connect resistor 17 in parallel with resistor 16, the totally effective resistance of the parallel resistors 16, 17 is known to be less than the smallest of these two resistors. Thus, the gain of the amplifier circuit 15, 16, 17, 19 is reduced and consequently the degree of attentuation is increased. The resistors 16 and 17 are advisably dimensioned in such a manner that the output voltage of the amplifier (or attenuating) circuit 15, 16, 17, 19 is reduced, for example, by the factor 10. The possible output voltage of rectifier circuit 6 is thus likewise reduced. This arrangement also avoids danger to transistor 7. Switches 11 and 18 are coupled (as shown) or may be combined. FIG. 4 shows a so-called noninverting amplifier circuit 15, 16, 17, 19. It is also possible to use here a so-called inverting amplifier circuit and to reduce its gain via a switch. 
     It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptions, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.