Abstract:
A level shifting circuit capable of operating with a wide range of impedances of a signal source without backflow of current and without a buffer circuit. A feedback element is coupled between an inverting input and an output of an operational amplifier. An output of a current source is connected to the inverting input of the operational amplifier while the signal to be level shifted is applied to the non-inverting input thereof. The current source may be a current mirror circuit and a temperature compensating diode may be coupled in series with the feedback element.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a level shifting circuit, and more particularly to a level shifting circuit using an operational amplifier. 
     A prior art level shifting circuit using an operational amplifier is shown in FIG. 1 in which an input voltage V i  to be level shifted is applied through a resistor R 1  to a non-inverting input of the operational amplifier 1 while applying thereto a level shifting voltage E through a resistor R 2 . Negative feedback using an output voltage V 0  of the operational amplifier 1 is provided through a negative feedback circuit including resistors R 3  and R 4 . 
     Provided that the operational amplifier 1 is substantially ideal, that is if its gain and its input impedance are essentially infinite, and the resistances of the resistors R 1  through R 4  are all equal to R, then the non-inverting and inverting input voltages V+ and V- can be respectively expressed as follows: 
     
         V+=(E+V.sub.i)/2, and 
    
     
         V-=V.sub.0 /2.                                             (1) 
    
     Since +V is approximately equal to V-, the following equation can be obtained from the above equations (1). 
     
         V.sub.0 =V.sub.i +E.                                       (2) 
    
     As is apparent from the above, the output voltage V 0  of the operational amplifier 1 is level shifted by the voltage E with respect to the input voltage V i . 
     In order to satisfy the above equation (2), it is necessary that the impedance of a signal source be zero or extremely small in comparison with the resistance value R of the resistors. 
     Due to the specific relation between the input voltage V i  and the level shifting voltage E, a current may flow back to the input side of the operational amplifier 1 from the level shifting power source E. Such a condition should be avoided, depending upon the nature of the signal source. Accordingly, in the case where the backflow of the current must be prevented or the impedance of the signal source is not neglegible, it is necessary to provide a buffer circuit. 
     Moreover, there is a requirement that the level shifting voltage E be varied in the circuit shown in FIG. 1. In order to comply with such a requirement, however, a disadvantage is introduced in that the construction of the level shifting voltage source becomes complicated. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the invention is to provide a level shifting circuit eliminating the aforementioned disadvantages. 
     In accordance with the invention, a feedback element is provided between an inverting input of an operational amplifier and an output thereof, an output of a current source is connected to the inverting input of the operational amplifier, and an input voltage to be level shifted is applied to the non-inverting input of the operational amplifier. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram showing a prior art level shifting circuit; 
     FIGS. 2-5 are circuit diagrams showing various embodiments of a level shifting circuit according to the invention; 
     FIGS. 6-9 are circuit diagrams showing various applications of the invention; and 
     FIG. 10 is a simplified diagram showing a operative section of a receiver in FIG. 9. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention will be described with reference to the drawings. 
     FIG. 2 is a circuit diagram showing a preferred embodiment of a level shifting circuit of the invention. An input voltage V i  to be level shifted is applied to a non-inverting input V+ of an ideal operational amplifier 1. A negative feedback resistor element R 5  is connected between an inverting input V- of the operational amplifier 1 and the output V 0  thereof. An output of a current source I 0  is connected to the inverting input V- of the operational amplifier. 
     Because the current of the current source I 0  flows through feedback resistor element R 5 , the following equation is established: 
     
         V.sub.0 =V-+I.sub.0 R.sub.5.                               (3) 
    
     Because V+=V i , V+=V-, the following equation can be obtained from equation (3). 
     
         V.sub.0=V.sub.i +I.sub.0 R.sub.5. 
    
     (4) 
     As can be appreciated from equation (4), the input voltage V i  is level shifted by a voltage expressed by the product of the current I 0  and the resistance of the resistor R 5 . If the direction of the level shifting is desired to be reversed, the connection of the current source I 0  is reversed. By varying the resistance value R 5  or the current I 0  of the current source, the amount of the level shifting can be adjusted easily and freely. 
     FIG. 3 is a circuit diagram showing a specific example of the current source I 0  shown in FIG. 2, in which a current mirror circuit of a common-base configuration is employed. The current output is varied by adjusting the resistance value of the resistor R 6  so that the level shifting voltage is varied. 
     Referring to FIG. 4, a diode D 1  is interposed in the feedback path of the circuit shown in FIG. 3 in order to provide temperature compensation for the transistors Q 1  and Q 2  forming the current mirror circuit. 
     FIG. 5 is a circuit diagram showing another embodiment of the invention where like numerals or like symbols used in FIG. 2 are used to indicate like parts or components. In FIG. 5, the input voltage V i  is applied to the input of the operational amplifier through an input attenuating circuit composed of resistors R 7  and R 8 . The input attenuating circuit acts to lower the gain with respect to the input voltage. 
     According to the invention, as the signal source is isolated from the current source circuit which provides the level shifting operation, the impedance of the signal source is unaffected. Further, the problem that the current flows backward to the signal source does not occur. Moreover, the invention is advantageous in that the amount of level shifting can be freely set to either a plus or minus level. 
     Application of the invention to an electronic tuning receiver will next be described with reference to accompanying FIGS. 6-10. 
     In the circuit diagram of FIG. 6, reference numerals 11 and 12 denote respectively a voltage signal source and tuning section, 13 indicates a circuit of the invention inserted between a voltage signal source 11 and a tuning section 12. A voltage V i  generated at an output of the signal source 11 is supplied to the non-inverting input of an ideal operational amplifier OP in the circuit 13 through a resistor R 1 . A resistor R 2  is connected between this non-inverting input and ground. A negative feedback resistor R F  is connected between the inverting input of the operational amplifier and the output thereof. A current output of a current source I 0  is connected to the non-inverting input. 
     FIG. 7 shows an application of the invention to an FM electronic tuning receiver capable of receiving general FM broadcasts and the audio portion of TV channels in the same frequency band. Here, the output voltage from the signal source 11 has a range covering all of the FM broadcast band signals and adjacent TV channels. 
     G 1 , G 2  and G 3  in FIG. 7 are gain setting circuits, and I 1 , I 2  and I 3  are constant current sources. These circuits and current sources are, respectively, selected and connected to the non-inverting input and the inverting input of operational amplifier OP by mechanically interlocked change-over switches SW 1  and SW 2 . 
     FIG. 8 is a circuit diagram of embodiments of the gain setting circuits G 1 , G 2 , G 3  and the current sources I 1 , I 2 , I 3 . The gain setting circuit includes resistors R 2  &#39;, R 2  &#34; and R 2  &#39;&#34; having values satisfying the following equations: ##EQU1## Resistor R 2  in FIG. 6 is here replaced by the resistors R 2  &#39;, R 2  &#34; and R 2  &#39;&#34; switched by switch SW 1 . The current sources I 1 , I 2  and I 3  make up a current mirror circuit with transistors Q 1  and Q 2  whose bases are commonly connected, and resistors R 3 , R 3  &#39;, R 3  &#34; that are selected and connected by a switch SW 2  to set the current output to one of I 1 , I 2  and I 3 . 
     FIG. 9 is a circuit diagram of a shortwave receiver having, for instance three presetting operative buttons P 1 , P 2  and P 3  for the 41 meter band, two buttons P 4  and P 5  for the 31 meter band, and two buttons P 6  and P 7  for the 25 meter band. The buttons are arranged as shown in FIG. 10. The presetting operative buttons P 1  to P 7  each are coupled to like numbered ones of switches S 1  to S 7 , S 1  &#39; to S 7  &#39; and S 1  &#34; to S 7  &#34; which are interlocked mechanically wherein each operative button is coupled to three switches. One terminal of each of switches S 1  to S 7  is connected to a corresponding wiper contact of variable resistors VR 1  to VR 7  with the second contacts of the switches S 1  to S 7  coupled together and to an output of the voltage generator 11. Switches S 1  &#39; to S 3  &#39; , S 4  &#39; to S 5  &#39; and S 6  &#39; to S 7  &#39; are respectively connected in parallel between corresponding ones of the gain setting circuits G 1 , G 2  and G 3  and the non-inverting input of an operational amplifier OP. Switches S 1  &#34; to S 3  &#34;, S 4  &#34; and S 5  &#34;, S 6  &#34; and S 7  &#34; are respectively connected between the current sources I 1 , I 2 , I 3  and the inverting input of the operational amplifier OP. Using this circuit, the operability of the receiver is improved as separate operations are not needed for band changing. 
     In the above-described embodiment, only two cases, namely the use of variable resistors as the voltage generator 11 and the use of fixed resistors coupled through switches, have been described. However, the voltage signal source is not limited to these circuits only and other arrangements may be used if desired. 
     In FIG. 6, if the minimum and maximum values of the DC voltage generated by the signal source 11 are represented, respectively, by V min  and V max  and the variable voltage of the tuning section 12 used for tuning over a predetermined frequency range is in the range of V f1  to V f2 , the following condition must be satisfied in order to apply to the tuning section 12 the voltage from V f1  to V f2  when the DC voltage generated by the signal source 11 changes within the range from V min  to V max  : ##EQU2## Specifically, the resistance values of the resistors R 1  and R 2  must be set so as to satisfy the above equation, and further the current I 0  of the constant current source may be determined to satisfy the following equation: 
     
         V.sub.f1 =I.sub.0 R.sub.F. 
    
     The non-inverting input voltage V+ is: ##EQU3## When the output voltage V of the signal source 11 varies in the range from V min  to V max , the input voltage V 0  applied to the tuning section 12 varies in the range from V f1  to V f2  satisfying the following equation: ##EQU4## Thus, the frequency of the tuning section 12 is adjusted in the range from f 1  to f 2 . 
     Therefore, even if the variable range of the DC voltage generated in the signal source 11 does not coincide with the variable range of the tuning voltage which is necessary for the tuning circuit to tune over the predetermined frequency range, the tuning in the circuit can be effected over the predetermined frequency range by using the full range of DC voltage generated in the signal source. 
     In accordance with this invention, it is possible to provide a receiver which is capable of reproducing FM broadcasts and sound or voice of TV signals by a single tuning circuit without lowering the tuning resolving power of the receiver. The same effect can be attained in a shortwave receiver in which plural bands are tuned with a single tuning circuit.