Abstract:
Conventional circuit configurations for range changeover between different bands (UHF, VHF) in television receivers utilize MOS tetrodes whose operating point adjustment is unproblematic. The MOS tetrodes of the inventive circuit configuration comprise a constant potential at their source electrode regardless of the band that is selected. Besides the savings in terms of components, the invention simplifies the dimensioning of the circuit configuration. The invention can be used in all high-frequency receivers having a plurality of frequency ranges between which a changeover is performed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a continuation of copending international application PCT/DE99/01165, filed Apr. 16, 1999, which designated the United States. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a circuit configuration for changing over the reception bands in high-frequency receivers, particularly in television and radio receivers. High-frequency receivers serve for selecting a desired frequency from a plurality of frequencies which are fed to the receiver, for instance via an antenna. The desired frequency is separated from the rest of the received frequencies, and the signal that is transmitted with that frequency is acquired by demodulation. The reception of different television or radio programs is possible by tuning the receiver to the frequency on which the respective program is broadcast. 
     The frequencies that are used for transmission purposes are divided into ranges, which are characterized as frequency bands. Receivers can typically be set to one frequency band or to several bands. The advantage of this is that a receiving unit need not be designed for all frequencies that are to be received; rather, subreceiver units that cover only one frequency band each can be provided. The tuning circuits in the subreceiver units are then optimized to the relatively small frequency spectrum of the respective band. Because one receiver unit is not provided to which all receivable frequencies are fed, but rather one of the subreceiver units must be selected according to the frequency that must be detected, a frequency band changeover is required. With the band changeover, that subreceiver unit is active which is allocated to the frequency band in which the desired frequency lies. The frequency that is detected by the respective subreceiver unit is then fed for further processing (e.g. demodulation) to a stage which is common to all subreceiver units. 
     The frequencies that are used in television transmission are divided into three bands. The lower VHF band (VHFI) encompasses the frequency spectrum from 45 to 126 MHz; the upper VHF band (VHFII) encompasses the frequencies from 133 to 407 MHz; and the UHF band encompasses the frequency interval from 415 to 860 MHz. For the VHF and UHF bands, electrically independent subreceiver units are usually used, which contain a respective mixer and oscillator. The band changeover is accomplished via specific control signals which switch on the desired mixer and oscillator. 
     Circuit configurations for band changeover in tuners are taught by EP 0 457 934 and EP 0 457 932. The circuit configuration according to EP 0457 932 is laid out for band changeover in tuners having at least two bands. For each band, an MOS tetrode is provided, which can be activated via a switching device. The MOS tetrode of one of the preliminary stages is active in that its source terminal is connected to the reference potential of the supply voltage via a switching device. The respective preliminary stage can thus be activated via the level of the potential at the source terminal of its tetrode. So that the tetrode is inactive when the source terminal does not lie at reference potential, a well defined potential must be set at a gate terminal and at the source terminal of the tetrode via voltage dividers. The disadvantage herein is that resistors with a low tolerance range must be used for the voltage divider. Another disadvantage is that the output signal at the drain terminal of the tetrode is dependent upon the dimensioning of the voltage divider. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a circuit configuration for band changeover in high-frequency receivers which overcomes the above-mentioned disadvantageous of the prior art circuits of this general type, that is compatible with existing high-frequency receivers in terms of function and software and that has a reduced wiring and design outlay as compared to prior art circuits. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, a circuit configuration for changing between at least two bands in a high-frequency receiver, that includes a plurality of amplifier stages with a first amplifier stage associated with one of the at least two bands and a second amplifier stage associated with another one of the at least two bands. A switching mechanism is included for activating one of the plurality of amplifier stages at a time and thereby selecting one of the at least two bands. Each one of the plurality of amplifier stages includes a semiconductor switch element having two terminals connected in a main current path and having two control electrodes. One of the two terminals of the semiconductor switch element of each one of the plurality of amplifier stages is connected to a constant potential irregardless of which of the at least two bands is selected. The switching mechanism activates one of the plurality of amplifier stages at a time via one of the two control electrodes of the semiconductor switch element of the activated one of the plurality of amplifier stages. 
     The invention has the advantage that the resistors for the voltage divider at the source terminal are forgone. Accordingly, the dimensions of the layout are also reduced. Lastly, the space consumption of the inventive circuit configuration is reduced compared to conventional circuit configurations, which is particularly important in realizations in an integrated circuit. 
     In accordance with an added feature of the invention, the output signal of the semiconductor switch element is not dependent upon the wiring of the source terminal. 
     In accordance with an additional feature of the invention, the reference potential of the supply voltage is used as constant potential. Potential fluctuations can then be minimized without additional measures. 
     In accordance with another feature of the invention, one of the bands is divided into sub-bands by connecting a selection circuit. The coupling of an amplifier stage to the selection circuit is advantageous particularly for VHF band, which comprises lower and upper ranges. 
     In accordance with a concomitant feature of the invention, when the selection circuit that is provided for one range is connected to the control electrode of the semiconductor component for the other range in such a way that the selection circuit is only active when this semiconductor component is switched out. To this end, the selection circuit can be active in such a way that it is connected to the reference potential in a switchable fashion and is continuously at the supply voltage. When the selection circuit is connected to the reference potential, the control electrode of this semiconductor component is simultaneously brought approximately to the reference potential, whereupon it blocks. Simple npn transistors whose emitters are connected to the reference potential can be used for the circuit. 
     Other features which are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as embodied in a circuit configuration for band changeover in high-frequency receivers, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 shows a circuit configuration for changing between at least two bands in a high-frequency receiver. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the sole drawing figure, FIG. 1, there is shown a circuit configuration that contains a first semiconductor switch element SE 1 , which is an element of a first amplifier stage VS 1 , and a second semiconductor switch element SE 2 , which belongs to a second amplifier stage VS 2 . The amplifier stages VS 1 , VS 2  are each allocated to a respective frequency band. In the exemplifying embodiment shown in FIG. 1, the first amplifier stage VS 1  is provided for the UHF band, and the second amplifier stage VS 2  is provided for the VHF band. 
     The semiconductor switch elements SE 1 , SE 2  each comprise two control electrodes GA 1 , GB 1 ; GA 2 , GB 2  for controlling a current in a respective main current path between a drain terminal DR 1 , DR 2  and a source terminal SO 1 , SO 2 . An input terminal IN via which a control signal is fed to the circuit configuration is connected to the first control electrode GA 1  of the first semiconductor switch element SE 1  via two resistors R 1 , R 2  that are connected in series. At the common junction of the resistors R 1 , R 2 , the anode terminal of a switching diode D 1  is provided. The cathode terminal of the switching diode D 1  is connected to a node KN. The second control electrode GB 1  of the first semiconductor switch element SE 1 , to which a first high-frequency signal can be fed, is connected to a terminal for a reference potential V ss  via a resistor R 4  which is connected in series with a switch S 1 . The node KN is likewise connected to the terminal for the reference potential V ss  via a second switch S 2 . A third switch S 3  connects the terminal for the reference potential V ss  to the second control electrode GB 2  of the second semiconductor switch element SE 2  via a resistor R 5 . The first control electrode GA 2  of this semiconductor switch element SE 2  is connected to the input terminal IN via a resistor R 3 . The second control electrode GB 2  can be fed a second high-frequency signal. The switches S 1 , S 2 , S 3  together form a switching mechanism SV. They can be realized as npn transistors which are connected on the emitter side to the terminal for the reference potential V ss . The control of the switching mechanism SV occurs via the base terminals of the transistors S 1 , S 2 , S 3 . 
     The source terminals SO 1 , SO 2  of the semiconductor switch elements SE 2 , SE 2  are likewise connected to the terminal for the reference potential V ss . Additional amplifier stages that are provided for other corresponding frequency bands, and that are not represented in FIG. 1, are connected in series with the drain terminals DR 1 , DR 2 . 
     The node KN is connected to the changeover resistors R 13  and via an adjusting resistor R 12  to a supply potential V DD . The changeover resistors R 13  are each connected to the cathode terminal of a respective changeover diode D 10 . The anode terminals of the changeover diodes D 10  are connected to each other. The interconnected anode terminals of the changeover diodes D 10  are connected to the common junction of a voltage divider that consists of a resistor R 10  and a resistor R 11 . The voltage divider R 10 , R 11  is connected between the reference potential V ss  and the supply potential V DD . 
     The selection circuit shown in FIG. 1 has four changeover resistors R 13  with four appertaining changeover diodes D 10 . Besides the semiconductor switch elements SE 1 , SE 2 , each of the amplifier stages VS 1 , VS 2  contains tuning circuits, which are not shown in the Figure. The selection circuit SK of the subreceiver for the VHF band makes it possible to switch over the resonant frequency of inductors of the VHF subreceiver between the lower and upper VHF bands by means of the changeover diodes D 10 . The inductors are not shown in FIG.  1 . 
     The range changeover is accomplished using specific control signals that are applied to the switches S 1 , S 2 , S 3  of the switching mechanism SV. In the present exemplifying embodiment, digital control signals are used which are sent to the base terminals of npn transistors that comprise the switches S 1 , S 2 , S 3 . For each npn transistor, a separate digital control signal is provided. The digital control signals comprise a first voltage level (control signal low) at which the npn transistors block, and a second voltage level (control signal high) at which they conduct. The base terminals are used as control inputs of the npn transistors. 
     The circuit configuration shown in FIG. 1 is laid out such that at a given point in time the control signal high can only be applied to exactly one of the switches S 1 , S 2 , S 3 . The switches to which the switching signal high is not being fed are controlled with the switching signal low. The switches S 1 , S 2 , S 3  are addressed via a control logic that controls a microprocessor in a television device, for example. The control device SV itself can be realized as part of a PLL circuit. 
     Three separate cases can be distinguished depending on the switch S 1 , S 2 , S 3  at which the switching signal high is present. This assumes that the first amplifier stage VS 1  is laid out for the UHF band, and the second amplifier stage VS 2  is laid out for the VHF band. 
     The UHF band is active when the control signal high is fed to the third switch S 3 , and the control signal low is fed to the switches S 1  and S 2 . The switch S 3  then conducts and connects the reference potential V ss  via the resistor R 5  to the second control electrode GB 2  of the second semiconductor switch element SE 2 , whereupon this blocks. Actuation cannot be accomplished by the second high-frequency signal, which is fed via the second control electrode GB 2 . This second amplifier stage VS 2  for the VHF band is then inactive. On the other hand, the first semiconductor switch element SE 1  is active, since its second control electrode GB 1  is separated from the reference potential V ss , and actuation is accomplished via its first control electrode GA 1  by the control signal at the input terminal IN via the path with the resistors R 1  and R 2 , and via its second control electrode GB 1  by the first high-frequency signal. The path across the switching diode D 1  does not carry a signal, since the switch S 2  is open, and the potential is not lower at the cathode of the switching diode D 1  than at the anode thereof. 
     The lower VHF band (VHFI) is active when the control signal high is fed to the first switch S 1 , and the control signal low is fed to the other switches S 2 , S 3 . Via the resistor R 4  the second control electrode GB 1  of the first semiconductor switch element SE 1  is then connected to the reference potential V ss , whereupon this blocks. Control cannot be accomplished via the control signal at the first control electrode GA 1  of the first semiconductor switch element SE 1 . The UHF band is inactive. On the other hand, the second semiconductor switch element SE 2  is actuated by the control signal at the input terminal IN, which is fed to the first control electrode GA 2  thereof via the resistor R 3 . 
     The second control electrode GB 2  of the second semiconductor switch element SE 2  is not connected to the low potential. The VHF band is active. Since the switch S 2  is open, the selection circuit SK is not active. The second amplifier stage VS 2  is then tuned to the lower VHF band. 
     The upper VHF band is active when the second switch S 2  is fed the control signal high, and the remaining switches S 1  and S 3  are fed the control signal low. The switching diode D 1  switches, since its cathode is connected to the reference potential V ss  and has a lower potential than its anode. Via the switching diode D 1  and the resistor R 2 , the first control electrode GA 1  of the first semiconductor switch element SE 1  is connected to the reference potential V ss . This consequently blocks, and the UHF band is inactive. The second semiconductor switch element SE 2  is controlled by the control signal at the input terminal IN via the first control electrode GA 2  of the second semiconductor switch element SE 2  and by the second high-frequency signal via its second control electrode GB 2 . The second control electrode GB 2  is not connected to the reference potential V ss ; the VHF band is thus active. Since the switch S 2  is closed, the selection circuit SK lies between the reference potential V ss  and the supply potential V DD . A current thus flows from the higher supply potential to the lower reference potential, and the selection circuit is active. The second amplifier stage VS 2  is tuned to the upper VHF band (VHFII). 
     It is advantageous when the control signal at the input terminal IN is provided by an AGC circuit (automatic gain control). By adjusting the operating point of the respective semiconductor switch element SE 1 , SE 2 , this type of control voltage source guarantees that the output signal of the amplifier stages VS 1 , VS 2  experiences only slight fluctuations, regardless of the field strength of the signal, which is received via an antenna, for instance. 
     The value of the resistor R 1  is selected large enough that the AGC circuit is loaded only slightly given a closed switch S 2  and thus a signal flow from the input terminal IN to the reference potential V ss . 
     The resistor R 11  together with the resistor R 10  of the selection circuit SK form a voltage divider, which holds the potential at the anodes of the changeover diodes D 10  to a value that is less than the supply potential V DD . When the switch S 2  is open, the changeover diodes D 10  are blocked. Only when the switch S 2  is closed—that is, upon activation of the upper VHF band—is the cathode potential of the changeover diodes D 10  suppressed, so that these conduct. As a result, connected inductors are shorted, and the resonant frequencies of the appertaining tuning circuit is adjusted to the upper VHF band. 
     The voltage divider R 10 , R 11  can also be replaced by a diode between the supply potential V DD  and the anodes of the changeover diodes D 10 . 
     The switching diode D 1  decouples the selection circuit SK from the first control electrode GA 1  of the first semiconductor switch element SE 1 . A dual gate MOSFET is a suitable semiconductor switch element. 
     The circuit configuration for band changeover as represented in FIG. 1 can be part of a tuner-Ics with mixer, oscillator, IF, preamplifier and PLL functions.