hereinafter , a television tuner of the present invention will be described with reference to fig1 to 5 . fig1 shows the entire circuit configuration of the television tuner . a television signal received by an antenna 1 is pre - selected by an antenna tuning unit ( ant tuning unit ) 2 and is then amplified by a radio - frequency amplifying unit ( rf amp ) 3 . subsequently , the amplified signal is input to a mixer unit ( mix ) 5 after unnecessary signals are further removed by a subsequent radio - frequency tuning unit ( rf tuning unit ) 4 . a local oscillating signal from an oscillator ( osc ) ( not shown ) is input to the mixer unit s . the television signal is mixed with the local oscillating signal and converted into an intermediate frequency signal . the intermediate frequency signal is balanced - output from the mixer unit 5 . the mixer unit 5 is provided in an integrated circuit 10 . the integrated circuit 10 is provided with balanced output terminals 10 a and 10 b for outputting the intermediate frequency signal from the mixer unit 5 and balanced input terminals 10 c and 10 d for inputting the intermediate frequency signal to the integrated circuit 10 . an intermediate frequency coupling circuit ( if coupling circuit ) 6 provided between the balanced output terminals 10 a and 10 b and the balanced input terminals 10 c and 10 d is composed of a balanced circuit . an intermediate frequency tuning circuit 6 a is connected between the balanced output terminals 10 a and 10 b . an adjacent channel trap circuit 6 b is connected between the output terminal 10 a and the input terminal 10 c via a capacitor 6 c . further , a capacitor 6 d connects the output terminal 10 b to the input terminal 10 d . the intermediate frequency tuning circuit 6 a is composed of a parallel tuned circuit in which two inductors 6 a 1 and 6 a 2 connected in series are connected parallel to a capacitor 6 a 3 and a varactor diode 6 a 4 that are connected in series . a voltage applied to a power source terminal vcc is supplied to the mixer unit 5 through the two inductors 6 a 1 and 6 a 2 . at this time , the voltage of the power source terminal vcc is applied to a cathode of the varactor diode 6 a 4 through the inductor 6 a 1 . further , an anode of the varactor diode 6 a 4 is connected to the input terminal 10 d through a resistor 6 e . the adjacent channel trap circuit 6 b is composed of a parallel resonance circuit comprising an inductor 6 b 1 and a capacitor 6 b 2 . the adjacent channel trap circuit 6 b attenuates a picture intermediate frequency of an adjacent channel by the resonance frequency . an integrated circuit 10 is provided with an intermediate frequency amplifier ( if amp ) 7 coupled to the balanced input terminals 10 c and 10 d and a bias circuit 8 for supplying a bias voltage to an amplifying element ( not shown ) inside the intermediate frequency amplifier 7 . the bias circuit 8 generates a bias voltage from the power source voltage . further , the integrated circuit 10 is provided with switching means 9 whose on or off state is controlled by the intermediate frequency signal output from the intermediate frequency amplifier 7 . the switching means 9 includes two transistors 9 a and 9 b whose emitters are connected to the ground and whose collectors are respectively pulled - up to the power source terminal vcc via resistors 9 c and 9 d . a bias voltage obtained by dividing the voltage of the power source terminal vcc by resistors 9 e and 9 f is applied to a base of the first transistor 9 a . the base is coupled to an output terminal of the intermediate frequency amplifier 7 via a coupling capacitor 10 j . a collector of the first transistor 9 a and a base of the second transistor 9 b are connected to each other and are also connected to the ground via a capacitor 9 g . a collector of the second transistor 9 b is connected to the input terminal 10 d via a resistor 9 h and is also connected to the ground via a resistor 9 i . the switching means 9 having the above - mentioned structure switches the voltage of an anode of the varactor diode 6 a 4 so as to convert the tuning frequency of the intermediate frequency tuning circuit 6 a into two kinds of high and low frequencies in accordance with the difference between television signal levels . in order to achieve this , when the intermediate frequency signal output from the intermediate frequency amplifier 7 is based on a television signal having at least a strong electric field or more , the first transistor 9 a is biased so as to be turned on by the intermediate frequency signal . on the other side , when the intermediate frequency signal is based on a television signal having at least a weak electric field , the first transistor 9 a is biased so as to be turned off by the intermediate frequency signal . therefore , when the level of a television signal is less than the weak electric field , the first transistor 9 a becomes an off state , and the second transistor 9 b is turned on . as a result , as shown in the equivalent circuit diagram of fig2 , the voltage of the anode of the varactor diode 6 a 4 becomes nearly 0 volt , and the voltage between two terminals thereof is equal to the voltage of the power source terminal vcc . therefore , the capacitance of the varactor diode 6 a 4 becomes small . a tuning frequency of the intermediate frequency tuning circuit 6 a at this time , for example , is set in accordance with the values of the inductors 6 a 1 and 6 a 2 and the capacitor 6 a 3 to have a frequency close to a picture intermediate frequency p ( 58 . 75 mhz ) of the television system of japan , as shown in fig3 . further , when the level of the television signal is more than the strong electric field , the first transistor 9 a is turned on , so that the second transistor 9 b is turned off . as a result , as shown in the equivalent circuit diagram of fig4 , the voltage obtained by dividing the voltage of the power source terminal vcc by the resistors 9 d and 9 i is applied to the anode of the varactor diode 6 a 4 . therefore , the voltage between both terminals of the varactor diode 6 a 4 becomes less than the voltage of the power source terminal vcc , and thus the capacitance of the varactor diode 6 a 4 becomes large . for example , as shown in fig5 , the tuning frequency of the intermediate frequency tuning circuit 6 a at this time is set in accordance with the values of the inductors 6 a 1 and 6 a 2 and the capacitor 6 a 3 to have a frequency close to an intermediate frequency c ( 56 . 5 mhz ) between the picture intermediate frequency p ( 58 . 75 mhz ) and a sound intermediate frequency s ( 54 . 25 mhz ) of the television system of japan . that is , the tuning frequency of the intermediate frequency tuning circuit 6 a is switched to two kinds of frequencies in accordance with the difference between input television signal levels . in case of a weak electric field , the tuning frequency is switched to a frequency close to the picture intermediate frequency . in case of a strong electric field , the tuning frequency is switched to a frequency close to the sound intermediate frequency . in the ntsc system adapted in japan , unite state of america , and so on , the picture intermediate frequency is higher than the sound intermediate frequency . thus , in case of the weak electric field , the tuning frequency is high . on the other side , in case of the strong electric field , the tuning frequency is low . when a system in which the picture intermediate frequency is lower than the sound intermediate frequency is adopted , the tuning frequency is low in case of the weak electric field . on the other hand , the tuning frequency is high in case of the strong electric field . further , when the level of the television signal is an intermediate level between the weak electric field and the strong electric field , that is , an intermediate electric field , the tuning frequency of an intermediate frequency tuning circuit may be set to a frequency close to a picture intermediate frequency , similar to the case of the weak electric field , or may be set to a frequency close to a sound intermediate frequency , similar to the case of the strong electric field . here , this setting is determined by the receiving sensitivity of the television receiver . however , since a sufficient receiving sensitivity is generally obtained in the intermediate electric field , it is advisable to set the tuning frequency of an intermediate frequency tuning circuit at a frequency close to the sound intermediate frequency , similar to the case of the strong electric field .