Frequency conversion apparatus

A frequency conversion apparatus for converting a high frequency signal into an intermediate frequency signal includes a mixer section for mixing a high frequency signal with a local oscillator signal so as to obtain an intermediate frequency signal, and an amplifier connected to the mixer section via a high frequency stopping circuit for supplying a constant direct current to the mixer section as a constant current source. A current flowing in the mixer section when the local oscillator signal is applied is kept substantially equal to a direct current flowing in the mixer section when no local oscillator signal is applied thereto. The amplifier is used for amplifying either the high frequency signal or the intermediate frequency signal, or both signals at the same time.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a frequency conversion apparatus which is 
effectively used for tuners and converters and the like. 
2. Description of the Prior Art 
In conventional frequency converters, especially one which is operated in 
switching mode by a local oscillator signal, the current flowing in the 
main terminals of component transistors suddenly increases by the 
injection of the local oscillator signal. Therefore, the difference in the 
operating current is considerable depending on the presence of the local 
oscillator signal to be injected. Accordingly, it was necessary to operate 
the frequency converter with a constant current. 
Consequently, an amplifier is directly connected to the mixer section to 
control the increase in the current and this amplifier is used as an 
amplifier for the high frequency input signal. Such configurations are 
shown in "GaAs FET up converter for TV tuner" by U. ABLASSMEIER et al, 
IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. ED-27, No. 6, June 1980. 
With such configurations, however, the amplifier is used only for 
amplification of the high frequency input signal. On the other hand, 
because the amplifier and the mixer section are directly connected, it was 
necessary to design the device while paying attention to the impedance 
matching therebetween. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a frequency conversion 
apparatus which is capable of reducing current consumption and improving 
conversion gain by using an amplifier as a constant current circuit so as 
to reduce the current consumption when the local oscillator signal is 
applied to a mixer. 
To achieve the above object, the frequency conversion apparatus of the 
present invention is so constructed that an amplifier is connected, in 
terms of direct current, to the high frequency signal input terminal of a 
mixer via a high frequency signal stopping circuit, so that the mixer and 
the amplifier are separated from each other with respect to high frequency 
signals so as to thereby suppress an increase of AC operating current 
accompanied by an increased local oscillation signal and so that the 
amplifier is also used as a high frequency input signal amplifier or as an 
intermediate frequency signal amplifier. 
With this configuration, an apparatus of low current consumption can be 
materialized. Constant and low current are realized by using an amplifier 
for a constant current circuit so as to output an amplified intermediate 
frequency signal from the mixer or so as to input an amplified high 
frequency signal into the mixer. The conversion gain is also improved. The 
frequency conversion apparatus of the present invention, when connected to 
other devices to construct a receiver, is effective to improve noise 
performance and does not considerably deteriorate the interference 
performance of the mixer. It can also be easily made into an integrated 
circuit (IC).

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 1, a mixer 5 is a double-balanced mixer, in which one main 
terminals of transistors 8 and 9 are connected to each other, one main 
terminals of transistors 10 and 11 are connected to each other, the other 
main terminals of transistors 8 and 10 are connected to each other, the 
other main terminals of transistors 9 and 11 are connected to each other, 
control terminals of transistors 8 and 11 are connected to each other, and 
control terminals of transistors 9 and 10 are connected to each other. A 
local oscillator signal supplied to a local oscillator signal input 
terminal 33 is converted by an unbalanced-to-balanced converter 4 to two 
balanced signals, which are applied to control terminals of transistors 8 
and 11 and of transistors 9 and 10, respectively. 
On the other hand, a high frequency input signal is inputted from a high 
frequency signal input terminal 34, flows through a DC blocking capacitor 
21, and enters the balanced-to-unbalanced converter 3, wherein the high 
frequency input signal is converted to two balanced signals, which are 
supplied to the one main terminals of transistors 8 and 11 and of 
transistors 9 and 10, respectively. The mixer 5 intermittently causes a 
current to flow through the main terminals by means of the local 
oscillator signal applied to the control terminals and also intermittently 
causes the high frequency input signal to be applied to one of the main 
terminals, thereby mixing the frequency. As a result of this, balanced 
intermediate frequency signals appear at the other main terminals, of 
transistors 8 and 10 and of transistors 9 and 11, respectively. The 
intermediate frequency signals are converted by a balanced-to-unbalanced 
converter 2 to an unbalanced signal, which is outputted from an 
intermediate frequency signal output terminal 37 via a DC blocking 
capacitor 16. 
The intermediate frequency signal appearing at intermediate frequency 
output terminal 37 is applied to a parallel resonator 63 composed of a 
tuning line 39 and a capacitor 23. The resonance frequency of the parallel 
resonator 63 is set to the intermediate frequency, so that the parallel 
resonator 63 acts as an intermediate frequency filter. By changing the 
position to input the signal into or the position to take out the signal 
from the tuning line 39, the input and output impedance of the 
intermediate frequency filter 63 can be changed. In this embodiment, a 
tuning line and a capacitor are used to configure the intermediate 
frequency filter 63, but any means which causes resonance may be used such 
as SAW filter, dielectric filter a crystal filter. 
The intermediate frequency signal passed through the intermediate frequency 
filter 63 enters into an input terminal 35 of an amplifier 6. The power 
supply terminal 36 of amplifier 6 is connected to one of the unbalanced 
side terminals of unbalanced-to-balanced converter 3 via a high frequency 
signal blocking circuit 7. Therefore, the overall direct current flowing 
through the main terminals of mixer 5 flow in this amplifier 6. A voltage 
is applied to a control terminal of transistor 38 which configures 
amplifier 6 to cause a direct current to flow through the main terminal of 
transistor 38, the direct current being is determined by the current 
characteristic of transistor 38 and a resistor 30. Thus, this direct 
current is equal to the overall direct current of mixer section 1. The 
potential difference between the control terminal of transistor 38 and the 
main terminal of transistor 38 connected to a grounded terminal via the 
resistor 30, i.e., the bias voltage, is fixed externally; however, the 
bias voltage of mixer 5 becomes a voltage corresponding to the overall 
direct current flowing through amplifier 6. In other words, this overall 
direct current is dominated by the current of amplifier 6. 
Therefore, the amplifier 6 operates as a constant current source unless it 
is not a large signal amplifier. If the main electrodes of transistors 8, 
9, 10 and 11 are intermittently conduct by the local oscillator signal, 
the mixer section 1 operates at the constant current by the constant 
current operation of amplifier 6, so that the operating current is not 
increased by the injection of the local oscillator signal. The 
intermediate frequency signal applied to the control terminal of 
transistor 38 via a DC blocking capacitor 18 is amplified by transistor 38 
and taken out from one main terminal of transistor 38 connected to the 
power supply terminal 36 via a choke coil 23. 
The other main terminal of transistor 38 is grounded for high frequency via 
a capacitor 20. The intermediate frequency signal taken out from the one 
main terminal of transistor 38 passes through a DC blocking capacitor 19, 
and is taken out from an output terminal 40 to be supplied to a circuit in 
the next stage. 
To prevent intermixing between the high frequency input signal supplied 
from high frequency signal input terminal 34 to mixer 5 via 
unbalanced-to-balanced converter 3 and the intermediate frequency signal 
appearing in choke coil 23 of amplifier 6, the high frequency signal 
stopping circuit 7 is connected between the balanced-to-unbalanced 
converter 3 and the current terminal 36 of amplifier 6. Since the high 
frequency input signal and the intermediate frequency signal are 
sufficiently attenuated by means of capacitors 14, 15 and a resistor 31, 
the balanced-to-unbalanced transducer 3 and amplifier 6 are completely 
separated from each other with respect to high frequency. Therefore, mixer 
section 1 and amplifier 6 are independent of each other for high frequency 
signals, allowing only a direct current to flow through both mixer section 
1 and amplifier 6 via the resistor 31. Power is supplied to mixer section 
1 from a power supply terminal 32 via a choke coil 24. The bias voltages 
of the control terminals of transistors 8, 9, 10 and 11 in mixer section 1 
and the control terminal of transistor 38 in amplifier 6 are respectively 
obtained by dividing the power source voltage by resistors 25, 27 and 29. 
That is, the bias voltages are applied from the connecting point of 
resistors 25 and 27 to the control terminals of transistors 8, 9, 10 and 
11 via a resistor 26, and from the connecting point of resistors 27 and 29 
to the control terminal of transistor 38 via a resistor 28, respectively. 
The bias voltages are determined so that the conversion loss at the mixer 
section becomes minimum and the gain and distortion characteristics of the 
amplifier become optimum. 
As above, with this embodiment according to the present invention, a 
considerable increase in AC operating current when the local oscillator 
signal is fed to the mixer section can be suppressed by operating the 
amplifier as a constant current circuit. Furthermore, since the operating 
current is shared by the mixer section and amplifier, the current 
consumption of the overall frequency conversion apparatus can be reduced, 
and the conversion gain of the frequency conversion apparatus can be 
improved by amplifying the intermediate frequency signal by the amplifier. 
Moreover, when the frequency conversion apparatus of the present invention 
is connected to other devices to configure a receiver, the effect of noise 
characteristics of the following stages can be suppressed, resulting in 
the reduced noise figure of the receiver. 
Another embodiment based on the first embodiment according to the present 
invention will now be explained with reference to FIG. 2. 
In FIG. 2, detailed descriptions of unbalanced-to-balanced converters 3, 4 
and mixer 5 are omitted because their configurations and operations are 
identical with those in FIG. 1. The intermediate frequency signals 
obtained by mixer 5, which are balanced signals, are taken out from 
intermediate frequency signal output terminals 54 and 55. 
Main terminals of the transistors composing mixer 5 are connected to the 
power supply terminal 32 via choke coils 43 and 44, respectively. The 
intermediate frequency signals taken out from the intermediate frequency 
signal output terminals 54 and 55 enter the input terminals 41 and 42 of 
the amplifier 6 via intermediate frequency filters 63, respectively. The 
intermediate frequency signal that entered through the input terminal 41 
is supplied to a control terminal of a transistor 49 via a DC blocking 
capacitor 45. The intermediate frequency signal that entered from the 
input terminal 42 is supplied to a control of a transistor 50 via a DC 
stopping capacitor 46. Here, the intermediate frequency signals may be 
directly supplied from intermediate frequency signal output terminals 54 
and 55 to input terminals 41 and 42. One main terminals of transistors 49 
and 50 are interconnected to each other and also to the ground via a 
resistor 51. The other main terminals of transistors 49 and 50 are 
connected to a power supply terminal 36 via choke coils 47 and 48, 
respectively. 
Therefore, the amplifier 6 is a differential amplifier comprising 
transistors 49 and 50 and resistor 51. The intermediate frequency signals 
that entered respective control electrodes of transistors 49 and 50 are 
amplified there and appear at the main terminals at the power source side 
of transistors 49 and 50, respectively. Since amplifier 6 is symmetrically 
configured, the intermediate frequency signal may be taken out from either 
of the main terminals of transistor 49 and 50. 
In FIG. 2, the intermediate frequency signal is taken out from the main 
terminal of transistor 50, and outputted from an output terminal 40 via a 
DC blocking capacitor 19. It is not particularly necessary to take out the 
signal from either of the transistors 49 and 50, but signals may be taken 
out from both the transistors and supplied to the next stage as balanced 
signals. The power supply terminal 36 is connected to the 
unbalanced-to-balanced converter 3 via high frequency signal blocking 
circuit 7 so that the current flowing through mixer section 1 passes 
through the high frequency signal blocking circuit 7 with the high 
frequency signal sufficiently attenuated and is fed to amplifier 6. 
Therefore, the sum of the currents flowing through the main terminals of 
transistors 49 and 50 is the very current of mixer section 1. The bias 
voltages of the transistors 49 and 50 are determined so that the 
conversion loss of the mixer section 1 becomes minimum and the gain and 
distortion characteristics of the amplifier 6 become optimum. The bias 
voltages are supplied from the connecting point of resistors 27 and 29 to 
the control terminals of transistors 49 and 50 via resistors 52 and 53, 
respectively. 
As described above, with this embodiment, since a balanced-to-unbalanced 
converter is not connected to the output of the mixer section, but the 
output of the mixer section is connected to the amplifier only via the 
intermediate frequency filter and DC blocking capacitor, it is 
advantageous for integrating the mixer section and the amplifier on a 
single semiconductor substrate. 
Now, still another embodiment based on the first embodiment according to 
the present invention will be explained with reference to FIG. 3. 
In FIG. 3, detailed description of unbalanced-to-balanced converters 3, 4 
and mixer 5 are omitted because they are constructed and operate in the 
same way as described in the case of FIG. 1. The intermediate frequency 
signals obtained by mixer 5, which are balanced signals, are taken out 
from intermediate frequency signal output terminals 54 and 55. 
Main terminals of the transistors composing mixer 5 are connected to power 
supply terminal 32 via choke coils 43 and 44, respectively. The 
intermediate frequency signals from intermediate frequency signal output 
terminals 54 and 55 enter the input terminals 41 and 42 of amplifier 6 via 
intermediate frequency filters 63, respectively. The intermediate 
frequency signal that entered through input terminal 41 is supplied to the 
control terminal of transistor 49 via DC blocking capacitor 45, and the 
intermediate frequency signal that entered through input terminal 42 is 
supplied to the control terminal of transistor 50 via DC blocking 
capacitor 46. Here, the intermediate frequency signals may be directly 
supplied from intermediate frequency signal output terminals 54 and 55 to 
input terminals 41 and 42, respectively. 
Main terminals of transistors 49 and 50 are interconnected to each other 
and further to the ground via a transistor 56. The transistor 56 may be 
either an enhancement type transistor or a depletion type transistor. The 
other main terminals of transistors 49 and 50 are connected to power 
supply terminal 36 via choke coils 47 and 48, respectively. 
Therefore, the amplifier 6 is a differential amplifier comprising 
transistors 49, 50 and 56. The intermediate frequency signals that entered 
respective control terminals of transistors 49 and 50 are amplified there 
and appear at the main terminals at the power source side of transistors 
49 and 50. Since the amplifier is configured so as to be symmetrical, the 
intermediate frequency signal is taken from either of the main terminals 
of transistors 49 and 50. The intermediate frequency signal is outputted 
from the output terminal 40 via DC blocking capacitor 19. The balanced 
intermediate frequency signals from both the transistors 49 and 50 may be 
supplied to the circuit in the next stage. The power supply terminal 36 is 
connected to the unbalanced-to-balanced converter 3 via high frequency 
signal blocking circuit 7 so that the current flowing through the mixer 
section 1 passes through the high frequency signal blocking circuit 7 with 
to the high frequency signal sufficiently attenuated and enters amplifier 
6. Therefore, the sum of the currents flowing through the main terminals 
of transistors 49 and 50 is the current flowing through transistor 56 or 
the very current of mixer section 1. 
The operating current of transistor 56 is determined so that the conversion 
loss of mixer section 1 becomes minimum and the gain and distortion 
characteristics of amplifier 6 are optimum. Since the static 
characteristic of transistor 56 is utilized by connecting the control 
terminal with one main terminal to be a constant current circuit, the 
operating current value is set by changing the static characteristic of 
transistor 56. This feature is advantageous, in addition to the features 
of the FIG. 2 embodiment, for integrating the mixer section and the 
amplifier on a single semiconductor substrate. 
Next, second embodiment of the present invention will be described. 
Referring to FIG. 4, mixer 5 is a double balanced mixer which is so 
constructed that main terminals of transistors 8 and 9 are connected to 
each other and main terminals of transistors 10 and 11 are connected to 
each other; the other main terminals of transistors 8 and 10 are connected 
to each other, the other and main terminals of transistors 9 and 11 are 
connected to each other; the control terminals of transistors 8 and 11 are 
connected to each other, and the control terminals of transistors 9 and 10 
are connected to each other. A local oscillator signal inputted to local 
oscillator signal input terminal 33 is converted by unbalanced-to-balanced 
converter 4 to two balanced signals, which are applied to the two 
interconnected control terminals of mixer 5, respectively. 
On the other hand, a high frequency input signal is supplied to input 
terminal 35 of amplifier 6, and then to the control terminal of transistor 
38 in amplifier 6 via DC blocking capacitor 18. The high frequency input 
signal is amplified by transistor 38, and taken out from the main terminal 
connected to power supply terminal 36 via choke coil 23, and applied to 
output terminal 40 via DC blocking capacitor 19. The operating current of 
transistor 38 is determined by a bias voltage applied to the control 
terminal via resistor 28, resistor 30 and the static characteristic of 
transistor 38. Capacitor 20 is used for grounding the high frequency 
signal. The high frequency input signal appearing at output terminal 40 
enters into high frequency signal input terminal 34 of mixer section 1, 
and is applied to unbalanced-to-balanced converter 3 via DC blocking 
capacitor 21 to be converted into two balanced signals, which are fed to 
mixer 5. 
The local oscillator signal and the high frequency signal are mixed by 
mixer 5, thereby producing two balanced intermediate frequency signals. 
The balanced intermediate frequency signals appear at the main terminals 
of transistors 8 and 10 and of transistors 9 and 11, respectively, which 
are connected to power supply terminal 32 via balanced-to-unbalanced 
converter 2 and choke coil 24. 
The balanced intermediate frequency signals are converted by 
balanced-to-unbalanced converter 2 into an unbalanced signal, which is 
applied to intermediate frequency signal output terminal 37 via DC 
blocking capacitor 16, and further to the next stage. 
Here, power supply terminal 36 of amplifier 6 is connected to 
balanced-to-unbalanced converter 3 via high frequency signal blocking 
circuit 7, and the direct current flowing through mixer 5 passes through 
high frequency signal blocking circuit 7 and flows into amplifier 6. The 
high frequency blocking circuit 7 sufficiently attenuates the high 
frequency signal by grounding capacitors 14 and 15, and controls voltage 
distribution to each transistor by resistor 31. The operating current 
value is determined in such a manner, as in the case of the first 
embodiment, that the conversion loss of mixer section 1 becomes minimum 
and the gain and distortion characteristics of amplifier 6 become optimum. 
The bias voltages are supplied by dividing the power source voltage by 
resistors 25, 27 and 29, from the connecting point of resistors 25 and 27 
to the control terminals of transistors 8, 9, 10 and 11 mixer 5 via 
resistor 26, and from the connecting point of resistors 27 and 29 to the 
control terminal of transistor 38 in amplifier 6 via resistor 28. 
In this embodiment, output terminal 40 of amplifier 6 and high frequency 
signal input terminal 34 of mixer section, are directly connected, but 
they may be connected via a matching circuit when their impedances are 
different from each other. 
As described above, by the second embodiment according to the present 
invention, a considerable increase in AC operating current when the local 
oscillation signal is injected into the mixer section can be suppressed by 
operating the amplifier as the constant current circuit. Further, since 
the same operating current is shared by the mixer section and the 
amplifier and the amplifier operates as a pre-amplifier of the mixer 
section, it becomes possible to improve the conversion gain of the 
frequency conversion apparatus and to suppress the influence of noise 
performance, thereby materializing the reduced noise figure of the overall 
apparatus. 
Another embodiment based on the second embodiment according to the present 
invention will hereafter be explained by referring to FIG. 5. 
In FIG. 5, detailed descriptions of balanced-to-unbalanced and 
unbalanced-to-balanced converters 2, 4 and mixer 5 are omitted because 
their configurations and operations are identical with those in FIG. 4. 
The high frequency input signal is supplied to input terminal 41 of 
amplifier 6, and further to the control terminal of transistor 49 via DC 
blocking capacitor 45. On the other hand, the control terminal of 
transistor 50 is grounded via DC blocking capacitor 46. 
Main terminals of transistors 49 and 50 are interconnected, and resistor 51 
is connected between the interconnecting point and the ground. The other 
main terminals of transistors 49 and 50 are connected to power supply 
terminal 36 via choke coils 47 and 48, respectively. Therefore, amplifier 
6 is a differential amplifier comprising transistors 49 and 50 and 
resistor 51. The high frequency input signal which is supplied to the 
control terminal of transistor 49 from input terminal 41 and accordingly 
appears also at the control terminal of transistors 50 is amplified, and 
appear at each of the main terminals at the power source side of 
transistors 49 and 50. The high frequency input signals appearing at the 
main terminals of transistors 49 and 50 are balanced signals, which are 
then outputted from output terminal 57 via DC blocking capacitor 19 and 
from output terminal 40 via DC blocking capacitor 62, respectively. 
The power source terminal 36 is connected to via high frequency signal 
stopping circuit 7 to the other main terminals of the transistors 
composing mixer 5 via resistors 58 and 59, respectively. The direct 
current flowing through mixer section 1 passes through the high frequency 
signal stopping circuit 7 so that the high frequency signal is 
sufficiently attenuated and enters into amplifier 6. Therefore, the sum of 
the currents flowing through main terminals of transistors 49 and 50 is 
the very current of mixer section 1. The bias voltages of transistors 49 
and 50 are determined so that the conversion loss of mixer section 1 
becomes minimum and the gain and distortion characteristics of amplifier 6 
become optimum. The bias voltages are supplied from the connecting point 
of resistors 27 and 29 to the control terminals of transistors 49 and 50 
via resistors 52 and 53, respectively. 
As described above, since any balanced-to-unbalanced converter is not 
connected to the input of the mixer section but the mixer section is 
connected to the amplifier only via a DC blocking capacitor, it is 
advantageous for integrating the mixer section and the amplifier on a 
single semiconductor substrate 
The resistor 51 in amplifier 6 may be replaced by a transistor. Such a 
modification is shown in FIG. 6. Referring to FIG. 6, a transistor 56 
connecting one main terminal with the control terminal is connected 
between the connecting point of the main terminals of transistors 49 and 
50 and the ground. The transistor 56 may be either an enhancement tube 
transistor or a depletion type transistor. 
A third embodiment according to the present invention will now be explained 
by referring to the drawing. 
In FIG. 7 mixer 5 is a double balanced mixer comprising transistors 8, 9, 
and 10 and 11 which are connected in the same way as described in the 
foregoing embodiments. A local oscillator signal inputted to local 
oscillator signal input terminal 33 passes through DC blocking capacitor 
22, and converted by unbalanced-to-balanced converter 4 into two balanced 
local oscillation signals. The two balanced local oscillation signals are 
fed to the control terminals of transistors 8 and 11 and of transistors 9 
and 10, respectively. At the main terminals of transistors 9 and 11 and of 
transistors 8 and 10 appear two balanced intermediate frequency signals, 
respectively, which are obtained by mixing the balanced local oscillator 
signals with the balanced high frequency signals fed to the other main 
terminals of transistors 8 and 9 and of transistors 10 and 11, 
respectively, as described before. The balanced intermediate frequency 
signals are converted by balanced-to-unbalanced converter 2 into an 
intermediate frequency signal, which is fed to an input terminal of an 
input filter 64 via DC blocking capacitor 16 and output terminal 37 of 
mixer section 1. 
On the other hand, a high frequency input signal inputted to a high 
frequency signal input terminal 66 is fed to the other input terminal of 
input filter 64. The high frequency input signal and the intermedite 
frequency signal are combined at input filter 64 to be combined signal. 
The combined signal is inputted to input terminal 35 of amplifier 6 and 
supplied to the control terminal of transistor 38 via DC blocking 
capacitor 18. 
The combined signal amplified by transistor 38, taken out from the main 
terminal of transistor 38 connected to power supply terminal 36 via choke 
coil 23, and passes through DC blocking capacitor 19 to be outputted from 
output terminal 40. The outputted combined signal is separated into a high 
frequency signal and an intermediate frequency signal by an output filter 
65. The intermediate frequency signal is supplied to the next stage 
through a terminal 67. 
On the other hand, the high frequency signal is supplied to high frequency 
signal input terminal 34 of mixer section 1. The high frequency signal 
then passes DC blocking capacitor 21, and enters into 
unbalanced-to-balanced converter 3 to be converted into the balanced high 
frequency signals, which are applied to mixer 5. 
The input filter 64 is a well-known filter is which the two input terminals 
for high frequency and intermediate frequency signals are adequately 
isolated from each other. Also, the output filter 65 is a well-known 
filter in which the two output terminals for high frequency and 
intermediate frequency signals are adequately isolated from each other. 
Accordingly, description as to specific circuit configurations of filters 
64 and 65 is omitted here. 
In view of the above, amplifier 6 acts as both a high frequency input 
signal amplifier and an intermediate frequency signal amplifier at the 
same time. 
The power source terminal 36 of amplifier 6 is connected to one of the 
unbalanced side terminals of balanced-to-unbalanced converter 3 via high 
frequency signal stopping circuit 7. Therefore, the overall direct current 
flowing through the main terminals of mixer 5 flows through amplifier 6. A 
voltage is applied to the control terminal of transistor 38 so as to cause 
a DC current to flow through the main terminals of transistor 38, the DC 
current being determined by resistor 30 and the current characteristic of 
transistor 38. Thus, this direct current is equal to the overall direct 
current of mixer section 1. The potential difference between the control 
terminal of transistor 38 and the main terminal of the same connected to 
the grounded via resistor 30, i.e., the bias voltage of transistor 38, is 
fixed externally. However, the bias voltage of mixer 5 is a voltage 
corresponding to the overall direct current flowing through the amplifier 
6. In other words, the overall direct current is dominated by the current 
of amplifier 6. Therefore, amplifier 6 operates as a constant current 
circuit as long as amplifier 6 is not a large signal amplifier. Although 
the main terminals of transistors 8, 9, 10 and 11 in mixer 5 conduct 
intermittently by the local oscillation signal, mixer section 1 is 
subjected to a constant current operation by the constant current 
operation of amplifier 6, so that the operating current is not increased 
by the injection of the local oscillation signal. 
To prevent interference between the high frequency input signal to be 
supplied to mixer 5 and the combined signal of high frequency input signal 
and intermediate frequency signal appearing in choke coil 23, a high 
frequency signal blocking circuit 7 is connected between 
unbalanced-to-balanced converter 3 and power supply terminal 36 of 
amplifier 6, causing to a sufficiently attenuation the combined signal 
consisting of the high frequency input signal and intermediate frequency 
signal by means of capacitors 14, 15 and resistor 31, thereby dividing 
completely unbalanced-to-balanced converter 3 and amplifier 6 with respect 
to high frequency. Therefore, mixer section 1 and amplifier 6 are 
independent of each other with respect to the high frequency signal, 
allowing only the direct current to flow through both mixer section 1 and 
amplifier 6 via resistor 31. 
Power is supplied to mixer section 1 from power supply terminal 32 via 
choke coil 24. The bias voltages of transistors 8, 9, 10 and 11 of mixer 
section 1 and transistor 38 of amplifier 6 are obtained by dividing the 
power source voltage by resistors 25, 27 and 29. The voltage at the 
connecting point of resistors 25 and 27 is supplied to the control 
terminals of transistors 8, 9, 10 and 11 via resistor 26, and the voltage 
at the connecting point of resistors 27 and 29 is supplied to the control 
terminal of transistor 38 via resistor 28. The bias voltages are 
determined so that the conversion loss at the mixer section becomes 
minimum and the gain and distortion characteristics of the amplifier 
become optimum. 
As above, by the third embodiment according to the present invention, 
considerable increase in AC operating current when the local oscillator 
signal is injected into the mixer section can be suppressed by operating 
the amplifier as the constant current circuit. Furthermore, since the same 
operating current is shared by the mixer section and the amplifier, 
current consumption of the overall apparatus can be reduced. The 
conversion gain of the frequency conversion apparatus can be improved by 
amplifying the high frequency input signal and intermediate frequency 
signal by same amplifier, thereby materializing the reduced noise figure 
of the overall apparatus. 
Another embodiment based on the third embodiment acccording to the present 
invention will hereafter be explained by referring to FIG. 8. 
In FIG. 8, detailed descriptions of balanced-to-unbalanced and 
unbalanced-to-balanced converters 2, 3, 4 and mixer section 1 are omitted 
because they are constructed and operated in the same way as described 
with respect to FIG. 7. 
A high frequency input signal inputted to input terminal 41 of amplifier 6 
passes through DC blocking capacitor 45, and enters into the control 
terminal of transistor 49. The amplifier 6 is a differential amplifier 
comprising transistors 49 and 50, main terminals of transistors 49 and 50 
being interconnected and further connected to the ground via resistor 51. 
The high frequency input signal at the control terminal of transistor 49 is 
amplified, taken out from the other main terminal of transistor 49, and 
outputted from output terminal 68 via DC blocking capacitor 70. The 
outputted high frequency input signal passes on input filter 69 to be 
removed of unnecessary band components, and is supplied to high frequency 
input terminal 34 of mixer section 1. The mixer section 1 mixes the high 
frequency input signal with the local oscillator signal, as described with 
respect to FIG. 7, and produces an intermediate frequency signal, which is 
taken out from intermediate frequency signal output terminal 37. The 
intermediate frequency signal passes through intermediate frequency filter 
63, and enters into input terminal 42 of amplifier 6. After this, the 
signal is supplied via DC blocking capacitor 46 to the control terminal of 
transistor 50, amplified by transistor 50, taken out from the other main 
electrode, and outputted via DC blocking capacitor 19 and output terminal 
40. At this time, since amplifier 6 is a differential amplifier, the high 
frequency input signal being amplified by transistor 49 also appears at 
output terminal 40. Accordingly, an intermediate frequency filter 63 is 
connected to output terminal 40 to remove the high frequency input signal, 
so that only the intermediate frequency signal is outputted from a 
terminal 71 to be supplied to the next stage. The other main terminals of 
transistors 49 and 50 are connected to power supply terminal 36 choke 
coils 47 and 48, respectively. 
As described above, amplifier 6 is a differential amplifier comprised by 
transistors 49 and 50 and resistor 51, wherein transistor 49 amplifies the 
high frequency input signal while transistor 50 amplifies the intermediate 
frequency signal. However, the reverse thereof may be applied because 
transistors 49 and 50 are symmetrical. 
The power supply terminal 36 is connected to unbalanced-to-balanced 
converter 3 via high frequency signal stopping circuit 7. The current 
flowing through mixer section 1 passes through high frequency signal 
blocking circuit 7 such that to be the high frequency signal is 
sufficiently attenuated and is supplied to amplifier 6. Therefore, the sum 
of currents flowing through the main terminals of transistors 49 and 50 is 
the very current of mixer section 1. The bias voltages of transistors 49 
and 50 are determined so that the conversion loss of mixer section 1 
becomes minimum and the gain and distortion characteristics of amplifier 6 
becomes optimum. The bias voltages are supplied from the connecting point 
of resistors 27 and 29 to the control terminal of transistor 49 via 
resistor 52 and to the control terminal of transistor 50 via resistor 53. 
Now, a further embodiment based on the third embodiment according to the 
present invention will be explained by referring to FIG. 9. 
Referring to FIG. 9, a transistor 56 is connected between the one main 
terminals of transistors 49 and 50 in amplifier 6 in place of resistor 51 
in FIG. 8. The one main terminal of transistor 56 is connected with the 
control terminal of the same. The transistor 56 may be an enhancement type 
transistor or depletion type transistor. 
Because the static characteristic of transistor 56 is utilized in 
connecting the control terminal and the one main terminal to be a constant 
current circuit, the operating current value is set by changing the static 
characteristic. 
FIGS. 10, 11, 12 and 13 show other embodiments of the high frequency signal 
blocking circuit used in the frequency conversion apparatus according to 
the present invention. 
FIG. 10 shows a configuration of high frequency signal blocking circuit 7, 
wherein mixer section 1 is directly connected with amplifier 6 and a 
grounding capacitor 14 is connected between the connecting point of mixer 
section 1 and amplifier 6. This configuration is the simplest 
configuration capable of attenuating the high frequency signal only by 
connecting the grounding capacitor 14 and is particularly advantageous for 
integrating the circuit. 
FIG. 11 shows a modified configuration of high frequency signal blocking 
circuit 7, wherein a coil 72 is used in place of resistor 31. When the 
resistor 31 is used, the direct current is consumed as heat generated by 
resistor 31 and the range of voltage supplied to mixer section 1 and 
amplifier 6 is narrowed by resistor 31 when the power source voltage is 
reduced. However, replacing resistor 31 with coil 72 can improve this 
problem considerably. Capacitors 14 and 15 are grounding capacitors. The 
values of coil 72 and capacitors 14 and 15 are selected to obtain the 
cut-off frequency which enables to sufficiently attenuate the unnecessary 
frequencies. 
FIG. 12 shows another configuration of high frequency signal blocking 
circuit 7, wherein a parallel circuit of a coil 74 and a capacitor 73 
replaces resistor 31. The coil 74 is used for passing direct current, and 
both ends of the high frequency signal blocking circuit 7 are grounded for 
high frequency signals by capacitor 73 and 14 or by capacitor 73 and 15. 
It is advantageous of this configuration, especially when integrating the 
circuit, that a signal grounding point may be provided, that the 
capacitance of capacitor 73 may be such a value as to sufficiently pass 
the desired frequency components, and that the parasitic capacity at coil 
74 can be utilized to a greater extent in high frequency band. 
FIG. 13 shows a configuration wherein a resistor 31 replaces coil 74 in 
FIG. 12. The capacitors 14 and 15 are grounding capacitors and either one 
thereof may be used. With this configuration, it is advantageous, 
especially when integrating the circuit, that it is easier to form the 
resistor than to form the coil. 
FIGS. 14 and 15 illustrate other embodiments of the unblanced-to-balanced 
or balanced-to-unbalanced converter of the frequency conversion apparatus 
according to the present invention. 
FIG. 14 shows a configuration wherein one main terminals of transistors 75 
and 76 are interconnected and further to one main terminal of the 
transistor 95. The sum of the currents flowing through the main terminals 
of transistors 75 and 76 can be changed by varying a voltage of a DC bias 
voltage source 80 applied to the control terminal of transistor 95. A 
power is supplied to the other main terminals of transistors 75 and 76 via 
passive elements 79 and 77 such as resistors and choke coils. To convert 
balanced signals into an unbalanced signal, the balanced signals having 
opposite phases are inputted into terminals 83 and 87 so that the 
unbalanced signal is taken out from a terminal 84 or a terminal 85. To 
convert an unbalanced signal into balanced signals, the unbalanced signal 
is inputted into one of the terminals 83 and 87, and the other terminal 
into which the signal is not inputted is grounded with respect to high 
frequency signals so that the balanced signals are taken out from 
terminals 84 and 85. The bias voltages of transistors 75 and 76 are 
supplied to this control terminals from a DC bias voltage source 82. The 
control terminals of transistors 75 and 76 are connected via a passive 
element 78 such as a coil or resistor. Since this arrangement can be 
semiconductorized, the whole part of the frequency conversion apparatus 
can be semiconductorized as an integrated circuit and therefore 
small-sized. Furthermore, it is possible to obtain a gain because of the 
active element configuration. 
FIG. 15 illustrates another configuration of the balanced-to-unbalanced or 
unbalanced-to-balanced converter which uses a hybrid circuit, wherein 
terminals 84 and 85 are for balanced signals and terminal 83 is for an 
unbalanced signal. The other side of the input/output terminal for the 
unbalanced signal of the hybrid circuit is terminated by a resistor 86. 
This configuration is suitable for super high frequency band and is 
advantageous for integrated-semiconductorization. 
As is apparent from the above descriptions, each embodiment shown above has 
a configuration wherein an amplifier acts as a constant current source for 
a mixer section, but is independent as an intermediate frequency amplifier 
and/or a high frequency signal amplifier. 
The effect of the invention will be described more specifically as to the 
amplifier which is operated as the intermediate frequency amplifier. 
FIG. 16 is illustrative of the relation between the current during DC 
operation and the current during AC operation when the local oscillator 
signal power is 10 dBm. A curve 88 represents the case of the first 
embodiment according to the present invention, indicating that the 
increase in AC operating current is within 2 mA with respect to the DC 
operating current of over 20 mA. On the other hand, a curve 87 represents 
the case of the conventional apparatus, which indicates the increase in 
the AC operating current as high as 30 mA. 
Characteristic of the drain current of AC operation versus input signal 
frequency is as shown in FIG. 17, wherein a curve 90 is the case of the 
first embodiment with the DC operating current set to 25 mA, indicating 
the current during AC operation. In this case, maximum current of AC 
operation is 26.5 mA, representing an increase of only 1.5 mA. On the 
other hand, a curve 89 is the current during AC operation in the case of 
the conventional apparatus, in which the DC operating current is set to 4 
mA. The current in the case of the conventional apparatus is increased by 
a maximum of 35 mA. Accordingly, the greater effect is achieved to reduce 
power consumption by the present invention. 
Furthermore, as shown in FIG. 18, a curve 92 represents the conventional 
apparatus and a curve 91 the embodiment according to the present invention 
which shows higher conversion gain of 6.5 dB. 
As shown in FIG. 19, a curve 93 represents the conventional apparatus and a 
curve 94 the embodiment according to the present invention, indicating a 
deteriorated 1% cross modulation characteristic of 6.5 dB with respect to 
the former. However, an excellent effect is achieved in the case of 94, in 
which the disturbing signal voltage is over 103.5 dB (of 50 ohm terminal 
value).