JPH0640604B2 - Frequency conversion circuit - Google Patents

Description

TECHNICAL FIELD The present invention relates to a frequency conversion circuit used in a radio receiver or the like.

2. Description of the Related Art In recent years, a radio receiving circuit including a frequency conversion circuit has been integrated into an IC. An example of a conventional frequency conversion circuit will be described with reference to the drawings.

FIG. 5 shows an electrical connection diagram of a frequency conversion circuit of a conventional receiving circuit. In FIG. 5, 1 is a frequency conversion circuit integrated into an IC, 2 is an intermediate frequency signal amplifier, 3 is a detection circuit, 6 is an antenna coil, 5 and 52 are variable capacitors, 56 is an intermediate frequency transformer, 55 is an intermediate frequency filter, and 53. Is a local oscillation coil, 9, 10, 11, 12, 13, 14,
24, 25 are transistors, 4, 7, 54, 51 are capacitors, 8, 19, 20, 90 are resistors 21, 22, 2
3 is a current source.

The operation of the frequency conversion circuit configured as described above will be described below.

The input signal tuned by the antenna 6, the capacitor 4, and the variable capacitor 5 is added to the base of one transistor 9 of the transistors 9 and 10 which are emitter-coupled to each other. The local oscillation signal of the local oscillation circuit composed of the transistors 24 and 25, the coil 53, the capacitor 51, and the tuning circuit of the variable capacitor 52 is applied to the transistor 1 through the resistor 20.
In addition to the bases of 1,14, transistors 11,12 and 1
The signals applied to the emitters of 3, 14 are switched to extract the intermediate frequency signal to the intermediate frequency transformer 56 connected to the collectors of the transistors 11 and 13, and the intermediate frequency signal is added to the intermediate frequency amplifier 2 via the ceramic filter 55 and amplified. The signal is detected by the detection circuit 3, and the detected signal is taken out to the terminal 71.

Problems to be Solved by the Invention However, in the configuration as described above, the primary impedance of the intermediate frequency transformer 56 is as high as 20 to 30 KΩ,
Further, the impedance of the filter 55 such as a ceramic filter is low and often has 1 to 3 KΩ. The impedance of the primary winding of this intermediate frequency transformer is designed to be high because the frequency conversion gain formed by the transistors 9, 10, 11, 12, 13, and 14 is increased and the intermediate frequency transformer 5 is designed.
This is because a large intermediate frequency signal is taken out in FIG. Then, the secondary winding of the intermediate frequency transformer is reduced to lower the impedance to perform impedance matching with the filter 55.

Therefore, the intermediate frequency transformer 56 is required, and the coil 58 and the capacitor 57 of the intermediate frequency transformer 56 are tuned to the intermediate frequency. However, since there are variations in the values of the coil 58 and the capacitor 57, the coil is generally a coil. 58
The value of is adjusted from the outside. Therefore, it was necessary to adjust the intermediate frequency transformer when making the receiver.

In view of the above problems, the present invention provides a frequency conversion circuit having good performance by eliminating the adjustment of the filter for extracting the intermediate frequency signal from the output of the frequency conversion circuit.

Means for Solving the Problems In order to solve the above problems, the frequency conversion circuit of the present invention includes a first and a second transistor whose emitters are connected to each other, and an emitter of the first and the second transistor. A fourth and fifth transistor connected to the collector of the third transistor and having their emitters connected to each other, and the emitters of the fourth and fifth transistors connected to the collector of the sixth transistor. The emitters of the six transistors are connected to each other, and an input signal or a local oscillation signal is applied to the bases of at least one of the third and sixth transistors, and a connection point of the bases of the first and fifth transistors is provided. , A local oscillation signal or an input signal is applied to at least one of the connection points of the bases of the second and fourth transistors, and the first and fourth transistors are connected. Frequency-converted two signals of opposite polarities are taken out from both the connection points of the connection point of the second transistor and the connection point of the collectors of the second and fifth transistors, and these two signals are output to the seventh node for the differential amplifier.
In addition to the bases of the 8th and 8th transistors, it is amplified, and this signal is taken out from at least one of the 7th and 8th transistors to an unadjusted intermediate frequency filter through an output circuit whose impedance is lowered. Be prepared.

Effect The present invention increases the gain by amplifying two signals of opposite polarities, which are frequency-converted by the above configuration, with a differential amplifier,
After that, the impedance is matched to the unadjusted intermediate frequency filter through the low output circuit, so that the S / N sensitivity is good, and the adjustment can be made by using the intermediate frequency filter such as a ceramic filter.

Embodiment Hereinafter, a frequency conversion circuit according to an embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is an electrical connection diagram of a frequency conversion circuit according to an embodiment of the present invention. In FIG. 1, 1 is a frequency conversion circuit, 2 is an intermediate frequency amplifier, 3 is a detection circuit, and 6
Is an antenna coil, 5, 52 are variable capacitors, 53 is a local oscillation coil, 55 is an intermediate frequency filter, 4, 7, 51,
54 is a condenser, 8, 15, 16, 19, 20, 2
7, 28, 29, 30, 35, 37, 40 are resistors, 2
1, 22, 23, 26, 36, 39 are current sources, 9 to 1
4, 24, 25, 31 to 34 and 38 are transistors.

The operation of the frequency conversion circuit configured as described above will be described with reference to FIG.

The input signal of the variable coil 5, the capacitor 4 and the antenna coil 6 which forms a tuning circuit is applied to the base of one transistor 9 of the transistors 9 and 10 which are emitter-coupled to each other and to the emitters of the transistors 11 and 12 from the collectors of the transistors 9 and 10. In addition to the emitters of the transistors 13 and 14, the local oscillation signal of the local oscillation circuit including the transistors 24 and 25, the coil 53, the capacitor 51, and the tuning circuit of the variable capacitor 52 is transmitted to the bases of the transistors 11 and 14 via the resistor 20. In addition, the signals applied to the emitters of the transistors 11, 12 and 13, 14 are switched to switch the frequency to the connection point 85 of the collectors of the transistors 11, 13 and the connection point 86 of the collectors of the transistors 12, 14 and have mutually opposite polarities. Take out the two signals of 17 and 18 is amplified in addition to the base of differential amplifier transistors 31 and 32 via the. The collector of the transistor 31 is connected to a diode-connected transistor 33 (which may be a diode), the collector of the transistor 32 is connected to the collector of a PNP transistor 34, and the diode- or diode-connected transistor 33 and the transistor 34 operate as a current mirror circuit. The signals amplified by the collectors of the transistors 32 and 34 are added to the base of the transistor 38 for the emitter follower, and the intermediate frequency signal (here, ceramic filter) is transmitted from the emitter of the transistor 38 to the intermediate frequency filter (here, ceramic filter). Take it out. Then, this intermediate frequency signal is amplified by the intermediate frequency amplifier circuit 2 and detected by the detection circuit 3 (here, the AM detection circuit) to extract a detection output signal. When the detection circuit 3 is an AM detection circuit, a signal whose DC voltage changes according to the magnitude of the signal level is used as a signal for automatic gain control, and line 72 in FIG. 1 is used to perform automatic gain control in addition to the intermediate frequency amplifier 2. In addition to the frequency conversion circuit 1 (bases of the transistors 10 and 9) via line 73, automatic gain control is performed. At this time, if the voltage of the line 73 drops, the currents of the transistors 9 and 10 drop and the gain is controlled.

Here, the values of the load resistors 15 and 16 of the frequency conversion circuit are lower than the primary impedance of the conventional intermediate frequency transformer 56 shown in FIG. If this resistance value in FIG. 1 is increased to 20 to 30 KΩ, the voltage drop becomes large,
It cannot be used when the power supply voltage of Vcc is low. Therefore, in order to use it at a low voltage, it is necessary to reduce the values of the resistors 15 and 16. For example, in order to operate at Vcc of 1.5V, it is necessary to set the voltage drop of the resistors 15 and 16 to about 0.5 to 0.7V. The current flowing through the resistors 15 and 16 is 200
If it is μA, it becomes 2.5 to 3.5 KΩ. Therefore, the conversion gain of the frequency conversion circuit is lower than that of the conventional intermediate frequency transformer 56. Therefore, in the embodiment of FIG. 1 of the present invention, the two signals of the load resistors 15 and 16 are connected to the capacitor 1
In addition to the bases of the transistors 31 and 32 via 7 and 18, signals are taken out to the collectors of the transistors 32 and 34 after being amplified by the differential amplifier transistors 31 and 32. At this time, the transistors 11, 13 and 1 of the frequency conversion circuit
The reason why both the collector signals 2 and 14 are applied to both the transistors 31 and 32 of the differential amplifier is to increase the gain. (If only one of these signals is amplified, the gain is small.) Even if only one signal is greatly amplified by the subsequent amplifier, the gain is increased. The ratio gets worse. Therefore, in order to increase the gain of the previous stage by using both outputs of the frequency conversion circuit, both signals are amplified by using a differential amplifier.

Next, using the emitter follower transistor 38,
Since the base impedance of the transistor 38 is high, the gain is substantially determined by the load resistance 35 of the differential amplifiers 31 and 32, and a large gain can be obtained. Further, since the impedance of the emitter of the transistor 38 is low, if the impedance of the filter 55 is 3 KΩ, impedance matching can be performed by setting the resistance 40 to about 3 KΩ. That is, the emitter follower transistor 38 is used for impedance conversion.

Further, the differential amplifiers 31 and 32 and the PNP transistor may be used, but since the noise of the PNP transistor is larger than that of the NPN transistor when integrated into an IC, the NPN transistor is used instead of the PNP transistor. And these NPN transistors 31, 32
Since the base voltage of and the collector voltage of transistors 11, 13, 12, and 14 could not be the same, capacitor 1
7 and 18 are used.

Further, the emitter resistor 30 of the transistors 31 and 32 can also be operated by a current source using a transistor, but if a transistor is used, noise of the transistor becomes large, so the resistor 3 is used.
0 is used.

Two signals of opposite polarities that have been frequency-converted as described above are amplified by a differential amplifier, an intermediate frequency signal is taken out to an unadjusted filter such as a ceramic filter having a low impedance, and the intermediate frequency filter is unadjusted. A frequency conversion circuit with good performance can be constructed.

FIG. 2 shows a frequency conversion circuit showing a second embodiment of the present invention. In this FIG. 2, even if a local oscillation signal is applied to the base of one of the transistors 9 and 10 of the frequency conversion circuit and an input signal is applied to one of the bases of the transistors 11, 14 and 12, 13, It works in the same way. In the automatic gain control at this time, when a positive voltage is applied to the line 73, the transistor 95 is turned on to reduce the currents of the transistors 9 and 10 to perform automatic gain control.

FIG. 3 shows a frequency conversion circuit showing a third embodiment of the present invention. In FIG. 3, the current mirror circuit shown in FIG.
5 only, and operates in the same way as the embodiment of FIG.

FIG. 4 is a frequency conversion circuit showing a fourth embodiment of the present invention. In the fourth embodiment, the emitter follower transistor 38 shown in FIG. 3 is eliminated. This load resistance 3
Impedance matching can be achieved by setting 5 to 3 KΩ, which has a low impedance almost the same as that of the resistor 40 shown in FIGS. However, the transistor 3
Although the gains of 1 and 32 are small, the two signals of the frequency conversion circuit are amplified by using the differential amplifier, so the gain is large to some extent. Therefore, even in the case of FIG. 4, it is almost the same as that of FIG. Can be operated.

EFFECTS OF THE INVENTION As described above, the present invention has a configuration in which two signals having opposite phases that have been frequency-converted by an input signal and a local oscillation signal are amplified by a differential amplifier and then extracted as a low impedance output to an unadjusted intermediate frequency filter. By setting S /
It is possible to eliminate the adjustment of the intermediate frequency when the N sensitivity is good.

[Brief description of drawings]

FIG. 1 is an electrical connection diagram of a frequency conversion circuit according to a first embodiment of the present invention, and FIG. 2 is an electrical connection diagram of a frequency conversion circuit according to a second embodiment of the present invention, FIGS. FIG. 5 is an electrical connection diagram of a frequency conversion circuit showing the third and fourth embodiments of the present invention, and FIG. 5 is an electrical connection diagram of a conventional frequency conversion circuit. 1 ... Frequency conversion circuit, 2 ... Intermediate frequency amplifier, 3 ...
Detector, 6 ... Antenna coil, 5, 52 ... Varicon, 53 ... Local oscillation coil, 55 ... Intermediate frequency filter, 56 ... Intermediate frequency transformer, 9-14, 24, 2
5,31-34,38,95 ... Transistor, 4,
7, 51, 54 ... Capacitors, 8, 15, 16, 1
9, 20, 27, 28, 29, 30, 35, 37, 4
0,92,93,94 ... Resistance 21,22,23,2
6, 36, 39, 91 ... Current source.

Claims (1)

[Claims] 1. A first and a second transistor whose emitters are connected to each other, and a fourth transistor whose emitters are connected to a collector of a third transistor and whose emitters are connected to each other. A fifth transistor, the emitters of the fourth and fifth transistors are connected to the collector of the sixth transistor, the emitters of the third and sixth transistors are connected to each other, and the third and sixth transistors are connected. An input signal or a local oscillation signal to the base of at least one of the transistors, and the connection point of the bases of the first and fifth transistors and the connection point of the bases of the second and fourth transistors of at least one of A local oscillation signal or an input signal is applied to the connection point, and the connection point between the collectors of the first and fourth transistors and the collector of the second and fifth transistors are connected. Two signals of opposite polarities, which are frequency-converted from both connection points of the points, are taken out, these two signals are added to the bases of the seventh and eighth transistors for the differential amplifier and amplified, and this signal is A frequency conversion circuit characterized in that an intermediate frequency signal is taken out to an unadjusted intermediate frequency filter such as a ceramic filter through an output circuit having a low impedance from at least one of the seventh and eighth transistors.