JPS6367769B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to radio receivers and other receiving circuits, and provides an excellent receiving circuit that can reduce signal distortion when receiving large input signals and significantly improve S/N with a simple configuration. The purpose is to provide

Another purpose is to make switching between AM and FM extremely easy by using an intermediate frequency amplification circuit that shares AM and FM signals.

An embodiment of the receiving circuit of the present invention will be described below with reference to the drawings. In the figure, 1 is
A frequency conversion circuit for AM signals; 2 is a local oscillation circuit that outputs a local oscillation output to be added to the frequency conversion circuit 1; 3 is an intermediate frequency amplification circuit for both AM and FM;
4 is an AM detection circuit, 5 is an FM intermediate frequency amplification circuit,
6 is an FM detection circuit.

First, we will explain the operation during AM reception. When switch 24 is turned to side a and turned on,
AM mode is activated. The received signal tuned by the antenna coil 13, variable capacitor 11, and capacitor 12 is applied to the AM frequency conversion circuit 1.
On the other hand, in the local oscillation circuit 2, the secondary winding of the tuning coil 19 is connected to the resistor 21 at the collector of the transistor 202 and the base of the transistor 205 in the local oscillation circuit 2.
4 and thus coil 1
The output appearing at the secondary winding of 9 is connected to the resistor 214, the base-emitter resistor 204 of the transistor 205,
203, it is amplified in the same phase as the emitter and collector of the transistor 202 to create a positive feedback loop and oscillate. This oscillation signal is applied to the transistor 103 of the frequency conversion circuit 1 via the emitter follower of the transistor 201. Then, the transistors 103 and 102 operate as an emitter-coupled differential amplifier, and the collectors of the transistors 103 and 102 take out oscillation outputs with opposite phases to each other, and the emitters of the frequency conversion transistors 104 and 105 and the emitters of the transistors 106 and 107 An oscillation signal is added to. On the other hand, the signal received by the antenna coil 13 is applied to the bases of transistors 104, 107, and the transistors 104, 105 and 106, 107 are mixed with the oscillation signal from the emitters coupled to each other.
Intermediate frequency coil 15 connected to the collector of 07
The intermediate frequency signal is extracted from the capacitor 16,
It is applied to the base of the transistor 301 of the intermediate frequency amplification circuit 3 via the resistor 20. The intermediate frequency signal to this intermediate frequency amplification circuit 3 is transmitted to the transistor 30.
It is amplified by three differential amplifiers 1,302, 312, 313, and 321, 322, and a capacitor 29 is connected from the collectors of transistors 321, 322 to the bases of AM detection transistors 401, 402, and to the mutually coupled emitters. AM detection is performed by transistors 401 and 402 connected to each other, and an AM detection output is taken out through a low-pass filter consisting of a resistor 30 and a capacitor 31. At this time, resistors 317, 318, and 316 are DC bias negative feedback resistors, and capacitors 22 and 23 are bypass capacitors.

The above is the operation for a small signal, and next we will explain the operation for a large input signal. When receiving a small signal, the differential amplification transistor 30 of the intermediate frequency amplification circuit 3
A current of 1,302 is supplied by a transistor 306, and a current of 1,302 is supplied by a transistor 104, 105, 106, 107, 102, 1 for AM frequency conversion.
03 is supplied with current by a transistor 110. When a small signal is received, the detected DC voltage obtained by the AM detection transistors 401 and 402 is small, but when a large input signal is received, the detected DC voltage value increases and becomes a higher voltage than when the small signal is received. Assuming that the voltage of +Vcc is 10V and the voltage drop across resistors 319 and 320 is 0.5V, AM detection transistors 401 and 402
The base voltage of is 9.5V. When there is no signal, if the base-emitter contact potential of transistors 401 and 402 is about 0.7V, the emitter voltage becomes 8.8V. On the other hand, the value of the resistor 418 is set to set the current of the transistor 417 so that the voltage drop across the resistor 415 is 0.3V, and since the contact potential of the diode 416 is approximately 0.7V, the base of the transistor 412 is approximately 9.0V. It is. Since the base of transistor 411 is about 8.8V, the base of transistor 411 is higher than transistor 412.
0.2V lower than the base of , so transistor 411 is conducting and transistor 412 is off. Therefore, there is no voltage drop across the resistor 413, the transistors 305 and 123 are in a cut-off state, and the transistors 306 and 110 are in a conductive state. However, when receiving a large input signal from a small signal input signal, the AM detection transistor 4
The DC voltage at the base of the emitter transistor 411 of 01,402 has become higher than the current 8.8V to 9.0V, and the transistor 4
11 gradually changes from a conductive state to a cut-off state, and the transistor 412 changes from a cut-off state to a conductive state.
Then, the value of the voltage drop across the resistor 413 increases, and the transistor 305 changes from a cut-off state to a conduction state. The base of transistor 306 is approximately
It is biased to 1.2V. This is done by dividing the voltage of about 1.4V between the two diodes 419 and 420 and the resistors 308 and 309 to set the base of the transistor 306 at about 1.2V. Further, the base of the transistor 110 is connected to two diodes 212,
213 voltage of 1.4V is applied. Therefore, when the voltage drop across resistor 413 becomes higher than 1.2V, transistor 305 becomes conductive and transistor 306 becomes cut off. Then, intermediate frequency amplification circuit 3
Since the currents in the transistors 301 and 302 are reduced, the gain of the intermediate frequency signal is reduced, and the signals at the collectors of the transistors 321 and 322 become too large and can be suppressed in amplitude. When an input signal is further input, the transistor 306 is completely cut off, making it impossible to control the gain of the intermediate frequency amplifier circuit 3, the intermediate frequency signal applied to the detection becomes even larger, the detected DC voltage becomes even higher, and the resistor 306
The voltage at No. 13 also becomes higher. and resistance 413
When the voltage drop becomes 1.4V or more, transistor 1
23 becomes conductive, the current of the transistor 110 decreases, and the transistor 1 of the frequency conversion circuit 1
02, 103, 104, 105, 106, 107
The current decreases, the gain of the frequency conversion circuit 1 is reduced, and the intermediate frequency signal applied to the detection circuit 4 is controlled to a certain value, thereby making it possible to extract a distortion-free detection output even in the case of a large input signal. The characteristics at this time are shown in FIG. At the output level, AGC of transistors 305 and 306 of intermediate frequency amplification circuit 3 starts from point A.
starts operating, and the AGC operation of the transistors 110 and 123 of the frequency conversion circuit 1 starts from point B.
Therefore, the distortion can be detected with a small value.
At this time, the resistors 121 and 12 in the frequency conversion circuit 1
2 functions to make it easier to completely shut off the transistors 104 to 107 by setting the emitters of the transistors 104 to 107 at a positive potential when the current in the transistor 110 decreases, thereby increasing the gain reduction effect. and transistor 306
The reason why the base voltage of the transistor 110 is lower than that of the transistor 110 is to speed up the AGC of the intermediate frequency amplification circuit 3.
This is to slow down the AGC operation of the frequency conversion circuit 1. If the AGC operation of the frequency conversion circuit 1 is made to operate faster and the AGC operation of the intermediate frequency amplifier circuit is made to operate later, the S/N will be in a poor state as shown by curve B in FIG. 3. This is because if the gain of frequency conversion circuit 1 is lowered while the S/N is poor, the S/N
doesn't get better. For this reason, the gain control (AGC) of the intermediate frequency amplifier circuit 3 is performed early and the AGC of the frequency conversion circuit 1 is operated only after the S/N ratio becomes good. Therefore, with this circuit, a signal with a good S/N ratio can be obtained as shown by curve A in FIG. Further, the resistors 211 and 124 are current limiting resistors.

Next, FM operation will be explained. switch 24
By injecting into the b side, FM from this b side
Supplies power to the tuner section. By this switching, the signal received by the tuner section is converted into an intermediate frequency signal (10.7M in this case).
Hz) to point A. The signal at point A is applied to the intermediate frequency amplification circuit 3 via the resistor 21, and also to the transistors 301, 302, 312, 31 of the three differential amplifiers.
3,321,322, and transistor 32
From the collector of transistors 505 and 506 of the FM intermediate frequency amplification circuit 5
It is added to the bases of transistors 509 and 510 of the FM detection circuit 6. The transistor 50 is added to the transistors 522 and 523 that constitute the FM detection circuit 6.
9,510, and a phase change capacitor 5 from the emitter of transistor 510.
11 and applied to the base of transistor 524. A tuning circuit 25 is provided between the base of this transistor 524 and +Vcc, and is tuned to an intermediate frequency signal of 10.7 MHz to change the phase to positive or negative depending on the frequency change due to FM demodulation from the intermediate frequency. The phase change capacitor 511 advances the phase of the signal at the emitter of the transistor 510, and the tuning circuit 25 changes the phase around this at a frequency that changes from positive to negative. This signal is applied to transistors 525 and 528 from the emitter of transistor 524. On the other hand, the signals at the emitters of transistors 510 and 509 are in opposite phase, and transistors 522 and 523 are turned on with opposite polarity.
OFF, and the collectors of these transistors 522 and 523 are connected to transistors 525, 526 and 527,
In addition to the emitter of transistor 528, a signal from transistor 524 differentially connects transistors 525,
526, 527, and 528 are turned ON and OFF, and FM detection is performed by multiplicative switching of ON and OFF of transistors 522 and 523.
The detection output signal can be taken out from the output end 27 by taking out the detection signal through the resistor 529 of the collector of 26 and 528, and by bypassing it with the capacitor 26. At this time, the base of the transistor 524 is +
Tuning circuit 2 from +Vcc so that it has the same potential as Vcc.
It is connected by the coil inside 5. Therefore, the bases of transistors 525 and 528 connected to the emitter of transistor 524 are 0.7V higher than +Vcc.
Low voltage. A diode 530 is provided to apply approximately the same voltage to transistors 526 and 527, and current flows through transistor 520. This diode 530 operates in the same way even if the base and collector of the transistor are set to +Vcc and the emitter is connected to the transistors 526 and 527. At this time, since the switch 24 is turned to the b side, no voltage is applied to the AM frequency conversion circuit 1 and the local oscillation circuit 2, so no AM operation is performed.
Only FM can be operated. Also, at this time, since the collector of the transistor 305 is connected to the power supply of the AM frequency conversion circuit 1 and the local oscillation circuit 2 via the resistor 211, the transistor 305
is in a cut-off state, and the current of the transistor 306 is constant during FM operation so that AGC operation is not performed. AM detection transistors 401 and 402 operate even during FM reception, and transistor 41
1,412 also operates, and since a DC voltage signal can be extracted from both ends of the capacitor 28 when the input signal becomes large, it can also be used as an instruction signal for a tuning indicator. On the other hand, during AM operation, voltage is applied to the base of the transistor 543 via the resistor 542, so the transistor 543 is turned on and the transistors 507, 512, 513, 516, 5
18 and 520 are turned off, and the FM intermediate frequency amplification circuit 5 and the FM detection circuit 6 do not operate. and,
During FM, no voltage is applied to the base of transistor 543, so transistors 507, 512,
513, 516, 518, 520 are turned on,
A current is made to flow so that the FM intermediate frequency amplification circuit 5 and the FM detection circuit 6 operate. Therefore, it has the advantage of requiring fewer switch circuits for switching between FM and AM. There is an advantage that it is easy to convert the area within this dashed line into an IC.

As described above, the present invention has the advantage of reducing distortion signals when receiving a large input signal and significantly improving the S/N ratio.

[Brief explanation of drawings]

FIG. 1 is an electrical connection diagram showing one embodiment of the receiving circuit of the present invention, and FIGS. 2 and 3 are diagrams explaining its operation. 1 is an AM frequency conversion circuit, 2 is a local oscillation circuit, 3 is an AM/FM intermediate frequency amplifier, 4 is an AM detection circuit, 5 is an FM intermediate frequency amplifier, and 6 is an FM detection circuit.

Claims (1)

[Claims] 1 An AM frequency conversion circuit, an intermediate frequency amplification circuit having at least two or more first and second differential amplification circuits, and an AM detection circuit, and a first a second transistor whose emitters are coupled to each other to the emitters of the first transistor that supplies current to the differential amplifier; and a fourth transistor whose emitters are coupled to the emitters of the third transistor that supplies current to the AM frequency conversion circuit. and a DC signal obtained from the output of the AM detection circuit is added to the bases of the second transistor and the fourth transistor to control the gains of the intermediate frequency amplification circuit and the AM frequency conversion circuit. In addition, the base bias of the first transistor is set to a lower voltage than the base bias of the third transistor, so that the gain control of the intermediate frequency amplifier circuit is performed prior to the gain control of the AM frequency conversion circuit. A receiving circuit characterized in that it is configured as follows.