JPH0312830B2 - - Google Patents

Description

[Detailed description of the invention] (a) Industrial application field The present invention is directed to intermediate frequency multiplication (IF) for television receivers.
This invention relates to automatic gain control circuits for audio IF systems in color television receivers. (b) Prior art In general, various systems are adopted in color television receivers in various countries, namely NTSC, PAL, and SECAM systems. For example, color television receivers that can receive both NTSC and PAL broadcasts have been proposed. ing. As an example, Japanese Patent Application Laid-open No. 58-141092
A technique for discriminating between the PAL system and the NTSC system is disclosed, but this merely discloses an automatic system switching circuit that automatically switches the image receiving circuit according to the received system. The three television broadcasting systems mentioned above can be divided into two types depending on the modulation method of the audio signal. NTSC and
In the PAL system, audio is modulated using FM modulation and video is modulated using AM modulation, and the signals are sent using negative modulation (with the synchronization signal on the outside). In addition, in the SECAM system, both audio and video are AM modulated and sent with positive modulation (with the synchronization signal on the inside). It is necessary to apply AGC to the audio signal of either method in the IF signal state.
Peak (first value) AGC for NTSC and PAL systems
was used, and the average value AGC was used in the SECAM method. In the NTSC and PAL systems, IF video and audio signals are amplified together by an IF amplifier and detected by a detector. Therefore, for the IF video signal,
If AGC is applied, AGC is also applied to the IF audio signal. In the IF video signal waveform, the synchronization signal appears on the outside, and the level at the synchronization tip indicates the electric field strength. Therefore, it is sufficient to perform level detection at the synchronization tip, but peak detection is optimal for detecting this. If average value detection were to be performed, there would be a problem in that the detection output would vary depending on the content of the video. On the other hand, in the SECAM system, both video and audio are AM.
Since it is modulated, it must be separated before applying AGC, and only the audio IF signal is separated by a filter. For separated audio IF signals, average value AGC is used, just like AM radio AGC.
All you have to do is apply Therefore, in a crisis where the above three systems of television broadcasting can be received, the peak AGC circuit and the average value
Two AGC circuits were required, which caused problems such as an increase in the number of elements. (c) Purpose of the invention The present invention provides peak AGC for NTSC and PAL systems, and average AGC for SECAM systems.
The purpose is to derive the AGC voltage so that it can be operated as a circuit. (D) Structure of the Invention The present invention comprises an input transistor to which an input IF signal from an input terminal is applied to the base via a resistor, and first and second transistors whose emitters are commonly connected. a differential amplifier to which an output signal is applied to the base of the first transistor; a voltage dividing circuit connected to the base of the second transistor of the differential amplifier to generate two types of divided voltages; first and second current mirror circuits for inverting output signals generated from the first and second transistors of the differential amplifier;
a control transistor for establishing a SEPP (single-ended push-pull) connection between the output signal of the first current mirror circuit and the output signal of the second current mirror circuit; and stopping the operation of the control transistor. and the switch for SEPP
The output signal originating from the connection point of the connection is applied
an AGC control circuit; an average value AGC capacitor connected in series between one end of the resistor and the connection point; and a Pook AGC connected in parallel (shunt type) between one end of the resistor and the connection point. It is characterized by comprising a capacitor for. (E) Embodiment The present invention will be explained with reference to the drawings. FIG. 1 is a basic circuit diagram of an automatic gain control circuit of the present invention, FIG. 2 is a circuit diagram showing an embodiment of the same circuit, and FIG. 3 is a circuit diagram showing an embodiment of the same circuit. It is a waveform diagram of each point. In FIG. 1, 1 is an AGC control circuit, 2 is a first switch, 3 and 4 are first and second transistors that make up a current mirror 5 , and 6 and 7 are third and second transistors that make up a current mirror 8 . 4 transistor, 9 a control transistor, 10 an input transistor constituting the active filter 11 , 1
2 and 13 are differential transistors constituting the differential pair 14 ; 15 is a first transistor consisting of resistors 16, 17, and 18;
19 is a second switch, 20 is a constant current source, 21 and 22 are integrating capacitors, 23 is a detection circuit, 35 is an input resistor, and 25 and 26 are an input terminal and an output terminal, respectively. In FIG. 2, 4' and 27 are Darlington-connected transistors corresponding to the first transistor 4 in FIG. 1, 28 is a current supply transistor, 29 and 30 are power failure current transistors, and 3
1, 32, and 33 are resistors and diodes that constitute the bias circuit 34 . Next, the present invention will be explained using FIG. First, it is assumed that an average value type AGC voltage is introduced, that is, that an audio signal is received from an AM system SECAM broadcast. In this case, the input signal Vi applied to the input terminal 25 in FIG. 1 is only an AM-modulated audio IF signal, as shown in FIG. 3A or 3B. The audio IF signal shown in Figure 3 A or B is as follows:
The detection circuit 23 detects the waves as shown in FIG. 3 (C) or (D). The output signal of the detection circuit 23 is applied to the base of the differential transistor 12 via the input resistor 35 and the active filter 11 , and is compared with the divided voltage V _C of the first voltage dividing circuit 15 . Here, the values of resistors 16, 17, and 18 are R ₁ , R ₂ ,
R ₃ and turn off the second switch 19,
The divided voltage V _C is as follows: V _C =V _CC (R ₂ +R ₃ )/(R ₁ +R ₂ +R ₃ ), and the divided voltage V _C and the detected input signal are compared. The comparison current output is inverted by current mirrors 5 and 8 , the output of which is applied to the base of control transistor 9. The collector of the control transistor 9 and the collector of the second transistor 4 are commonly connected, and the common connection point is connected to the output terminal 26. The collector current of the control transistor 9 and the collector current of the second transistor 4 are
When switch 2 is off and there is no signal, the current is equal, and the current I ₁ =I ₂ . Therefore, the current generated at point B flows into the integrating capacitors 21 and 22, and average value integration is performed. Here, the output voltage V _B at the output terminal 26 is a DC voltage centered on the divided voltage V _C and is applied to the AGC control circuit 1 . The dashed lines in FIGS. 3C and 3 indicate the level of the AGC voltage appearing as the average value mentioned above. Next, the circuit of the present invention is based on the peak (leading value) type.
When deriving the AGC voltage, it is assumed that the audio signal is an FM format NTSC or PAL broadcast. In this case, the input signal applied to input terminal 25 in FIG.
Vi is a mixture of an FM-modulated audio signal and an AM-modulated video signal, as shown in Figure 3 (e).
It is an IF signal. The first switch 2 and the second switch 19 are turned on, contrary to the above-mentioned state.
The input signal shown in FIG .
1 to the base of the differential transistor 12. The active filter 11 is for trapping the audio signal, and only the video signal is applied to the base of the differential transistor 12. In response to turning on of the second switch 19, the divided voltage V _C ' at point C becomes V _C '=V _{C C} R ₂ /(R ₁ +R ₂ ). Further, in response to the first switch 2 being turned on, the control transistor 9 is turned off. Therefore, only when the base voltage of the differential transistor 12 falls below the divided voltage V _C ', a current I ₁ is generated from the first transistor 4, and the integrating capacitor 22 is rapidly charged. On the other hand, when the voltage applied to the base of the transistor 12 is higher than the voltage V _C ', I ₁ =0. Therefore, the AGC voltage V _B appears as a peak AGC voltage that is applied to the video signal at the tip of synchronization with reference to the voltage V _C ' at point C. Note that when the voltage Vi at the point C is equal to the average AGC voltage, the detection level is small, so the second switch 19 is turned on to change the reference potential at the point C. By doing so, it is possible to always obtain a constant audio detection output regardless of the broadcasting method. Next, FIG. 2 shows an embodiment showing a specific configuration of FIG.
When obtaining the average AGC voltage, the first switch 2 and the second switch 19 are connected in the same way as in FIG.
is turned off, I ₁ =I ₂ is set, and the average value voltage obtained by integrating the detection output with an integrating circuit consisting of the input resistor 35 and capacitors 21 and 22 is derived. At this time, the current I ₃ is almost zero (I ₁ =I ₂ ≫I ₃ ). On the other hand, when deriving the peak type AGC voltage, the first switch 2 and the second switch 19 are set to ON, so the voltage at point C is
When a voltage lower than V _C is applied to the base of differential transistor 12, current I ₁ flows into capacitor 22 and rapidly charges it. On the contrary, the voltage applied to the base of the transistor 12 is the voltage
When higher than V _C , I ₁ =0 and current I ₃ is discharged from capacitor 22 . The purpose of the discharge using the current I ₃ is to speed up the responsiveness of the AGC. Since the current _I3 at this time is miniaturized as mentioned above, it is almost _zero with respect to the charging current.
AGC voltage appears. In the above description, it is assumed that the input impedance of the AGC control circuit 1 is sufficiently large. (F) Effects of the Invention According to the automatic gain control circuit of the present invention, the average value AGC and the peak AGC can be adjusted regardless of whether the audio modulation method in the television broadcast signal is FM, that is, NTSC and PAL method, or AM, or SECAM method. The voltage can be derived by switching means, especially the embodiment of FIG.
When integrated into an IC, the IC can be used in combination, so the desired purpose can be achieved without having to prepare an IC suitable for each AGC method as in the past.

[Brief explanation of the drawing]

FIG. 1 is a basic circuit diagram of an automatic gain control circuit of the present invention, FIG. 2 is a circuit diagram showing an embodiment of the same circuit, and FIG. 3 is a waveform diagram at each point. Explanation of main drawing numbers, 1...AGC control circuit, 2...
...first switch, 5 , 8 ...current mirror,
9... Control transistor, 11 ... Active filter, 14 ... Differential pair, 19... Second switch, 21, 22... Integrating capacitor, 23...
Detection circuit.

Claims (1)

[Claims] 1. Consists of an input transistor to which an input IF signal from an input terminal is applied to the base via a resistor, and first and second transistors whose emitters are commonly connected, and the output signal of the input transistor is is applied to the base of the first transistor; a voltage dividing circuit connected to the base of the second transistor of the differential amplifier and generating two types of divided voltages; first and second current mirror circuits for inverting output signals generated from the first and second transistors of the amplifier; and an output signal of the first current mirror circuit and an output signal of the second current mirror circuit are SEPP. (single·
A control transistor for establishing an end-push-pull connection relationship, a switch for stopping the operation of the control transistor, and an AGC control circuit to which an output signal generated from the connection point of the SEPP connection is applied. , a capacitor for average value AGC connected in series between one end of the resistor and the connection point, and a capacitor for peak AGC connected in parallel (shunt type) between one end of the resistor and the connection point. , and applies a peak AGC voltage or an average AGC voltage to the AGC control circuit from the connection point in response to switching of the divided voltage of the voltage dividing circuit and switching of the switch. Gain control circuit.