JPS60223270A - Automatic gain control circuit - Google Patents

Abstract

PURPOSE:To eliminate the need to prepare two AGC circuits independently of the original one by securing a peak AGC circuit function to both NTSC and PAL systems and then a mean value AGC circuit function to an SECAM system with just a single AGC circuit. CONSTITUTION:The 1st and 2nd switches SW2 and 19 are turned on in an NTSC or PAL broadcast reception mode, and a modulated signal Vi is applied to an input terminal 25. The base of a control transistor TR27 is set at an earth potential by the SW2 and kept cut off. Therefore a signal VA underwent the wave detection through a wave detection circuit 23 is supplied to the base of a TR12 of a differential pair 14 via a TR10 for active filter. The collector potential of the TR12 varies in response to the signal VA, and the outut of the TR12 is supplied to the base potential of a TR7. A capacitor 22 is connected to a terminal 26. Thus a current I1 flows to the capacitor 22 when the voltage lower than the voltage V'C set at a point C is applied to the base of the TR12. Then the capacitor 22 is quickly charged. While I1=0 is satisfied when the voltage applied to the base of the TR12 is kept higher than the V'C. Therefore the AGC voltage VB is produced as the peak AGC voltage based on the V'C set at the point C.

Description

Detailed Description of the Invention (a) Industrial Application Field The present invention relates to an automatic gain control circuit for an intermediate frequency amplification (IF) system of a television receiver, and particularly to an automatic gain control circuit for an intermediate frequency amplification (IF) system of a color television receiver. Regarding control circuits.

(b) Prior art In general, in color television receivers, various system parts and N
The TSC, PAL, and SECAM systems have been adopted in many countries, and for example, color television receivers that can receive both NTSC and PAL broadcasts have been proposed.

As an example, Japanese Patent Laid-Open No. 58-141092 discloses a technique for distinguishing between PAL and NTSC systems.
This merely discloses an automatic system switching circuit that automatically switches the image receiving circuit according to the received system.

On the other hand, the same color television system includes NTSC, PAL and SE.
Among the CAM systems, NTSC and PAI.

The audio system uses the FM system, and the SECAM system uses the AM system, and each of these IF circuits is required separately, and in particular, beagle (leading value) AGC and average value AGC are used as automatic gain control. I had to.

(c) Purpose of the invention The present invention provides peak AG control for NTSC and PAL systems.
The purpose of this invention is to derive an AGC voltage so that O can be operated as an average value AGO circuit for the SECAM system.

B) Structure of the Invention The present invention provides an input transistor to which an input signal is applied;
a differential amplifier consisting of a pair of transistors in which the output side of the input transistor is connected to a first transistor and the pace of the second transistor is connected to a voltage dividing circuit; a first shunt and a second shunt provided at the output load ends of each of the transistors; an integrating circuit connected between the output terminal and the input terminal connected to the first shunt; A switching means connected between one end of the shunt No. 2 and the ground, and a switching means connected to the output terminal A.
A GC control circuit is configured to apply a peak AGC voltage or an average value AGC voltage to the AGC control circuit from the output terminal by opening and closing the opening/closing means.

(E) Embodiment To explain the present invention according to 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 each point. FIG.

In FIG. 1, (1) is an AGC control circuit, (2) is a first switch, (3H4) is a current mirror provided as a first shunt (the first and second transistors forming a clause, 6) (The force is transmitted through a current mirror provided as a second shunt (the third and fourth transistors forming J8, (9
) is the control transistor, the input transistor that constitutes the tilt active filter I, the u needle ■ is the differential transistor that constitutes the differential pair 0, and the H+1 resistor (161 (17
>C1 second voltage divider circuit, (tit statement second switch, (4) is a constant current source, υ (2 bottles 1 integrating capacitor, (2) is a detection circuit, (24) is an input Resistance, (c)
(26) indicate an input terminal and an output terminal, respectively.

In FIG. 2, (4) and (5) are Darlington-connected transistors corresponding to the first transistor (4) in FIG.
7) is a constant current transistor, and I3 is a resistor and diode that constitute a bias circuit (b).

Next, the present invention will be explained using FIG. 1. First, when an average value type AGC voltage is derived, that is, when an audio signal
When receiving SRCAM broadcasting using the SRCAM method, if the first switch (2) and the second switch Ql are turned off, the input signal ■i applied to the input terminal (c) will be as shown in Figure 3 (a) or (b).
As shown in the figure, the AM wave changes depending on the size of the signal (carrier). The output (vA) of the detection circuit (c) is a demodulated signal that changes in magnitude of the signal (carrier) as shown in Figure 3 (c).
As shown in (2), it appears on the output side of the detection circuit (C) and is applied to the input resistor (C).

At this time, since the second switch ill is off, the divided voltage (V,) obtained by the voltage dividing circuit turns on the transistor α of the differential pair 0.

, :, : Te! PK: (ll (17) (1M value wo R1,
Rt, R3 tor, vc=v, c (R2+R
,)/(RI+R2+R,), so the transistor a dark 1 is on, (15 is off and the transistor (450) is at almost ground potential and the transistor (4') is on. At the same time, the pace potential of the control transistor 07) is also high, so the control transistor (27) is also turned on, and from the current
The voltage (V,) at the output terminal (261) is the differential pair Qll
When the voltage vA is integrated by the resistor c39 and the capacitor an123 and the DC component is taken out as a value (average value) centered on the pace potential (VC) of one transistor (131), when the amplitude is large, the When the amplitude is small, the waveforms are as shown in FIGS. 3(a) and 3(c), and as shown in FIGS. 3(b) and 2).

The broken lines in FIGS. 3(c) and 3(c) indicate the level of the AGC voltage appearing as the above-mentioned average value.

Next, when the circuit of the present invention is used to derive the peak (leading value) type AGC voltage, that is, when the audio signal is an FM system NTSC or PAL broadcast, the first switch (2) and the second switch (4) are turned on. do. A modulated signal ■i is applied to the input terminal Q1 as shown in FIG. 3(E).

At this time, since the first switch (2) is on, the pace of the control transistor η is at the ground potential and is cut off, so that the pace of the control transistor η is cut off.
The signal (V,) detected at is applied to the base of the transistor a2 of the differential pair Qj) via the active filter transistor (10), and the collector potential of the transistor a4 changes accordingly. , the output of the transistor α2 is added to the pace potential of the transistor (7). Here, since a capacitor 0 is connected to the terminal 2, the voltage at the 0 point is ■.

'a '''' VccRt / (R+ + R2), and the voltage ■. When a lower voltage is applied to the base of the transistor (14), the current ■.
21 and is rapidly charged.

On the other hand, when the voltage applied to the base of the transistor @ is higher than the voltage N, I=0. Therefore, the AGC voltage (Vl) appears as a peak λGC voltage such that AGC is applied at the synchronization tip with reference to the zero point voltage (V, ).

Note that when the voltage (VL) at the 0 point is equal to the average AGC voltage, the detection level is small, so the button and the second switch 4 are turned on to change the reference potential at the 0 point.

Next, FIG. 2 is an example showing the specific configuration of FIG. 1,
The constant current source (a) is composed of a resistor (ell), a 3L diode (to), and a transistor (to), and when obtaining the average value AGC voltage, the first switch (2) is used as in Fig. 1 above. and the second switch 11 is turned off, I,
= I, and the average value of the voltage is derived by integrating the detection output with an integrating circuit consisting of an input resistor (total) and a Conde/Sashi 1) (Company).

At this time, the electrical current is almost zero (II ” I2 >> I
s).

On the other hand, when deriving the peak type A cx c voltage, the first switch (2) and the second switch I are set to ON, so the voltage at the 0 point (Vc
When a lower voltage is applied to the base of differential transistor u2, current 1 flows into capacitor 4 and charges it quickly. On the contrary, the voltage applied to the base of the transistor a2 is the voltage (2).

When it is higher, I=0, and the current flows into capacitor 4.

However, since the electric current at this time is reduced to a minute current as mentioned above, it is almost zero, so the voltage at the 0 point (
The AGC voltage appears at the synchronization tip with reference to Vc).

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 a switching means, and especially when the embodiment shown in FIG. can be achieved.

[Brief explanation of the drawing]

Figure 1 is a basic circuit diagram of the automatic gain control circuit of the present invention;
The figure is a circuit diagram showing one 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, (Spinning 8)...Current mirror, (9)...
・Control transistor, I...active filter,
σ\...Differential pair, 01...Second switch, Qυ
@...Integration capacitor, (231...Detection circuit. Applicant: Sanyo Electric Co., Ltd. and 1 other representative: Patent attorney: Yasuo Sano

Claims (1)

[Claims] (1) An input transistor to which an input signal is applied to the pace, and a pair of transistors, the output side of which is connected to a first transistor, and the base of a second transistor connected to a voltage divider circuit. a dynamic amplifier, a first shunt and a second shunt connected to the output load end of each of the first or second transistor of the differential amplifier, and an output provided in the first shunt. It consists of an integrating circuit connected between the terminal and the input terminal, a switching means connected between one end of the second shunt and the ground, and an AGC control circuit connected to the output terminal, and the switching means Peak AGC voltage or average value AGC from the output terminal
An automatic gain control circuit characterized in that a voltage is applied to the AGC control circuit.