JPS61147611A - Agc circuit - Google Patents

Abstract

PURPOSE:To decrease number of external terminals of an IC and also number of externally mounted components by using a voltage generated at a low cut circuit as an AGC signal to control the gain of an intermediate frequency amplifier. CONSTITUTION:A connecting point between a resistor R4 and an inverting input terminal is connected to an external terminal 31 and connected to an external terminal 32 and an intermediate frequency amplifier 4 via a resistor R5. A capacitor C4 is connected between the external terminal 31 and ground and a capacitor C5 is connected between the external terminal 32 and ground. Further, a signal at a connecting point A between the resistor R4 and an inverting input of a differential amplifier 30 receives a low pass characteristic by the resistor R4 and the capacitor C4, the signal at the point A is smoothed by the resistor R6 and the capacitor C5 and the result is fed to the intermediate frequency amplifier 4 and the gain of the amplifier 4 is controlled. The output of an AM demodulation circuit 5 is subject to low cut-off characteristic by only the connection of the capacitors C4, C5 to the external terminals 31, 32 and AGC is applied to the amplifier 4.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to an AGC circuit for AM signal reception that is suitable for integration into an integrated circuit.

[Technical background of the invention] Currently, integrated circuits (hereinafter referred to as ICs) of AM/FM receivers are used.
The mainstream is generally one that shares the AM/FM output terminal.

That is, as shown in FIG. 5, an FM modulation signal is input into the IC 1, demodulated by the FM demodulation circuit 2, and introduced into the output terminal 3. Further, the intermediate frequency signal is amplified by an intermediate frequency amplifier 4, further demodulated by an AMM modulation circuit 5, and its output is led out to an external terminal 6. This external terminal 6 is led out to an output terminal 8 via a low-cut circuit 7 consisting of an external capacitor C1 and a resistor R1 inside IC1.
This output terminal 8 and the output terminal 3 from the FM demodulation circuit 2
is selectively switched by the movable contact 9, and an AM or FM demodulated output is outputted via the external output terminal 0UTi.

In addition, a resistor R2 is connected to the external terminal 6 from which the AM demodulated signal is derived.
A two-stage low-pass filter consisting of R3 capacitors C2 and C3 is connected to the intermediate frequency amplifier 4 via the external terminal 10.
is being applied.

The reason for inserting the low-pass circuit 7 in the rear terminal of the AM demodulator is that FM broadcasting requires a wide band from low to high frequencies and sufficient amplification to the low range, whereas AM broadcasting requires sufficient amplification between stations. This is because the high range cannot be extended very much due to restrictions, and for this reason, if the low range is amplified too much, it will be very difficult to open up, and the low range will need to be cut.

By the way, as shown in FIG. 6, the output signal of the AM demodulation circuit 5 is led out to the external terminal 6, the input voltage input to the low-pass circuit 7 is Ifi, and the capacitance of the capacitor C1 is CI,
When the resistance value of resistor R1 is R1, external output terminal OUT
The output voltage Vouy+ generated at + is as shown in Equation 1.

[Problems with the background technology] In the conventional AGC circuit when you want to switch and share the output terminal of AM/FMI [II signal], the problem is that external components increase when you want to low-cut only the AMI tone signal. There is. Furthermore, even if a low-cut circuit for AM is constructed inside an IC, the number of external terminals of the IC and the number of external connections also increase. Furthermore, the low-pass filter for generating the AGC signal is sometimes a two-stage low-pass filter for reducing ripples in the AGC signal, which also increases the number of external components.

[Object of the Invention 1] The present invention eliminates the above-mentioned problems and provides an AGC circuit in which the number of external terminals of an IC and the number of external parts are reduced.

[Summary of the Invention] The AGC circuit of the present invention demodulates a widened modulation signal output from an intermediate amplifier that amplifies an intermediate frequency signal using a demodulation circuit,
This demodulated signal is applied to a low-cut circuit having one terminal of a capacitor that is grounded, and the voltage generated in this low-cut circuit is used as an AGC signal to control the gain of the intermediate frequency amplifier.

[Embodiment of the Invention] An embodiment of the present invention will be described in detail below with reference to the drawings.

In this embodiment, in FIG. 1, the signal is led out from the AM demodulation circuit 5 via the differential amplifier 30 to the output terminal 8, and is outputted via the movable terminal 9 from the external output terminal 0tJT2. In addition, the negative input terminal of the differential amplifier 30 and the AMtl11
A resistor R4 is connected between the circuits 5, and is connected to the external terminal 31 from between this resistor R4 and the negative input terminal, and is also connected to the external terminal 32 and the intermediate frequency amplifier 4 via a resistor R5. A capacitor C is connected between external terminal 31 and ground.
4, a capacitor C5 is externally connected between the external terminal 32 and the ground.

Further, the signal at the connection point A between the resistor R4 and the negative terminal of the differential amplifier 30 has a low-pass characteristic due to the resistor R4 and the capacitor C4, and the signal at this point A is further smoothed by the resistor R5 and the capacitor C5. It is applied to the intermediate frequency amplifier 4 to control the gain of this amplifier 4.

Incidentally, the differential amplifier 30 is constructed as shown in FIG. That is, the output from the AM demodulation circuit 5 is applied to the differential amplifier 30 directly or via the resistor R4.

This differential amplifier 30 mainly consists of transistors Q+-Q+, resistors R6 and R7 with equal resistance values, and a current source It, 12, and includes a converting section for converting V- from voltage to current, and a current output of this converting section. It has the function of an IV converter that converts into voltage. That is, when the base potential of the transistor Q3 becomes higher than the base potential of the transistor Q4, a current flows from the emitter of the transistor Q3 to the emitter of the transistor Q4 via the resistor R6. At this time, the same level of current also flows through the resistor R7. As a result, the voltage generated across the resistor R7 is connected to the output terminal 0UT2 as an output voltage.

Note that the external capacitor C4 provides a low pass based on the resistor R4, so that the output of the output terminal 0UT2 is low-cut. Also, the power supply Ve is the output terminal 0LI7
This is for setting the second output voltage.

Here, a low-cut circuit consisting of a resistor R4 connected between the output of the AM demodulation circuit 5 and the output terminal 0LJT2 shown in FIGS. 1 and 2, a differential amplifier 30, and an external capacitor C4 is connected to the AM demodulation circuit shown in FIG. Let the output voltage of the demodulation circuit 5, in other words, the input voltage of the differential amplifier 30, be Vt, the resistance value of the resistor R4 be R4, the capacitance of the capacitor C4 be C4,
Output terminal 0UT2 when the gain of differential amplifier 30 is 1
The output voltage VIXIT2 generated in the following equation (2) has a value expressed by the following equation (2).

=V+ −[1−(1/jωC4·R4+1>1) Here, the equation (1) showing the relationship between the input and output power of the conventional low-pass circuit 7 and the equation (2) of the present invention are completely (identical). From this, it can be understood that they have the same frequency characteristics.In other words, the conventional low-cut characteristics shown in FIG. 6 and the low-cut characteristics of the present invention shown in FIG. 3 have the same transfer functions in equations (1) and (2). Therefore, they are exactly the same.

Therefore, according to this embodiment, the output of the AM demodulation circuit 5 can be low-cut by connecting only the capacitors C4 and C5 to the external terminals 31 and 32, and the AGC
can be applied.

Next, another embodiment of the present invention will be described with reference to FIG.

In this embodiment, the output of the AM demodulation circuit 5 is connected to the positive input of the -I converter 40 that converts voltage into current, and to the negative input external terminal 41 of this V-1 converter 40. It is connected to one output of converter 40. V-
The other output of converter 40 is I-, which converts current to voltage.
The V converter 42 is connected to an external output terminal 0LIT3 via an output terminal 8 and a movable contact 9, from which an output is derived.

Roughly speaking, the negative input of the V-1 converter 40 is also connected to the intermediate frequency amplifier 4 via the AGC up 43, and the gain of the intermediate frequency amplifier 4 is controlled by applying a control signal of the AGC up 43. Also AGC up 4
The output of No. 3 is also connected to an external terminal 44. Incidentally, a capacitor CIi is externally connected to the external terminal 41, and a capacitor C7 is externally connected to the external terminal 44.

By the way, the V-1 converter 40 in this embodiment has the same function as the resistor R4 shown in FIG. The cutoff frequency is [1/2π·(1/c+s)·Cs].

However, gl is the conversion conductance of the V-1 converter 40,
C6 is the capacitance of an external capacitor C6.

Therefore, even in this embodiment, since the voltage generated across the capacitor Cs is a high-cut signal, it is
It can be a C signal. Further, an AGC amplifier 43 is used in place of the resistor Rs in FIG. 1, but the operation is exactly the same.

In this embodiment as well, capacitor C6 is connected to external terminals 41 and 44.
By connecting only C7, the output of the AM12 circuit 5 can be low cut, and the intermediate frequency amplifier 4 can be subjected to AGC.

[Effects of the Invention] As described above, according to the present invention, while outputting an AM/FM demodulated signal from one terminal, it is possible to reduce the number of IC bins and the number of external components, thereby achieving low cut only for AM use.

Furthermore, since the AGC control signal is configured to pass through a two-stage low-pass filter, ripples can be reduced, and distortion caused by applying AGC can also be reduced.

[Brief explanation of drawings]

FIG. 1 is a circuit configuration diagram of the present invention, FIG. 2 is a circuit diagram of the main part of FIG. 1, FIG. 3 is a circuit diagram of the main part of FIG. FIG. 5 is a conventional circuit diagram, and FIG. 6 is a circuit diagram showing the main parts of FIG. 4......Intermediate frequency amplifier 5...
......AM demodulation circuit 30...
... Differential amplifier R4, R5 ... Resistor 04, Cs ... Capacitor 40 ... V-1 converter 42 ...
......I-V converter 43...
...AGO amplifier C6, C7...Capacitor attorney Sa Suyama - Figure 1 Figure 3 Figure 4

Claims (1)

[Claims] The amplitude modulation signal output from the intermediate frequency amplifier that amplifies the intermediate frequency signal is demodulated by the demodulation circuit, and this demodulated signal is applied to the low-cut circuit with one terminal of the capacitor grounded, and the voltage generated in this low-cut circuit is used as the AGC signal. An AGC circuit characterized by controlling the gain of an intermediate frequency amplifier.