JP2595732B2 - AM detection circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an AM detection circuit suitable for being integrated in an integrated circuit (hereinafter, referred to as an IC).

2. Description of the Related Art In recent years, AM detection circuits are often integrated in ICs.
In order to facilitate integration, a circuit with few external components is required.

Hereinafter, a conventional AM detection circuit will be described with reference to FIG. In FIG. 3, reference numeral 1 denotes an input terminal to which an AM modulation signal is input, 6 denotes an output terminal to which an AM detection signal is output, 7 denotes a power supply terminal, 9 denotes a smoothing capacitor, and 10 denotes a resistor. A time constant circuit is formed by the resistor 10 and 18 is a rectifying transistor.

Next, the operation of the conventional circuit will be described. When the AM-modulated input signal is input from the input terminal 1, the transistor 18 performs rectification, and the capacitor 9 smoothes the voltage of the peak value of the input signal and holds the charge. When the peak value of the input signal is higher than the immediately preceding peak value, the capacitor 9 is further charged, and an output signal that increases for each input peak value is output. Conversely, when the peak value of the input signal is lower than the immediately preceding peak value, the voltage between the base and the emitter of the transistor 18 does not reach the level at which the transistor 18 is turned on.
Is discharged through the resistor 10, and the output signal decreases. Usually, the time constant of the discharge is determined by the product of the resistor 10 and the capacitor 9, and is set to be a value larger than one cycle of the carrier of the input signal and smaller than one cycle of the original signal. Therefore, an output signal detected along the envelope of the AM modulation signal can be extracted from the emitter of the transistor 18.

Problems to be Solved by the Invention Normally, when detecting an AM modulated signal input at a carrier frequency of 455 KHz, the discharge time constant is set to about several μsec, and in the conventional circuit configuration, a high value resistor and Connect a high-value capacitor to the external terminals of the IC
By integrating only 18 in the IC, the integration of the AM detection circuit was realized. However, there is a disadvantage that the two components of the capacitor and the resistor are externally attached, and the integration efficiency is poor.

An object of the present invention is to eliminate such inconveniences and to provide an AM detection circuit that can reduce the value of a capacitor and can integrate a discharge circuit and a capacitor in an IC.

Means for Solving the Problems An AM detection circuit according to the present invention includes a first transistor having a base connected to an input terminal to which an AM modulation signal is input and a collector connected to a power supply terminal; and an emitter of the first transistor. A current source connected between the power supply terminal and a current source connected between the power supply terminal and the other end of the second resistor connected to the power supply terminal; A structure comprising: a second transistor to which a signal of an emitter output of the first transistor is input to a base; and a capacitor connected between an emitter of the first transistor and a collector of the second transistor. It is.

According to the above configuration, the amplification degree A of the transistor amplifier constituted by the second transistor is determined by the resistance ratio between the first resistor and the second resistor. The change in the output voltage is fed back to the input side to perform a Miller integration operation, and the equivalent capacitance added to the emitter of the first transistor is made (1 + A) times the value C of the capacitor. Then, the signal rectified by the first transistor is smoothed by its equivalent capacitance, and the operation as an AM detector is performed. Therefore, the value of the capacitor for obtaining the same time constant as the conventional example can be reduced to 1 / (1 + A).

Hereinafter, embodiments of the AM detection circuit according to the present invention will be described.
This will be described with reference to the drawings.

FIG. 1 is a circuit configuration diagram of a first embodiment of an AM detection circuit according to the present invention. In FIG. 1, 1 is an input terminal, 6 is an output terminal, 7 is a power supply terminal, 9 is a capacitor, 10, 11, and 13 is a resistor,
12 and 18 are transistors.

In FIG. 1, a transistor 18 is connected through an input terminal 1.
A DC bias and an AM modulation signal are input to the base of the transistor 18, and the transistor 18 performs a rectification operation of the AM modulation signal. The transistor 12 forms a transistor amplifier, and the amplification A of the amplification stage
Is determined by the resistance ratio between the collector load resistors 13 and 11. The amplifier and the capacitor 9 operate as Miller capacitance. An equivalent capacitance 9 'having a value (1 + A) C as viewed from the emitter side of the transistor 18 is equivalently connected between the base of the transistor 12 and the ground point. State. Here, C is the capacitance value of the capacitor 9.

Therefore, the transistor 18 that rectifies the peak value of the input signal
When the voltage of the emitter output is input, the equivalent capacitance 9 'is charged. When a signal whose level is lower than the peak value of the input signal is input, the time constant is determined by the product of the equivalent capacitance 9' and the resistor 10. The accumulated charge is discharged, and the base potential of the transistor 12 decreases according to the time constant.

Here, considering the time constant of discharge, the equivalent capacity (1+
A) Since it is given by the product of C and the value of the resistor 11, to obtain the same time constant as in the conventional example, the value of the capacitor is set to (1+
A) The value can be reduced to one-fifth, which is a circuit suitable as a circuit built in the IC.

Next, a second embodiment of the AM detection circuit of the present invention will be described.
This will be described with reference to the drawings.

FIG. 3 is a circuit configuration diagram of a second embodiment of the AM detection circuit, in which 1 is an input terminal, 6 is an output terminal, 7 is a power supply terminal, 9 is a capacitor, 13 to 15 are resistors, 16 -18 are transistors.

The difference from the first embodiment is that the transistor 17
On the input side of the transistor amplifier consisting of
An emitter follower circuit in which a resistor 14 is connected to an emitter is added, and a discharge path is configured by a constant current source 19.

Also in this case, the equivalent capacitance seen from the emitter side of the transistor 18 can double the capacitance value of the capacitor 9 by the Miller integration effect as in the first embodiment. Further, the DC impedance of the constant current source 19 is determined depending on the value of the current, and a large impedance value is easily obtained. From this, the time constant determined by the product of the DC impedance of the constant current source 19 and the equivalent capacitance has a large value, and the required capacitance value of the capacitor 9 can be reduced by increasing the impedance of the discharge circuit.

As described above, in the AM detection circuit of the present invention, the transistor amplifier having the capacitor connected between the input and output feeds back the change of the output voltage to the input side and performs Miller integration operation, so that the equivalent capacitance value is obtained. Can be increased in proportion to the degree of amplification, the required capacitor value can be significantly reduced, and the capacitor and the discharge circuit can be integrated in the IC.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a first embodiment relating to an AM detector of the present invention, FIG. 2 is a circuit diagram of a second embodiment of the present invention, and FIG. 3 is a circuit diagram of a conventional AM detector. is there. 1 ... input terminal, 6 ... output terminal, 7 ... power supply terminal, 9
... capacitors, 10, 11, 13 ... resistors, 12, 16-18 ... transistors.

Claims (1)

(57) [Claims] 1. A first transistor having a base connected to an input terminal to which an AM modulation signal is input and a collector connected to a power supply terminal, and a current connected between an emitter of the first transistor and a ground point. A source, the other end of a first resistor having one end grounded, connected to the emitter, the other end of a second resistor having one end connected to the power supply terminal connected to a collector, and an emitter output of the first transistor connected to a base. And a capacitor connected between an emitter of the first transistor and a collector of the second transistor.