JPH0346574Y2 - - Google Patents

Description

[Detailed explanation of the invention] (a) Industrial application field This invention is for detecting AM (amplitude modulation) signals.
Regarding AM detection circuits, especially IC (integrated circuits)
This paper relates to an AM detection circuit suitable for

(b) Prior art Regarding AM detection circuits, various circuits have been proposed for a long time, but the following are suitable for IC implementation.
For example, there is a circuit as shown in FIG. In FIG. 1, an AM signal applied to an input terminal 1 is applied to the base of a first transistor 3 via an input capacitor 2, and is detected by the base-emitter junction of the first transistor 3 and a detection capacitor 4. The signal is led out to the output terminal 7 via the second and third transistors 5 and 6. In that case, the capacitor that can be built into the IC is usually the capacitance of the capacitor connected externally to the IC (about 10,000PF).
Since the capacitance is small (approximately 1/500 to 1/1000 of PF), the operating current of the detection transistor must also be extremely small in order to set the detection time constant to a predetermined value. In order to satisfy this condition, in the circuit shown in FIG. 1, a constant current source 8 of I ₀ is provided, and the base current of the third transistor 6 is set to I ₀ /β (where β is the current amplification factor of the transistor). Furthermore, the base current of the second transistor 5 is set to I ₀ /β ² to keep the collector-emitter current of the first transistor 3 at a small value. Since the above β is usually about 100,
Sufficient detection could not be achieved with a two-stage connection, and the desired output could not be obtained unless a three-stage connection was made using the first to third transistors 3 to 6. In the detection circuit shown in Fig. 1, at least three transistors and a constant current source are connected in series, so the minimum voltage is 3V _BE + _V
V _BE is the base-emitter voltage of the transistor,
An operating voltage (V _X is the operating voltage of a constant current source) is required. For example, if V _BE = V _X = 0.7 V, a power supply of 2.8 V or higher was required. Furthermore, if four diodes 9 to 12 are used to obtain the operating voltage,
Since the current flowing through the diodes 9 to 12 and the current flowing through the first and second transistors 3 and 5 are significantly different, this causes a difference in rising voltage and poor temperature characteristics, resulting in a detection output obtained at the output terminal 7. This has the drawback that the initial setting voltage varies when the signal variation becomes large or when the detection output signal is used for gain control.

(c) Purpose of the invention The present invention was created in view of the above points, and the circuit configuration has been devised so that it can operate with a low power supply voltage.
It is an object of the present invention to provide an AM detection circuit, and also to provide an AM detection circuit capable of obtaining a stable detection output signal.

(d) Structure of the invention The AM detection circuit according to the invention includes a first transistor to which an input signal is applied to the base, a capacitor having one end connected to the emitter of the first transistor, and a base of the first transistor. a second transistor connected to the emitter, a current inversion circuit for inverting the collector current of the second transistor, a third transistor whose base is connected to the output terminal of the inversion circuit, and a current inversion circuit for inverting the collector current of the second transistor; A constant current source connected to a common emitter.

(E) Embodiment Figure 2 shows an embodiment of the present invention.
3 is an input terminal to which an AM input signal to be detected is applied; 14 is a first transistor to which the input signal is applied to its base via an input capacitor 15; and 16 is a connection between the emitter of the first transistor 14 and ground. 17 is a second transistor whose base is connected to the emitter of the first transistor 14; 18 is a diode-connected transistor 19;
and a transistor 20 whose base and emitter are commonly connected, and the second transistor 17
The collector current of the transistor 20 is inverted and the collector current of the transistor 20 is inverted.
21 is a third transistor whose base is connected to the collector of the transistor 20 of the inversion circuit 18; 22 is a constant whose base is connected to the common emitter of the second and third transistors 17 and 21; A current source and 23 are output terminals from which a detected output signal is obtained.

AM applied to input terminal 13 to be detected
The input signal is subjected to AM detection by the first transistor 14 and the capacitor 16. The detected signal is applied to the base of the second transistor 17 and is led out from the common emitter of the second transistor 17 and the third transistor 21 to the output terminal 23.

Now, if the constant current flowing through the constant current source 22 is _I0 , in a steady state, the third transistor 21
The collector current of the second transistor 17 is I ₀ / _β , the base current of the third transistor 21 is I ₀ /β, and the collector current of the second transistor 17 is I ₀ /β, The collector current of the transistor 14 becomes I ₀ /β ² , and this state flows from the collector of the second transistor 17 to the current inversion circuit 18 and the base/emitter of the third transistor 21 to the second transistor 1 .
It is kept stable by the negative feedback of the loop to the emitter of 7. Since the collector current of the first transistor 14 has a small value of I ₀ /β ² , it is suitable for performing AM detection together with the small-capacity capacitor 16 integrated into an IC. Furthermore, since _the operating voltage from the base of the first transistor 14 to _the ground is 2V _BE + _V It is possible to operate with

Furthermore, in the present invention, a bias circuit is configured by the fourth, fifth, and sixth transistors 24, 25, and 26, and a bias circuit is formed at the base of the first transistor 14.
It supplies an operating voltage of 3V _BE (=2.1V). The current of the constant current source 27 is made equal to the current of the constant current source 22.
If _I0 , the collector current of the fourth transistor 24 is _I0 , the base current is _I0 /β, the collector current of the fifth transistor 25 is _I0 /β, the base current is _I0 / ^β2 , and the sixth The collector current of the transistor 26 is I ₀ /β ² , and the current flowing through each transistor is
The transistor 14 and the sixth transistor 26, the second
transistor 17, fifth transistor 25, third
The transistor 21 and the fourth transistor 24 can be made to correspond to each other. Therefore, by creating corresponding transistors in a pair structure within an IC, it is possible to match the rise characteristics and temperature characteristics.
Variations in the output voltage and gain control initial setting voltage can be suppressed.

(f) Effects of the invention As described above, according to the invention, it is possible to obtain an AM detection circuit that has a built-in capacitor and operates at low voltage. Further, if a circuit as shown in FIG. 2 is used as a bias circuit, a stable output voltage can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing a conventional AM detection circuit, and FIG. 2 is a circuit diagram showing an embodiment of the present invention. Explanation of main figure numbers, 14...first transistor,
16...Capacitor, 17...Second transistor, 18 ...Current inversion circuit, 21...Third transistor, 22...Current source.

Claims (1)

[Scope of utility model registration request] The first signal to be detected is applied to the base.
a transistor, a capacitor connected between the emitter of the first transistor and ground, a second transistor whose base is connected to the emitter of the first transistor, and obtaining an inversion current of the collector current of the second transistor. a third transistor to which the output of the current inversion circuit is applied to the base; a current source connected to a common emitter of the third transistor and the second transistor; a bias circuit connected to the base of the second transistor, so that a current of 1/β ² (where β is the current amplification factor of the transistor) of the current I ₀ flowing through the current source flows through the base of the second transistor; An AM detection circuit characterized in that a detection time constant is set by a capacitance of the capacitor and a base current of the second transistor.