JPS6246326Y2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement of a detection circuit, and particularly to preventing changes in the DC output voltage at the output end of the detection circuit. Conventionally, a detection circuit as shown in FIG. 1 has been known. In FIG. 1, reference numeral 1 denotes an intermediate frequency amplification circuit 3 that amplifies an AM intermediate frequency input signal applied to an input terminal 2. The intermediate frequency amplification circuit 3 is composed of first and second transistors 4 and 5, and is a two-stage amplifier that constitutes a pre-stage amplifier of a detection circuit. 6 is a detection transistor whose base is connected to the emitter of the second transistor 5; 7 is a low-pass filter connected to the output terminal of the detection transistor 6; 8 is an AGC according to the output signal of the low-pass filter 7; In the AGC signal generation circuit that generates a signal, a signal applied to an input terminal 2 is amplified by an intermediate frequency amplification circuit 1, and detected by a detection transistor 6 via a two-stage directly connected amplifier 3. In FIG. 1, the detection output signal is applied to an AGC signal generation circuit 8 via a low-pass filter 7, and the output signal of the AGC signal generation circuit 8 is applied to the intermediate frequency amplification circuit 1 to perform AGC. The two-stage connected amplifier 3 has a series circuit consisting of first and second resistors 9 and 10 between the emitter of the second transistor 5 and the ground, and the middle point of the series circuit and the base of the first transistor 4 are connected to each other. A negative feedback resistor 11 is provided between them. However, in a two-stage direct-coupled amplifier as shown in FIG. changes, and this change causes the low-pass filter 7
The signal is applied to the AGC signal generating circuit 8 via the AGC signal generating circuit 8, which has the drawback of generating an inaccurate AGC signal and causing variations in the detection output. The present invention has been developed in view of the above points, and will be described below based on embodiments with reference to the drawings. Second
The figure shows an embodiment of the present invention, in which 13 is a detection transistor, 14 is a two-stage directly connected amplifier constituted by first and second transistors 15 and 16, and 17 is a constant current connected to the first transistor 15. a diode 18 and a first diode arranged to supply
A first constant current circuit 20 includes a constant current transistor 19, a diode 21 and a second constant current transistor 22, and the first constant current circuit 20 includes a diode 21 and a second constant current transistor 22, and
a second constant current circuit that supplies a constant current to the base of 2 ;
3 is a control circuit consisting of a resistor 24, a diode 25, and transistors 26 and 27, and controls the first and second constant current circuits 17 and 20 ; 28 is a constant voltage for supplying a constant voltage to the control circuit 23 ; It is a circuit. Now, a predetermined value is output from the output terminal of the constant voltage circuit 28.
If a constant voltage of V ₀ is generated, the control circuit
A constant current of a predetermined value I ₀ flows through the diode 25 of No. 23 , and a constant current of I ₀ also flows through the collector of the transistor 26 which is mirror-connected to the diode 25.
Therefore, the current _I0 flowing to the collector of the transistor 26 is supplied from the power supply 29 via the transistor 27 and the diode 18 of the first constant current circuit 17 , and the current _I0 also flows through the transistor 27 and the diode 18. . Since the first constant current transistor 19 is mirror-connected to the diode 18, a current of _I0 also flows through the first constant current transistor 19, and the current _I0 is supplied to the collector of the first transistor 15 as a bias current. be done. On the other hand, in order for a constant current of I ₀ to flow through the collector of the transistor 27 of the control circuit 23 , a base current of I ₀ /β (where β is the current amplification factor of the transistor 27) must flow through its base. The base current also flows through the diode 21 of the second constant current circuit 20 . In the second constant current circuit 20 , since the diode 21 and the second constant current transistor 22 are connected in a current mirror, a current of I ₀ /β also flows through the collector of the second constant current transistor 22. , the current I ₀ /β is supplied to the base of the first transistor 15 . Therefore, since a constant current of I ₀ /β is supplied to the base of the first transistor 15 and a constant current of I ₀ is supplied to the collector, the first transistor 15 has no bias current even if β changes. does not change, and the collector voltage of the front-stage transistor 15 and the emitter voltage of the rear-stage transistor 16 also do not change, so that a constant DC voltage is obtained at the output terminal 30 connected to the emitter of the detection transistor 13. The present invention has the advantage that even if the voltage of the power supply 29 changes, the collector and base currents of the first transistor 15 do not change, so the DC output voltage does not change. In addition, in an IC, the β values of transistors in the same chip are matched and can be considered to be approximately the same value, so even if β varies, the transistor 27 that flows through the negative feedback resistor 31 of the two-stage connected amplifier 14 Since only a small amount of correction current is required to correct the slight difference in β between the first transistor 15 and the first transistor 15, there is almost no fluctuation in the DC output voltage. Therefore, it is possible to provide a detection circuit in which the detection output signal can be kept constant and which does not adversely affect the AGC, and it is also possible to prevent deterioration of the distortion factor. As described above, the detection circuit according to the present invention is a practical one that can improve characteristics compared to conventional detection circuits.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing a conventional detection circuit, and FIG. 2 is a circuit diagram showing an embodiment of the detection circuit according to the present invention. Explanation of main figure numbers, 13...detection transistor,
14 ...Two-stage direct-coupled amplifier, 17 ...First constant current circuit, 20 ...Second constant current circuit, 23 ...Control circuit.

Claims (1)

[Scope of utility model registration request] A detection circuit including a two-stage directly connected amplifier before a detection transistor, which includes a transistor, generates a collector current of the transistor as a first constant current, and has a value of 1/β of the first constant current. a current generating circuit that generates a base current as a second constant current; a first constant current circuit connected in a current mirror type for supplying the first constant current to the collector of the front stage transistor of the second stage direct-coupled amplifier; and a second constant current circuit connected in a current mirror type for supplying a second constant current to the base of the front-stage transistor, and the collector current and base current of the transistor are used as the bias current of the front-stage transistor. detection circuit.