JPH04196706A - Amplifier for limiter - Google Patents

Abstract

PURPOSE:To obtain a high gain even at a low frequency by connecting an output circuit differentially receiving an output signal outputted differentially from a feedback type limiter amplifier to a post-stage of the feedback type limiter amplifier. CONSTITUTION:Each of amplifier circuits 11-14 has two input terminals and outputs two output signals proportional to a difference between two input signals inputted to the two input terminals, and feedback resistors 15, 16, 18 are connected between the input terminal of the 1st stage amplifier 11 and the output terminal of the final stage amplifier 14. An output circuit 20 is an output circuit comprising a differential amplifier or a comparator or the like connecting to the output terminal of the final stage amplifier 14 and receiving the two output signals extracted differentially from the output terminal of the amplifier 14. Thus, the gain of the feedback type limiter amplifier is lowered by increasing the gain of the output circuit 20. Thus, the capacitance of a bypass capacitor 19 is decreased, the leading characteristic of the limiter amplifier is improved and a high gain at a low frequency is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention is directed to a direct conversion type FSK (Frequency) with good sensitivity characteristics and two frequency band characteristics.
cy 5hift Keying) relates to a limiter amplifier used in a receiver, etc.

BACKGROUND OF THE INVENTION Recently, in FSK receivers such as pagers, direct conversion systems have begun to be used in which most of the receiving section can be integrated circuits in order to reduce the size and cost of devices.

The limiter amplifier used in this type of FSX receiver needs to be able to obtain high gain even at low frequencies.

FIG. 2 shows the configuration of a conventional limiter amplifier.

In FIG. 2, 1a to 1f are amplifiers connected in six stages, 2a is a capacitor connected between the input terminal IN and the input terminal of the amplifier 1a, and 2b to 2f are amplifiers 13, respectively.
It is a capacitor connected between the front stage and the rear stage of ~1f,
Amplifiers 13 to 1f are connected in multiple stages, for example 6 stages, between the input terminal IN and the output terminal OUT until the required amplification degree is obtained.
The capacitors 23 to 2f are connected in cascade between them.

Further, FIG. 3 shows the configuration of another conventional limiter amplifier. In FIG. 3, 33 to 3f are cascade-connected amplifiers that output two output signals proportional to the difference between two input terminals and the input signals input thereto, and one input terminal of the first stage amplifier 3a. A feedback resistor 5.6 is directly connected between the input terminal and one output terminal of the final stage amplifier 3f, and a feedback resistor 8 is connected between the other input side of the amplifier 3a and the other output terminal of the amplifier 3f. Connected.

7 is a bypass capacitor connected between the connection point of feedback resistors 5 and 6 and the ground, and 9 is a bypass capacitor connected between the other input side of the amplifier 3a and the ground.

Problem to be Solved by the Invention However, in the limiter amplifier shown in Fig. 2 above, when the signal frequency to be amplified is high, the cutoff frequency of the bypass characteristic determined by the input impedance and coupling capacitor of each amplifier is set to a high frequency. Since the settings can be made, it is relatively easy to incorporate the coupling capacitor into an IC.

However, in order to obtain high gain at low frequencies required by a direct conversion method such as a PSK receiver that uses an intermediate frequency, it is necessary to increase the capacitance of the coupling capacitor.
becomes inappropriate.

In addition, in the limiter amplifier shown in Figure 3 above, the bypass capacitor of the feedback circuit is generally connected externally, and in order to obtain high gain at low frequencies, the resistor and capacitor of the feedback circuit must be connected externally. It is necessary to make the low-pass characteristic determined by the capacitance of the capacitor sufficiently low.

However, increasing the capacitance of the bypass capacitor makes the rise of the limiter amplifier unstable and takes a long time, making it unsuitable for devices such as pagers that require intermittent operation.

The present invention solves these conventional problems, and aims to provide a limiter amplifier that can obtain high gain even at low frequencies.

Means for Solving the Problems In order to achieve the above object, the present invention includes an output circuit that differentially receives the output signal differentially output from the limiter amplifier at the downstream stage of the feedback type limiter amplifier. It is connected.

Therefore, in the present invention, the gain of the limiter amplifier can be lowered by increasing the gain of the output circuit connected to the subsequent stage, such as a differential amplifier or a comparator. Accordingly, the capacitance of the capacitor of the feedback circuit can be reduced, so that the rise characteristics can be improved. Since the gain of the output circuit connected to the subsequent stage is added to the gain of the feedback limiter amplifier, it is possible to obtain a high gain at a low frequency required for the limiter amplifier as a whole.

Example FIG. 1 shows the configuration of an embodiment of the present invention.

In FIG. 1, reference numerals 11 to 14 denote two input terminals and amplifiers that output two output signals proportional to the difference between two input signals input thereto;
4 are cascaded. Further, 15 and 16 are feedback resistors connected in series between one input terminal of the first stage amplifier 11 and the other output terminal of the final stage amplifier 14. Reference numeral 17 denotes a bypass resistor externally connected between the connection point with the second feedback resistor 15 and 16 and the ground. Furthermore, 18
is a feedback resistor connected between the other input terminal of the amplifier 11 and the other output terminal of the amplifier 14. Further, 19 is a bypass capacitor 19 externally connected between the other input terminal of the amplifier 11 and the ground.

On the other hand, 20 is connected to the output terminal of the final stage amplifier 14,
This is an output circuit such as a differential amplifier or a comparator that differentially receives two output signals that are differentially extracted from the output signal.

Next, the operation of the above embodiment will be explained.

In the above embodiment, in order to obtain the high gain at low frequencies required for the limiter amplifier, a part of it is configured by the feedback type limiter amplifier consisting of amplifiers 11 to 14, and the remaining part is is configured by the output circuit 20. Therefore, the output circuit 20 connected to the subsequent stage
That is, by increasing the gain of the differential amplifier or comparator, the gain of the feedback type +l emitter amplifier can be lowered. For example, a normal feedback limiter amplifier requires a gain of about 100 dB, but in this embodiment, the gain is 60 dB or less.

Therefore, the capacitance of the bypass capacitor connected to the feedback circuit of the feedback limiter amplifier can be reduced, the rise characteristics of the limiter amplifier are improved, and the limiter amplifier as a whole has a high gain at low frequencies. is obtained.

Effects of the Invention As is clear from the above embodiments, the present invention comprises a limiter amplifier consisting of a feedback limiter amplifier and an output circuit that differentially receives its differential output signal. By lowering the gain of the limiter amplifier, the capacitance of the feedback circuit capacitor can be reduced.As a result, a limiter amplifier with good rise characteristics can be provided.In addition, the output circuit connected to the subsequent stage can have a high gain. Therefore, the limiter amplifier as a whole has the effect of being able to return a high gain at a low frequency.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a limiter amplifier according to an embodiment of the present invention, and FIGS. 2 and 3 are circuit diagrams of a conventional 1j limiter amplifier. 11-14...Amplifier, 15,16.18...Feedback resistor, 17.19...Bypass capacitor, 20...
Output circuit.

Claims (1)

[Claims] It has a feedback type limiter amplifier in which multiple stages of amplifiers are connected in series and has a feedback circuit that feeds back the output signal to the input side. A limiter amplifier characterized by connecting an output circuit that operates by receiving output signals differentially.