JPH04267610A - Radio reception circuit - Google Patents

Abstract

PURPOSE:To prevent deterioration in reception sensitivity by using an output impedance of a mixer circuit in place of an input resistor of a buffer amplifier so as to eliminate thermal noise caused in the input resistor. CONSTITUTION:An output impedance of output circuits 30I, 30Q of mixer circuits 30I, 30Q is nearly equal to collector load resistance R11', R12' because the collector output impedance of transistors(TRs) 13-16 is very high. Moreover, the TRs 13-16 act like a current source when viewing the TRs from a collector terminal. In the equivalent circuit, the relation of R12'=R11' and Z12=Z11 is in existence. In this case, the gain of buffer amplifiers 60I, 60Q is set to a desired value by selecting the Z11, Z12. That is, the buffer amplifiers 60I, 60Q are formed to have a feedback impedance circuit Z11 between an output terminal and an inverted input terminal of an operational amplifier OP and the gain is selected in a ratio of the impedance Z11 to an output impedance of the mixer circuits 30I, 30Q. Through the constitution above, the noise generated in the buffer amplifier is reduced and the reception sensitivity is improved.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention is used for receiving high-frequency signals and converting them into baseband signals in mobile radio communication devices such as mobile/automobile radio telephones, cordless telephones, and selective call receivers. The present invention relates to a wireless receiving circuit.

0002

2. Description of the Related Art Conventionally, as this type of radio receiving circuit, a circuit employing a direct conversion method (direct conversion method) for directly converting a received high frequency signal into a baseband signal is known. FIG. 5 is a circuit block diagram showing an example of the configuration.

[0003] In the figure, a high frequency signal received by an antenna 1 is amplified by a high frequency amplifier 2 and then branched and input to a pair of mixing circuits 3I and 3Q, respectively. In these mixing circuits 3I and 3Q, the received high-frequency signal is mixed with a local oscillation signal generated from the local oscillator 4 and a local oscillation signal obtained by shifting the phase of this local oscillation signal by π/2 by a phase shifter 5 to adjust the frequency. converted. Here, the frequency of the local oscillation signal is set approximately equal to the carrier frequency of the received high frequency signal. For this reason, the mixing circuits 3I and 3Q output not intermediate frequency signals but baseband signals as they are. And the mixing circuit 3I,
Each received baseband signal output from 3Q is buffered and amplified by buffer amplifiers 6I and 6Q, and unnecessary frequency components are removed by low-pass filters 7I and 7Q, and then input to demodulation circuit 8. The demodulation circuit 8 compares or calculates these two baseband signals, thereby demodulating the modulated signal. Compared to superheterodyne type receiving circuits, this type of receiving circuit can be made smaller by eliminating the need for intermediate frequency amplifiers and their filters, and is easier to integrate. It is attracting attention especially for use in portable receivers.

By the way, the mixing circuits 3I, 3Q and the buffer amplifiers 6I, 6Q used in this type of receiving circuit have conventionally been configured as follows, for example. FIG. 6 is a circuit diagram showing its configuration. That is, first, the mixing circuit 3I,
3Q is bipolar transistor Tr11, Tr12
, Tr13, Tr14, Tr15, Tr16 and a constant current source I, collector load resistors R11, R12 connected to the transistors Tr13, Tr16, and supplying base current to the transistors Tr13, Tr16. resistor R13 and transistors Tr11, Tr1
A diode D for supplying a base current according to a bias voltage lower than the power supply voltage Vcc by a certain voltage to
1. The received high frequency signal output from the high frequency amplifier 2 is input to the bases of the transistors Tr11 and Tr12 via the transformer 21, and the local oscillation signal output from the local oscillator 4 is input to the bases of the transistors Tr11 and Tr12 via the local oscillation signal input terminal 31. Tr13, Tr
16 is input to the base. Then, the multiplication output of the received high frequency signal and the local oscillation signal is obtained by the transistor Tr.
A differential signal is extracted from the collector connection point of transistors Tr13 and Tr15 and the collector connection point of transistors Tr14 and Tr16, and is supplied to buffer amplifiers 6I and 6Q.

On the other hand, the buffer amplifiers 6I and 6Q consist of an operational amplifier OP, its input resistances R21 and R22, and a period resistance R2.
4 and a voltage dividing resistor R23. The gain of the buffer amplifiers 6I and 6Q is determined by the value of each resistor mentioned above: R21=R2
2, R23=R24, it is expressed by -R24/R21. The signals output from the mixing circuits 3I, 3Q are output to the low-pass filters 7I, 7Q after their signal levels are controlled according to the gains.

[0006]

[Problems to be Solved by the Invention] However, such conventional circuits have the following problems that should be improved. That is, in the buffer amplifiers 6I and 6Q, the mixing circuit 3I
, 3Q, input resistors R21 and R22 are inserted in series in the signal path of the received baseband signal output from 3Q.
Thermal noise generated from these input resistors R21 and R22 is superimposed on the received baseband signal, deteriorating the signal-to-noise ratio (S/N) of the received baseband signal, resulting in deterioration of reception sensitivity. There was a problem.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a radio receiving circuit which can reduce the noise generated from a buffer amplifier, improve receiving sensitivity, and further reduce the circuit scale and facilitate integration. With the goal.

[0008]

[Means for Solving the Problems] In order to achieve the above object, the present invention branches a received high frequency signal into two and inputs it into a pair of mixing circuits, and each of these mixing circuits converts the received high frequency signal into a local oscillation signal. The first and second signals whose phases are orthogonal to each other are obtained by mixing with In a wireless receiving circuit equipped with a frequency conversion circuit for obtaining a baseband signal, the buffer amplifier is combined with an operational amplifier into which the signal output from the mixing circuit is input without going through at least a resistor circuit, and an output of the operational amplifier. It has a configuration including a feedback impedance circuit provided between the terminal and the inverting input terminal, and the gain is set by the ratio of the impedance of the feedback impedance circuit and the output impedance of the mixing circuit. be.

The present invention also provides a buffer amplifier in which each of the differential output signals of the mixing circuit is connected to an inverting input terminal and a non-inverting input terminal without passing through a resistor circuit, when the mixing circuit is of a differential output type. a first impedance circuit for feedback provided between the output terminal of the operational amplifier and the inverting input terminal, and the non-inverting input terminal of the operational amplifier and the reference voltage input terminal. and a second impedance circuit provided between the two, and the gain is determined by the ratio of the impedance of the first impedance circuit to the output impedance of one of the mixing circuits, and the impedance of the second impedance circuit. It is also characterized in that it is set by the ratio of the output impedance of the mixing circuit and the other output impedance of the mixing circuit.

[0010]Further, the present invention includes a capacitor circuit inserted between the output terminal of the mixing circuit and the operational amplifier of the buffer amplifier for AC coupling therebetween, and a first impedance circuit and a first impedance circuit. Each of them is also characterized in that the second impedance circuit is configured by a circuit in which a resistor and a capacitive element are connected in parallel.

[0011]

As a result, according to the present invention, the gain of the buffer amplifier is set by the ratio of its feedback impedance to the output impedance of the mixing circuit. In other words, the input resistance of the buffer amplifier is replaced by the output impedance of the mixing circuit. Therefore, it is possible to omit the input resistance of the buffer amplifier, thereby eliminating thermal noise generated by the input resistance and preventing deterioration of receiving sensitivity. Furthermore, by eliminating the input resistance, the circuit scale can be reduced accordingly, and integration can be facilitated. Further, according to the present invention, even when the mixing circuit is of a differential output type, the same implementation is possible.

Furthermore, since the output of the mixing circuit and the operational amplifier of the buffer amplifier are separated in direct current and coupled in alternating current by the capacitance circuit, the dynamic range of the buffer amplifier is narrowed, and as a result, the operational amplifier is It becomes possible to drive with a power supply voltage. In other words, the present invention can be sufficiently applied to battery-powered wireless communication devices with low power supply voltage.

Furthermore, by configuring the first and second impedance circuits of the buffer amplifier by parallel circuits of a resistor and a capacitive element, these impedance circuits can also have a function as a low-pass filter. This makes it possible to reduce the burden on the low-pass filter disposed after the buffer amplifier and simplify the configuration of the low-pass filter.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows the circuit configuration of a mixing circuit and a buffer amplifier of a radio receiving circuit according to an embodiment of the present invention. In this figure, the same parts as those in FIG. 6 are given the same reference numerals and detailed explanations will be omitted.

Mixing circuits 30I and 30Q include bipolar transistors Tr11, Tr12, Tr13, and Tr1.
4, Tr15, Tr16 and a constant current source I, collector load resistors R11', R12' connected to the transistors Tr13, Tr16, and the transistor Tr13.
, a resistor R13 for supplying base current to Tr16
And the power supply voltage V is applied to the transistors Tr11 and Tr12.
and a diode D1 for supplying a base current according to a bias voltage lower than cc by a certain voltage.

On the other hand, the buffer amplifiers 60I and 60Q include an operational amplifier OP, a first impedance circuit Z11 connected between its output terminal and an inverting input terminal, and a supply path for a reference voltage Vref to the non-inverting input terminal. The second
, and capacitors C11 and C12 inserted in the signal supply path to the inverting input terminal and the non-inverting input terminal. Of these, the first and second impedance circuits Z11 and Z12
is constituted by a parallel circuit of a capacitor Cz and a resistor Rz, as shown in FIG. In addition, each of the above capacitors C1
1, C12 are the mixing circuits 30I, 30Q and the operational amplifier O.
This is to separate the connection with P in a direct current manner and couple it in an alternating current manner.

By the way, in the mixing circuits 30I, 30Q and buffer amplifiers 60I, 60Q configured in this way, the output impedance of the mixing circuits 30I, 30Q is as follows.
Transistors Tr13, Tr14, Tr15, T
Since the collector output impedance of r16 is very high, the resistance value is approximately equal to that of the collector load resistors R11' and R12'. Also, transistors Tr13, Tr14, T
r15 and Tr16 appear as current sources when viewed from the collector terminal. Therefore, if the output portions of the mixing circuits 30I, 30Q and the buffer amplifiers 60I, 60Q are represented by an equivalent circuit, it will be as shown in FIG. 2, for example. Further, this equivalent circuit can also be expressed as shown in FIG. 3 using Norton's theorem.

In these equivalent circuits, if R1
2'=R11' and Z12=Z11, the output voltage v0 of the buffer amplifiers 60I and 60Q becomes v0 = Z11/R11'×(v2 - v1). Therefore, the gains of the buffer amplifiers 60I, 60Q are determined by the impedances of the first and second impedance circuits Z11, Z12 and the output impedances of the mixing circuits 30I, 30Q. Furthermore, v1 = R11'
Since .i1, v2 = R12'.i2 = R11'.i1, the output voltage v0 of the buffer amplifiers 60I and 60Q can also be expressed as v0 = Z11(i2 - i1). Therefore, the total gain of the mixing circuits 30I, 30Q and the buffer amplifiers 60I, 60Q can be set to a desired value by appropriately determining the values of the first impedance circuit Z11 and the second impedance circuit Z12 of the buffer amplifiers 60I, 60Q. Can be set. That is, signals amplified to a desired signal level can be obtained from buffer amplifiers 60I and 60Q.

Therefore, with the circuit of this embodiment, it is possible to obtain signals at desired levels from the buffer amplifiers 60I, 60Q, and to eliminate the need for input resistors of the buffer amplifiers 60I, 60Q. As a result, thermal noise generated by the input resistance is superimposed on the received signal, resulting in a S/N ratio of the received signal.
It is possible to prevent problems such as deterioration of reception sensitivity due to deterioration of reception sensitivity. In addition, by eliminating the input resistance, the circuit scale can be made smaller, and
Integration can be facilitated. These effects are useful for mobile radio communication devices such as mobile telephones, cordless telephones, and selective call receivers, where radio wave reception conditions are subject to change and miniaturization and weight reduction are important issues. Extremely effective.

Furthermore, in the circuit of this embodiment, the buffer amplifier 6
A capacitor C is connected to the input path of the operational amplifier OP of 0I, 60Q.
11 and C12, the common-mode input range of the operational amplifier OP can be narrowed, thereby making it possible to drive the operational amplifier with a low power supply voltage Vcc. In other words, the present invention can be sufficiently applied to battery-powered wireless communication devices with low power supply voltage.

Furthermore, the first buffer amplifiers 60I and 60Q
Since each of the second impedance circuits Z11 and Z12 is constituted by a parallel circuit of a resistor Rz and a capacitor Cz, these impedance circuits Z11 and Z1
2 can have a function as a low-pass filter, thereby reducing the burden on the low-pass filters 7I, 7Q disposed after the buffer amplifiers 60I, 60Q. It becomes possible to simplify the configuration.

It should be noted that the present invention is not limited to the above embodiments. For example, the first and second impedance circuits may be configured with only resistors, and the power supply voltage Vc
If c can be set high enough, there is no need to insert a capacitor in the input path to the operational amplifier OP. others,
The circuit configurations of the buffer amplifier and mixing circuit, the types of systems and wireless devices to which they are applied, etc. can be modified in various ways without departing from the gist of the present invention.

[0023]

As described in detail above, according to the present invention, a buffer amplifier is combined with an operational amplifier into which a signal output from a mixing circuit is input without going through a resistor circuit, and an output terminal of this operational amplifier. A buffer impedance circuit is provided between the inverting input terminal and the feedback impedance circuit, and the gain is set by the ratio of the impedance of the feedback impedance circuit and the output impedance of the mixing circuit. It is possible to provide a radio reception circuit that reduces noise generated from an amplifier, improves reception sensitivity, and further reduces circuit scale and facilitates integration.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing the configuration of a mixing circuit and a buffer amplifier of a radio receiving circuit in an embodiment of the present invention.

FIG. 2 is a diagram showing the mixing circuit and buffer amplifier equalization circuit shown in FIG. 1;

FIG. 3 is a diagram showing another equalization circuit of the mixing circuit and buffer amplifier shown in FIG. 1;

FIG. 4 is a circuit diagram showing the configuration of an impedance circuit in the buffer amplifier shown in FIG. 1.

FIG. 5 is a circuit block diagram showing an example of the configuration of a direct conversion type radio receiving circuit.

FIG. 6 is a circuit diagram showing the configuration of a conventional mixing circuit and buffer amplifier.

[Explanation of symbols]

1...Antenna, 2...High frequency amplifier, 3I, 3Q, 30I
, 30Q...Mixing circuit, 4...Local oscillator, 5...π/2 phase shifter, 6I, 6Q, 60I, 60Q...Buffer amplifier, 7I,
7Q...Low pass filter, 8...Demodulation circuit, 21...Transformer, 31...Local oscillation signal input terminal, R11', R12'
...Collector load resistance, OP...Operation amplifier, Z11...1st
Z12...second impedance circuit, C11, C12...capacitors for AC coupling.

Claims (4)

[Claims] 1. A received high frequency signal is branched into two and input into a pair of mixing circuits, and each of these mixing circuits mixes the received high frequency signal with a local oscillation signal, thereby producing first and second signals whose phases are orthogonal to each other. a radio receiving circuit comprising a frequency conversion circuit that obtains a first and second baseband signal, amplifies these first and second signals with a buffer amplifier, and then passes them through a low-pass filter to obtain first and second baseband signals. In the buffer amplifier, the buffer amplifier includes an operational amplifier into which the signal output from the mixing circuit is input without going through at least a resistor circuit, and a feedback amplifier provided between the output terminal and the inverting input terminal of the operational amplifier. a first impedance circuit, and a gain is set by a ratio of the impedance of the feedback first impedance circuit and the output impedance of the mixing circuit. 2. The mixing circuit is of a differential output type, and the buffer amplifier is such that each of the differential output signals of the mixing circuit is inputted to an inverting input terminal and a non-inverting input terminal, respectively, without passing through at least a resistor circuit. an operational amplifier; a first impedance circuit for feedback provided between the output terminal and the inverting input terminal of the operational amplifier; and a first impedance circuit provided between the non-inverting input terminal of the operational amplifier and the reference voltage input terminal. a second impedance circuit, the gain of which is a ratio between the impedance of the first impedance circuit and one output impedance of the mixing circuit, and the impedance of the second impedance circuit and the other output impedance of the mixing circuit. 2. The radio receiving circuit according to claim 1, wherein the radio receiving circuit is set based on a ratio of . 3. A capacitive circuit is inserted between the output terminal of the mixing circuit and the operational amplifier of the buffer amplifier for alternating current coupling between the output terminal and the operational amplifier of the buffer amplifier. wireless receiving circuit. Claim 4: A first impedance circuit and a second impedance circuit.
Claim 1, wherein the impedance circuit is constituted by a circuit in which a resistor and a capacitive element are connected in parallel.
3. The wireless receiving circuit according to 2 or 3.