JP3822993B2 - Integrated mixer circuit - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for improving linearity in a common-source frequency mixing circuit (mixer circuit) integrated on the same semiconductor substrate and a circuit configuration therefor, and in particular, a bipolar transistor and a CMOS transistor (complementary electric field). The present invention relates to a method for improving linearity in a BiCMOS hybrid mixer circuit using a combination of an effect transistor and a circuit configuration therefor.
[0002]
[Prior art]
With the rapid spread of mobile communication, studies on integration of high-frequency circuits used for portable terminals and the like are being actively conducted. There are a wide variety of devices applied in this integration, such as Si bipolar transistors and CMOS transistors.
[0003]
As a typical example of a mixer circuit to which a CMOS device is applied, Lofogallan et al., I, E, E, E, Journal, Ob, Solid, State, Circuit, Vol. 31, No. 7 (July 1996) 1GHz CMOS / RF circuit for direct conversion wireless receivers described on pages 880-889 [A. Rofougaran; et al. "A 1GHz CMOS RF Front-End IC for a Direct-Conversion Wire-less Receiver" IEEE Journal of Solid-State Circuits, Vol. 31, No. 7, (July 1996), pp. 880-889].
[0004]
FIG. 2 shows a circuit configuration of the CMOS / RF circuit disclosed in the above document. In the conventional example, the mixer circuit using the bipolar device so far has been configured by using a differential pair, whereas the circuit portion of the RF (high frequency) signal input side is grounded at the source terminal 1. A pair of field effect transistor pairs M1 and M2 is configured, and an RF signal from the terminal pair 6 is input to the gates of the field effect transistor pairs M1 and M2. Since the source terminals of the transistors M1 and M2 are directly grounded, the current flowing through the circuit is not limited by the current source or the resistance unlike the case of using the conventional differential pair. For this reason, when a signal with a large amplitude is input, a large current can flow following the signal, and a circuit with high linearity can be realized.
[0005]
In the circuit of FIG. 2, the circuit portion that receives the local oscillation signals from the input terminals 4 and 5 and performs the switching operation is also configured by using the MOS field effect transistors M3, M4, M5, and M6. Are output from output terminals 2 and 3.
[0006]
[Problems to be solved by the invention]
As described above, a mixer circuit with high linearity can be realized by using a common source circuit on the RF signal input side. By the way, in order to reliably perform the switching operation of the mixer circuit by the output signal from the local oscillation signal buffer amplifier circuit, the transistor responsible for this switching operation is required to have a high current driving capability. However, since the relationship between the input voltage and the output current of the MOS transistor has a square characteristic, the current driving capability is inferior to that of a bipolar transistor having an exponential characteristic. In the conventional circuit of FIG. 2, as described above, the MOS field effect transistor is used as the transistor responsible for the switching operation. Therefore, the switching operation based on the local oscillation signal becomes incomplete, and the linearity is eventually deteriorated. This has been clarified by the analysis of the present inventors. Therefore, improving the switching characteristics of the transistors responsible for the above switching operation is a problem in improving the linearity of the entire mixer circuit. However, conventionally, no recognition has been made on this point.
[0007]
Accordingly, an object of the present invention is to provide a method for improving the switching characteristics of a transistor responsible for a switching operation in an integrated mixer circuit, thereby improving linearity as a mixer circuit, and a circuit configuration therefor.
[0008]
Another object of the present invention is to provide a method for improving linearity in a BiCMOS hybrid mixer circuit using a combination of a bipolar transistor and a CMOS transistor, and a circuit configuration therefor.
[0009]
[Means for Solving the Problems]
The object of the present invention described above is that the RF signal input circuit portion of the mixer circuit is MOS by using BiCMOS technology capable of integrating bipolar transistors and complementary metal oxide field effect transistors on the same semiconductor substrate. The transistor is configured by using a source-grounded pair, and the switching circuit portion is configured by using a bipolar transistor.
[0010]
By adopting the above-described characteristic configuration of the present invention, it is possible to effectively improve the switching characteristics of the circuit portion responsible for the switching operation in the integrated mixer circuit, and to greatly improve the linearity of the integrated mixer circuit. Can do.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0012]
<Example 1>
FIG. 1 shows a circuit configuration of an integrated mixer circuit according to a first embodiment of the present invention. In this embodiment, the circuit portion responsible for the input of the RF signal uses a grounded source pair of FETs (field effect transistors) M1 and M2, and the circuit portion that receives the local oscillation signal and performs the switching operation has two sets. A switching transistor differential pair consisting of bipolar transistors Q3, Q4 and Q5, Q6 is used.
[0013]
The source terminals 1 of the FETs (field effect transistors) M1 and M2 are directly connected to the ground terminal, and the RF input signal from the input terminal pair 6 is input to the gates of the FETs (field effect transistors) M1 and M2. A local oscillation signal for performing a switching operation of the differential pair of switching transistors composed of bipolar transistors Q3, Q4 and Q5, Q6 is input from input terminal 4/5 to the bases of bipolar transistors Q3, Q6 / Q4, Q5, respectively. Entered. Further, the mixer output from both switching transistor differential pairs is output from output terminals 2 and 3.
[0014]
FIG. 3 shows a case where an FET is used as the switching transistor differential pair as shown in FIG. 2 (conventional example), and a case where a bipolar transistor is used as shown in FIG. 1 (the present invention). Shows the input voltage (differential voltage) -output current characteristics of each differential pair. As can be seen from FIG. 3, the bipolar transistor differential pair according to the present invention starts the switching operation with a smaller input voltage signal than the conventional FET differential pair.
[0015]
In the FET, between the gate-source voltage V _GS and the drain current I _d ,
I _d = β (V _GS −V _th ) ² (1)
There is a relationship. Here, V _th is a threshold voltage of the FET, and β is a coefficient indicating the current drive capability of the FET, and is generally proportional to the gate width. On the other hand, in the bipolar transistor, between the base-emitter voltage V _BE and the collector voltage I _c ,
I _c = I _s exp (qV _BE / kT) (2)
There is a relationship. Here, I _s is the reverse leakage current, q is the electron charge, k denotes the Boltzmann constant, T is the absolute temperature.
[0016]
As is apparent from the above-mentioned numbers 1 and 2, the FET has a square voltage-current characteristic, and the bipolar transistor has an exponential voltage-current characteristic. For this reason, in the bipolar transistor, the collector current can be greatly changed by a slight change in the base-emitter voltage. This characteristic difference brings about the superiority of the switching characteristics of the differential pair using bipolar transistors. The switching characteristics when using FETs can be improved by increasing the gate width, since the coefficient β in Equation 1 is proportional to the gate width. However, this increase in gate width also increases the parasitic capacitance. This means an increase, and there is a limit in devices that require high-frequency operation.
[0017]
As is apparent from the above description, the switching characteristics can be improved by using a bipolar transistor for the switching differential pair of the integrated mixer circuit. As a result, the linearity of the mixer circuit can be improved. Although FIG. 1 shows the circuit configuration of the mixer circuit alone, as shown in FIG. 4, a local signal buffer amplifier is also provided on the same semiconductor substrate, and the local signal is amplified by the amplifier. It is also possible to configure so as to supply to the mixer circuit shown in the first embodiment (FIG. 1).
[0018]
In FIG. 4, the buffer amplifier for amplifying the local oscillation signal includes field effect transistors M7, M8, M9, M10 and M11, M12, resistors R3, R4, R5, R6, a bias circuit, and the like. Note that a bias circuit is connected to the RF signal input terminal of the mixer circuit in FIG. 4 via resistors R1 and R2. In FIG. 4, the local signal buffer amplifier is composed of FETs, but it goes without saying that it can also be composed of bipolar transistors.
[0019]
In the circuit configuration shown in FIG. 4, the following results were obtained as a result of performing a simulation when a BiCMOS device with a 0.35 μm rule was used and a received signal in the 900 MHz band was input. The comparison was made between the circuit of FIG. 4 using a bipolar transistor for the switching differential pair and the circuit of FIG. 2 using a MOSFET for the conventional switching differential pair according to the present invention. As a result, in the circuit configuration of FIG. 4 according to the present invention, good values of conversion gain of 10.7 dB and ICP of −6.5 dB were obtained. However, in the conventional circuit configuration of FIG. The values were as low as .6 dB and ICP as -8.5 dB. Thus, according to the present invention, it was confirmed that the ICP can be improved by 2 dB by adopting a source-grounded mixer circuit configuration using a bipolar transistor pair as a switching transistor differential pair. The ICP referred to here is an evaluation scale called “−1 dB compression point”. On the input / output characteristic curve as shown in FIG. 5, the characteristic curve obtained when a large signal is input is the characteristic obtained when a small signal is input. It is defined by the input signal power at the point where the gain decreases by 1 dB from the ideal characteristic on the extension of the curve, and the larger this value, the higher the linearity. From this, it was confirmed that the linearity is greatly improved by the present invention.
[0020]
<Example 2>
FIG. 6 shows a circuit configuration of an integrated mixer circuit according to the second embodiment of the present invention. In the first embodiment, an example in which the present invention is applied to a so-called double balance type mixer circuit having a differential RF signal input terminal pair and performing a switching operation by two differential pairs is shown. . On the other hand, in the second embodiment, as shown in FIG. 6, the present invention is applied to a single balance type mixer circuit having a single RF signal input terminal and performing a switching operation with a single differential pair. An applied example is shown. Even in the case of a single-balanced mixer circuit, FETs are used for the voltage-current converter in the RF signal input stage, and bipolar transistors are used for the differential pair that performs switching operations for frequency conversion. Realize and improve linearity.
[0021]
In FIG. 6, an RF input signal from a single input terminal 6 is input to the gate of an RF signal input stage composed of a single field effect transistor M1 whose source terminal 1 is grounded, and the switching circuit is a set of switches. The switching differential pair is composed of bipolar transistors Q3 and Q4. A local signal for switching from input terminals 4 and 5 is input to the bases of bipolar transistors Q3 and Q4 and output from mixers from output terminals 2 and 3 Is obtained. A mixer circuit having such a circuit configuration is called a single balance type mixer circuit.
[0022]
In the case of the single-balanced mixer circuit shown in FIG. 6 as well, a FET (field effect transistor) is used for the voltage-current conversion section of the RF signal input stage, and a bipolar transistor is used as a differential pair for performing switching operation for frequency conversion. It has been confirmed that the linearity of the mixer circuit can be improved by using.
[0023]
【The invention's effect】
As is apparent from the above detailed description, according to the present invention, the RF signal input stage of the integrated mixer circuit is constituted by an FET (field effect transistor), while the differential pair for switching is constituted by using a bipolar transistor. As a result, the linearity of the mixer circuit can be greatly improved.
[Brief description of the drawings]
FIG. 1 is a diagram showing a circuit configuration of a common source mixer circuit according to an embodiment of the present invention.
FIG. 2 is a diagram showing a circuit configuration of a common-source mixer circuit composed of a conventional MOS transistor.
3 is a diagram showing a comparison between input voltage (differential voltage) -output current characteristics obtained in the circuit configuration according to the present invention shown in FIG. 1 and the conventional circuit configuration shown in FIG. 2;
4 is a diagram showing a detailed circuit configuration of a mixer circuit in which a local signal buffer amplifier is also provided on the same semiconductor substrate in accordance with the circuit configuration according to the present invention shown in FIG. 1. FIG.
FIG. 5 is a diagram for explaining the definition of ICP (−1 dB compression point).
FIG. 6 is a diagram showing a circuit configuration of a common source mixer circuit according to another embodiment of the present invention.
[Explanation of symbols]
M1, M2... NMOS transistors for RF signal input,
Q3 to Q6... Npn bipolar transistors for switching,
M3 to M6... NMOS transistors for switching,
M7 to M12... NMOS transistor for buffer amplifier,
R1, R2... Bias resistors,
R3 to R6 ··· Load resistance.

Claims (4)

A Gilbert-type mixer circuit comprising an input differential pair for inputting a first signal and two differential pairs for switching operation by a second signal;
A buffer amplifier that amplifies the second signal and outputs an amplified signal to each of the differential pairs;
The input differential pair is composed of a field effect transistor pair in which a source electrode is grounded and a gate is an input terminal,
Each of the two differential pairs is composed of a bipolar transistor,
2. The integrated mixer circuit according to claim 1, wherein the buffer amplifier comprises a differential pair of field effect transistors. An integrated mixer circuit having a Gilbert-type mixer circuit including an input differential pair for inputting an RF signal and two differential pairs for switching operation by a local oscillation signal,
A buffer amplifier that amplifies the local oscillation signal and outputs an amplified signal to each of the differential pairs;
The input differential pair is composed of a differential pair by a first field effect transistor,
Each of the two differential pairs is composed of a bipolar transistor pair,
The buffer amplifier is composed of two differential pairs of second field effect transistors,
A source of the first field effect transistor is connected to a ground terminal;
The RF signal is input from the gate of the first field effect transistor,
An emitter of a bipolar transistor pair constituting each of the differential pairs is connected to a drain of each of the field effect transistor pairs;
Each of the collectors of the bipolar transistor pair is connected to the output terminal of the mixer circuit;
An integrated mixer circuit, wherein the local oscillation signal is inputted to each base of the bipolar transistor pair.   A semiconductor integrated circuit device comprising the integrated mixer circuit according to claim 1.   A portable terminal device for mobile communication, comprising the semiconductor integrated circuit device according to claim 3.

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