JP2000059147A - Mixer circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a mixer circuit for simultaneously reducing distortion and reducing noise. SOLUTION: This mixer circuit is provided with a circuit 42 for which four transistors are double balance connected and the circuit for adding differential high frequency signals 47 and 48 to the two sets of common emitters of the double balance connected circuit by a differential amplifier 41. The operation currents of the double balance connected circuit and the differential amplifier are mutually independent. The operation currents for respectively performing an optimum operation are supplied to the respective ones of the differential amplifier and a Gilbert cell pair (circuit for which four transistors are double balance connected), the noise is reduced and the distortion is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-frequency mixer circuit used in a video transmission device, a mobile communication device and the like, and more particularly to a device capable of outputting a low-noise and low-distortion signal.

[0002]

2. Description of the Related Art Conventionally, in a mobile communication device or the like, a Gilbert cell type mixer circuit formed of a semiconductor integrated circuit is used to extract an IF signal by mixing a local oscillation signal (LO) with an RF signal. I have.

This Gilbert cell type mixer circuit has two
This is a double balanced mixer circuit having the configuration shown in FIG. First, the operation principle of the mixer circuit will be described.

In this mixer circuit, an RF signal is applied to differential amplifier 21 from RF input terminals 27 and 28,
The signal is input from the LO signal input terminals 29 and 30 to the Gilbert cell pair 22
, And an IF signal is output from IF output terminals 31 and 32.

The Gilbert cell pair 22 comprises four transistors in a double balanced connection, the common collectors of which are connected to Vcc via output loads 25,26.

The differential amplifier 21 comprises two NPN bipolar transistors 23 and 24 and a resistor 33 disposed between the emitters of these transistors, each emitter being composed of a differential amplifier 21 and a Gilbert cell pair. It is connected to a constant current source 34 that supplies an operating current to 22.

The RF input signal is applied to the differential amplifier 21 as a balanced signal from the RF input terminals 27 and 28, or as an unbalanced signal by grounding one of the RF input terminals 27 and 28 at high frequency. It is applied to the differential amplifier 21.

[0008] The applied RF input signal is applied to a differential amplifier 21.
To the differential RF current outputs, which are obtained at the collectors of transistors 23 and 24, respectively.

On the other hand, the LO signal is supplied to an LO signal input terminal 29,
The signal 30 is applied to the Gilbert cell pair 22 as a balanced signal, or is applied to the Gilbert cell pair 22 as an unbalanced signal by grounding one of the LO signal input terminals 29 and 30 at high frequency.

If the applied LO signal is moderately large, the Gilbert cell pair 22 operates as a mere current switch controlled by the LO signal, as shown at 91 in FIG. Accordingly, by connecting the collector of the transistor constituting the differential amplifier 21 to the emitter of the Gilbert cell pair 22, a current including the product of the input RF signal and the LO signal appears at the collector of the Gilbert cell pair.

[0011] The IF output signal is supplied to the I
Obtained as balanced outputs at the F output terminals 31 and 32.

In order to reduce the intermodulation distortion in the conventional mixer circuit, the resistance 33 between the emitters of the transistors constituting the differential amplifier 21 is increased to increase the amount of feedback, or A method of increasing the bias current 34 of the transistor is adopted.

The method of increasing the resistance 33 between the emitters is basically to suppress the distortion by lowering the gain of the differential amplifier 21, which leads to deterioration of the sensitivity, that is, the noise figure. Not preferred.

The method of increasing the bias current 34 of the differential amplifier 21 reduces distortion and noise generated in the differential amplifier 21.

[0015]

When the operating current of the differential amplifier 21 is increased, the distortion and noise generated in the differential amplifier 21 are reduced. On the other hand, the noise generated from the Gilbert cell pair 22 is mainly shot noise, and when the operating current is increased, the noise generated from the Gilbert cell pair 22 becomes very large. For these reasons, differential amplifiers
When the operating currents of the pair 21 and the Gilbert cell 22 are simultaneously increased, the intermodulation distortion is reduced.
There is a problem that the noise generated at 22 increases and the noise figure of the entire mixer circuit deteriorates.

In order to simultaneously realize low distortion and low noise in the entire mixer circuit, the optimum operating current value in each of the Gilbert cell pair 22 and the differential amplifier 21 is different.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and realizes low distortion and low noise at the same time, and can cope with a wideband input signal containing a DC component such as a video signal. An object is to provide a mixer circuit suitable for an integrated circuit.

[0018]

Therefore, the mixer circuit of the present invention is configured so that the Gilbert cell pair and the differential amplifier can set independent operating currents.

Therefore, it is possible to supply an operating current for performing an optimal operation to each of the differential amplifier and the Gilbert cell pair, thereby realizing a low distortion and low noise mixer circuit.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention according to claim 1 of the present invention provides a circuit in which four transistors are double-balanced and a differential high-frequency signal is applied to two sets of common emitters of the double-balanced circuit. In a mixer circuit including a circuit added by a differential amplifier, the operation currents of the double-balanced circuit and the differential amplifier are configured to be independent of each other, and the differential amplifier and the Gilbert cell pair ( An operation current for performing an optimum operation can be supplied to each of the four transistors (a circuit in which four transistors are double-balanced and connected).

According to a second aspect of the present invention, there is provided a current source for supplying a current corresponding to a difference between operating currents of the double balanced circuit and the differential amplifier to collectors of two sets of transistors constituting the differential amplifier. It can supply an operating current for performing an optimal operation to the Gilbert cell pair, and can be applied to a wideband input signal including a DC component to produce an output with low noise and low distortion. Obtainable.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(First Embodiment) In a mixer circuit according to a first embodiment, as shown in FIG. 1, bias current sources for a differential amplifier 41 and a Gilbert cell pair 42 are separately provided. Is coupled to the emitters of the transistors constituting the Gilbert cell pair 42 via coupling capacitors 43 and 44 in an alternating manner.

If the LO signals applied to the LO input terminals 49 and 50 are sufficiently large, the Gilbert cell pair 42
Similarly, operates as a simple changeover switch. The input RF signal is applied to RF terminals 47 and 48,
Since the low-impedance emitter of the transistor constituting the Gilbert cell pair 42 is added to the low-impedance emitter of the transistor constituting the Gilbert cell pair 42 from the coupling capacitor 43, 44
Is very small even with a small operating current.

Therefore, the distortion as a whole of the mixer circuit is mainly the distortion generated by the differential amplifier 41,
By setting the operating current of the differential amplifier 41 large so that distortion and noise are reduced, distortion and noise in the differential amplifier 41 can be reduced.

On the other hand, the distortion generated in the Gilbert cell pair 42 is sufficiently small even with a small operating current for the above reason.
The noise generated by the Gilbert cell pair 42 is mainly shot noise, and the noise generated from the Gilbert cell pair 42 can be reduced by reducing the operating current.

According to the mixer circuit shown in FIG.
The operating currents of the 41 and the Gilbert cell pair 42 can be independently set so as to optimize each other, and a low distortion and low noise mixer circuit can be realized.

(Second Embodiment) In the second embodiment,
A description will be given of a mixer circuit obtained by further improving the circuit of the first embodiment (FIG. 1).

In the configuration shown in FIG. 1, since the differential amplifier 41 and the Gilbert cell pair 42 are AC-coupled using the capacitors 43 and 44, a wide-band input signal including a low frequency is passed. Therefore, the values of the coupling capacitors 43 and 44 must be sufficiently large.

Therefore, when the mixer circuit of FIG. 1 is realized by a semiconductor integrated circuit, a very large chip area is occupied by the capacitor.

Also, since the capacitors realized by the semiconductor integrated circuits have large variations in capacitance values, the signals applied to the emitters of the Gilbert cell pair 42 are unbalanced, causing distortion.

Further, the mixer circuit shown in FIG. 1 cannot essentially handle an input signal containing a DC component such as a video signal.

The mixer circuit according to the second embodiment improves on these points. This circuit, as shown in FIG.
Current sources 3 and 4 for making the operating current of the Gilbert cell pair 2 different from the operating current of the differential amplifier 1 are provided. Other configurations are the same as those of the conventional mixer circuit (FIG. 9).
There is no change in how to apply the RF signal and the LO signal and how to extract the IF output signal.

In this circuit, the operating current sources 14 and 15
The optimum operating current of the differential amplifier 1 is supplied to the differential amplifier 1,
The current sources 3 and 4 supply the difference current between the operating current supplied from the operating current sources 14 and 15 and the optimum operating current of the Gilbert cell pair 2 to the collectors of the two transistors of the differential amplifier 1. Therefore, the Gilbert cell pair 2 is supplied with the optimum operating current of the Gilbert cell pair 2.

FIG. 3 shows that the bias current of the differential amplifier 1 is 5
mA, the operating current of the Gilbert cell pair 2 (Icc
4 shows the conversion gain (Gain) and the noise figure (NF) when G.I.Gilbert is changed from 1 mA to 5 mA. FIG. 4 shows the third-order input intercept point (IP3in) and noise under the same conditions. Exponent (N
F). In these figures, the operating current (Icc Gilbert) of the Gilbert cell pair 2 is 5 m.
At the time of A, the characteristic becomes that of the conventional mixer circuit shown in FIG.

FIG. 4 shows that as the operating current (Icc Gilbert) of the Gilbert cell pair 2 is increased, the noise figure (NF) of the entire mixer circuit monotonically deteriorates. Third input intercept point (I
P3in) is the operating current of the Gilbert cell pair 2 (Icc
Gilbert), and reaches a maximum at about 2 mA.

According to FIG. 3, the conversion gain (Gain) is
It is almost constant within 0.4 dB within the operating current range of the Gilbert cell pair from 1 mA to 5 mA.

As described above, by setting the bias current of the differential amplifier 1 to 5 mA and the operating current of the Gilbert cell pair 2 to 2 mA, the noise figure (NF) is about 1.6 dB as compared with the conventional mixer circuit. Improvement The third-order input intercept point (IP3in) has an improvement effect of about 1.5 dBm. Regarding the conversion gain (Gain), the same value as that of the conventional mixer circuit can be realized.

As described above, in the mixer circuit of FIG. 2, the optimum operating current of the differential amplifier 1 is supplied from the operating current sources 14 and 15 to the differential amplifier 1, and the differential current from the current sources 3 and 4 is The supply provides the Gilbert cell pair 2 with the optimal operating current of the Gilbert cell pair 2. Therefore, as compared with the conventional mixer circuit, the noise figure and the third-order input intercept point (IP3in) can be improved without increasing the current consumption and without deteriorating the conversion gain. There is something great.

Further, since the mixer circuit of FIG. 2 does not use an element for limiting the input frequency, the input RF
DC is also allowed as a signal.

(Third Embodiment) FIG. 5 shows a circuit of a third embodiment. FIG. 5 shows that the current sources 3 and 4 of the mixer circuit of the second embodiment (FIG. 2) are connected to a PNP-type bipolar transistor 8.
This shows a case where the present invention is realized by a current mirror of 1, 82 and 83.

Although the circuit diagram of FIG. 5 requires an extra current source 80 as compared with FIG. 2, for example, as shown in FIG.
, The current source 100 is originally required due to the bias of the active load 102. Also, when the output of the mixer is output via the differential amplifier 203 whose input stage is formed of a PNP transistor as shown in FIG. 7, the current source 200 is originally required due to the bias of the active loads 201 and 202. It is. Alternatively, as shown in FIG. 8, the current source 34 of the mixer section is constituted by a current mirror 301, and the current source 80 is
With the PN transistor 300, the NPN transistor 300 forms a current mirror with the constant current source 301, and can also serve as the current source 80. As shown in FIGS. 6 to 8, this current source 80 can be used together with a current source used in another circuit part in an actual circuit, so that an extra increase in current consumption of the entire integrated circuit is caused. No

[0043]

As is apparent from the above description, the mixer circuit of the present invention can set the operating currents of the Gilbert cell pair and the differential amplifier to optimal values, respectively, and achieve low noise and low distortion. Can be obtained.

In a circuit provided with a current source for supplying a difference current between operating currents of a differential amplifier and a Gilbert cell pair,
Since there is no means for limiting the frequency, the present invention can be applied to a wideband input signal including a DC component, and low noise and low distortion can be realized without increasing current consumption.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a mixer circuit according to a first embodiment of the present invention,

FIG. 2 is a circuit diagram of a mixer circuit according to a second embodiment of the present invention;

FIG. 3 is a characteristic diagram showing Gilbert cell versus operating current dependence of conversion gain (Gain) and noise figure (NF);

FIG. 4 is a tertiary input conversion intercept point (IP3
Characteristic diagram showing Gilbert cell versus operating current dependence of In) and noise figure (NF),

FIG. 5 is a circuit diagram of a mixer circuit according to a third embodiment of the present invention;

FIG. 6 shows a first circuit to which the mixer circuit according to the third embodiment is applied,

FIG. 7 shows a second circuit to which the mixer circuit according to the third embodiment is applied,

FIG. 8 shows a third circuit to which the mixer circuit according to the third embodiment is applied,

FIG. 9 is a circuit diagram of a conventional mixer circuit,

FIG. 10 is an equivalent circuit diagram of a conventional mixer circuit when an LO signal is sufficiently large.

[Explanation of symbols]

1, 21, 41, 61, 101, 203 Differential amplifier 2, 22, 42, 62 Gilbert cell pair 3, 4, 14, 15, 34, 80, 301 Constant current source 5, 6, 25, 26, 55 ~ 58, 65, 66 Output load 7, 8, 27, 28, 47, 48, 67, 68 RF signal input terminal 9, 10, 29, 30, 49, 50, 69, 70 L0 signal input terminal 11, 12, 31 , 32, 51, 52, 71, 72 IF signal output terminal 23, 24 NPN bipolar transistor 33, 84 to 86 Resistor 43, 44 Coupling capacitor 81 to 83 PNP bipolar transistor 91 Current equivalent to the operation of Gilbert cell pair Selector switch 100, 200 Current source 102, 201, 202 Active load 300 NPN transistor 301 Current mirror

Claims (2)

[Claims] 1. A mixer circuit comprising: a circuit in which four transistors are double-balanced connected; and a circuit in which a differential high-frequency signal is applied to two sets of common emitters of the double-balanced circuit by a differential amplifier. A mixer circuit, wherein operating currents of a double balanced connection circuit and the differential amplifier are independent of each other. 2. A current source for supplying a current corresponding to a difference between operating currents of the double balanced circuit and the differential amplifier to collectors of two sets of transistors constituting the differential amplifier. The mixer circuit according to claim 1, wherein: