JP3334815B2 - Frequency conversion circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a frequency conversion circuit,
For example, it is suitable for application to those used in television sets.

[0002]

2. Description of the Related Art Conventionally, an integrated circuit having a circuit configuration shown in FIG. 3 is generally used as a frequency conversion circuit used in a television set. This frequency conversion circuit 1 is composed of a mixer 2 and a buffer amplifier 3.
Mixer 2 is provided with R input to lower differential pair (Q5, Q6).
F signal and a pair of differential pairs provided in the upper stage (Q7, Q8)
And the local oscillation signal OSC input to (Q9, Q10)
In order to convert the frequency of the RF signal efficiently, it is desirable that the transistors Q7 to Q10 perform a complete switching operation. This can be seen from the following.

That is, a signal current flows through the collector of the transistor Q5 by the RF signal input to the input terminal RFIN1. This signal current is split by the upper transistor Q7 or Q8 and is taken out as a voltage from the load resistor R5 or R6. On the other hand, a signal current having a phase opposite to that of the transistor Q5 flows through the collector of the transistor Q6. This signal current is also divided by the upper transistor Q9 or Q10,
It is taken out as a voltage from the load resistor R5 or R6.

At this time, if the transistors Q7, Q10 and the transistors Q8, Q9, to which signals having opposite phases are inputted, are in a balanced state, the current flowing through the load resistor R5 becomes one half of the collector current of the transistor Q5 and the transistor Q6.
Of the collector current. Load resistance R6
Are also canceled out, and the output terminals OUT1 and O2
No signal is generated in UT2. Conversely, it can be seen from the fact that when the transistors Q7, Q10 and the transistors Q8, Q9 are completely turned on or off, the maximum amplitude is obtained at the output terminal.

[0005]

If frequency conversion can be performed efficiently as described above, conversion gain and noise figure characteristics will be improved. By the way, when the operating frequency increases, the voltage amplitude for performing the switching operation of the transistor must be increased. This is because the higher the frequency, the lower the voltage applied to the intrinsic base-emitter junction due to base diffusion resistance and diffusion capacitance.

Particularly, in the case of a television tuner, the input frequency is about 50 [MHz] to 900 [MHz], so that the voltage amplitude must be increased in a high band.
However, in the case of the buffer amplifier 3 formed by the transistors Q1 to Q4, since there is no such compensation function, there has been a problem that the conversion gain decreases as the frequency increases and the noise figure deteriorates (FIG. 4).

The present invention has been made in view of the above points, and it is an object of the present invention to propose a frequency conversion circuit including a buffer amplifier capable of obtaining an optimum mixer drive amplitude over a wide band.

[0008]

According to the present invention, a high frequency (RF) signal and a local oscillation (OSC) signal are mixed, and the frequency of the high frequency (RF) signal is converted and output. Mixer circuit 12 and mixer circuit 1
2, a buffer amplifier 13 for injecting a local oscillation (OSC) signal into the load Q3, Q4 of the buffer amplifier 13, and peaking circuits R12, R1.
1 is connected.

[0009]

[Function] A peaking circuit R12 is connected to the buffer amplifier 13.
R11 is connected to increase the amplitude of the injection into the mixer circuit 12 when the frequency of the local oscillation signal is high. Thereby, frequency conversion can always be performed efficiently.

[0010]

BRIEF DESCRIPTION OF THE DRAWINGS FIG.

In FIG. 1, in which parts corresponding to those in FIG. 3 are assigned the same reference numerals, reference numeral 11 denotes a frequency conversion circuit as a whole, and a current-to-voltage conversion among transistors Q1 to Q4 constituting buffer amplifier 13 for injecting a local oscillation signal. , Except that resistors R12 and R11 are connected to the bases of the transistors Q3 and Q4. Here, the other ends of the resistors R12 and R11 are respectively connected to the common potential V1.

This local oscillation signal injecting buffer amplifier 1
3 is adapted to be peaked by resistors R11 and R12, and has a high frequency local oscillation signal OSC.
Is made larger. Hereinafter, the operation principle will be described. Therefore, the transistors Q7 to Q10
Is considered.

The signal voltage at point A which supplies the base signal voltage to transistors Q7 and Q10 is determined by the collector current of transistor Q1 and the impedance ZA at point A.
Similarly, the signal voltage at point B, which supplies the base signal voltage to transistors Q8 and Q9, is determined by the collector current of transistor Q2 and the impedance ZB at point B. Here, the frequency characteristic of the impedance ZA at the point A will be described.

The impedance ZA at the point A can be considered almost as the impedance when the transistor Q3 is viewed from the emitter, and the differential resistance r of the bipolar transistor can be considered.
Using e and the current amplification factor h _FE ,

(Equation 1) It can be expressed as.

Here, the value of the current amplification factor h _FE at a high frequency is calculated by the following equation using the transition frequency f _T.

(Equation 2) Can be obtained as

Here, it will be explained that a specific numerical value is substituted, and that a higher impedance in which the amplitude of the local oscillation signal is reduced can provide a higher impedance. In this example, the current value of the constant current source I1 connected to the differential pair of the buffer amplifier 13 is 5 [mA], and the resistance values of the resistors R12 and R11 connected to the bases of the transistors Q3 and Q4 are each 200 [mA]. Ω]. Also, the transition frequency f _T is set to 4.5 [GHz].

First, when the frequency of the local oscillation signal OSC is low (that is, 50 [MHz]), the value of the current amplification factor h _FE is 90 (= 4500/50), so that the impedance ZA (50) becomes formula

(Equation 3) It can be seen that the value is 12.6 [Ω].

Next, when the frequency of the local oscillation signal OSC is the intermediate frequency (that is, 450 [MHz]), the value of the current amplification factor h _FE is 10 (= 4500/450), so that the impedance ZA (450) Is

(Equation 4) 30.4 [Ω], which is about 2.
It turns out that it becomes about 4 times larger.

Further, when the frequency of the local oscillation signal OSC is very high (that is, 900 [MHz]), the value of the current amplification factor h _FE is 5 (= 4500/900).
Therefore, the impedance ZA (900) is given by the following equation.

(Equation 5) 50.4 [Ω], which is about four times the value at the lowest frequency.

Thus, the frequency f of the local oscillation signal OSC
Becomes higher, the impedance ZA at the point A looks larger, and at a lower frequency f, it looks smaller. Therefore, the frequency characteristic of the conversion gain becomes almost flat in any range of the gain after frequency conversion from the low band to the high band (FIG. 2). As described above, a sufficiently large gain can be obtained even in the high band, so that the frequency characteristic of the noise figure hardly deteriorates in the high band, and almost flat characteristics can be obtained over the entire band.

It is desirable that the signal amplitude for switching the transistors Q7 to Q10 be as small as possible. If the signal amplitude is too large, spurious characteristics such as unnecessary radiation will be degraded. In addition, since the frequency characteristic of the conversion gain becomes almost flat over the entire band, a frequency conversion circuit having good spurious characteristics can be obtained.

According to the above configuration, peaking is applied to the buffer amplifier 13 which supplies a local oscillation signal to the mixer 12, and when the frequency is high, the input amplitude to the mixer is increased, so that the RF signal is always efficiently converted in frequency. can do. This makes it possible to realize flat frequency characteristics in both the conversion gain and the noise figure over a wide band.

In the above embodiment, the case where peaking is performed by connecting the resistors R12 and R11 to the bases of the transistors Q3 and Q4, which are loads of the buffer amplifier 13, has been described. However, the present invention is not limited to this. Peaking may be applied by another connection. For example, peaking may be performed by connecting resistors to the emitters of the transistors Q3 and Q4, respectively. Further, the present invention can be applied to a case where a coil element is connected as a load of the differential pair (Q1, Q2) constituting the buffer amplifier 13.

In the above-described embodiment, the current value of the current source I1 for supplying a constant current to the buffer amplifier 13 is 5
[MA]. In this case, the peaking resistors R11 and R1
2 has been described as 200 [Ω], but the present invention is not limited to this, and the current value of the current source I1 and the resistance values of the peaking resistors R11 and R12 can take other values.

Further, in the above-described embodiment, the case where the present invention is applied to the frequency conversion circuit of a television set and the frequency band is set to 50 [MHz] to 900 [MHz] has been described. However, the present invention can be applied to a television tuner that receives an RF signal in a frequency range higher than this frequency band.

Further, in the above-described embodiment, the case where the present invention is applied to the frequency conversion circuit of the television set has been described. However, the present invention is not limited to this, and the present invention is applied to a mixer circuit for converting the frequency of an RF signal belonging to a high frequency band. Widely applicable.

[0027]

As described above, according to the present invention, the peaking circuit is connected to the buffer amplifier to increase the injection amplitude into the mixer circuit when the frequency of the local oscillation signal is high. As a result, the high-frequency signal can always be efficiently frequency-converted over a wide band.

[Brief description of the drawings]

FIG. 1 is a connection diagram showing one embodiment of a frequency conversion circuit according to the present invention.

FIG. 2 is a characteristic curve diagram for explaining the frequency characteristic.

FIG. 3 is a connection diagram showing a conventional frequency conversion circuit.

FIG. 4 is a characteristic curve diagram for explaining the frequency characteristic.

[Explanation of symbols]

1, 11, a frequency conversion circuit, 2, 12, a mixer,
3, 13 ... Buffer amplifier.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) H03D ^7/ 00-7/14 H04B 1/26

Claims (3)

(57) [Claims] 1. A frequency conversion circuit comprising: a mixer circuit for mixing a high-frequency signal and a local oscillation signal to convert and output the frequency of the high-frequency signal; and a buffer amplifier for injecting the local oscillation signal into the mixer circuit. A frequency conversion circuit, wherein a peaking circuit is connected to a load of the buffer amplifier. 2. The frequency conversion circuit according to claim 1, wherein said peaking circuit is connected to a base of an active load. 3. The load of the buffer amplifier comprises first and second transistors connected to the collectors of an emitter-coupled differential amplifier for inputting a local oscillation signal output from a local oscillator to a base. 2. The frequency conversion circuit according to claim 1, wherein the frequency conversion circuit comprises a resistance element connected at one end to bases of the first and second transistors and connected to a common potential at the other end.