JPH04269005A - Negative feedback amplifier - Google Patents

Abstract

PURPOSE:To improve the ternary cross modulation distortion characteristic considerably while the size and the gain of a transistor(TR) are kept constant and to design the gain and the distortion characteristic independently. CONSTITUTION:Let an input voltage be Vi, an output voltage Vo, a voltage amplification factor of an amplifier 10 be A, a transfer function of a feedback circuit 22 be B22, and an attenuation ratio of an attenuator 30 be (n), then a voltage gain G of a negative feedback amplifier is expressed as G=Vo/Vi=nA /(1-nAB2). When a quadratic nonlinear characteristic expressed as Vo=AVi+DVi<2> is in existence between the input voltage Vi and the output voltage Vo in a main amplifier section (comprising an amplifier 10 and an attenuator 30), since a distortion output voltage VH is expressed as VH=DVi<2>n<2>/(1-nAB2), then the distortion is suppressed into n<2>/(1-nAB2) by negative feedback. That is, the distortion is suppressed by a product between the suppression component n<2> by the attenuator 30 and the (1-nAB2) by negative feedback.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a negative feedback amplifier that maintains constant gain, reduces distortion, and improves linearity.

0002

2. Description of the Related Art FIG. 6 is a block diagram showing the basic configuration of a conventional feedback amplifier. In the figure, the feedback amplifier includes an amplifier 10 and a feedback circuit 21 connecting its input and output. Here, if the input voltage is Vi, the output voltage is Vo, the voltage amplification degree of the amplifier 10 is A, and the transfer function of the feedback circuit 21 is B1, the voltage gain G of the feedback amplifier is:
G=A/(1-AB1). In the case of negative feedback, |1-AB1|>1.

Therefore, when the feedback amplifier shown in FIG. 6 is used as a negative feedback amplifier, the amplifier 10
If there is a second-order nonlinearity between Vo = AVi + DVi2 between the output voltage Vo and the output voltage Vo, the distorted output voltage VH
is VH = DVi2 + AB1VH (1-AB1)VH = DVi2, therefore, VH = DVi2 / (1-AB1), so the distortion is suppressed to 1/(1-AB1) by negative feedback. . In other words, the distortion rate is improved by the amount of gain reduction due to negative feedback.

0004

[Problem to be Solved by the Invention] However, in conventional negative feedback amplifiers, when the amount of feedback B1 is increased to improve distortion, the gain decreases, making it impossible to achieve the specifications of a device using this negative feedback amplifier. . That is, gain and distortion characteristics cannot be designed independently, and the effect of improving distortion characteristics is limited to a range that satisfies a predetermined gain.

Furthermore, in order to simultaneously achieve significant improvements in gain and distortion characteristics, it is necessary to increase the size of the transistors constituting the main amplifier several times, resulting in an increase in the size of the circuit and a significant increase in power consumption. An increase was inevitable. An object of the present invention is to provide a negative feedback amplifier in which the third-order intermodulation distortion characteristics can be significantly improved while the transistor size and gain are held constant, and the gain and distortion characteristics can be designed independently. shall be.

[0006]

[Means for Solving the Problems] The present invention provides an amplifying means for amplifying a signal input from a signal input terminal, and a feedback means for feeding back a signal at an output terminal of the amplifying means to an input terminal of the amplifying means. In the negative feedback amplifier, a linear attenuation is provided between the signal input terminal and the input terminal of the amplifying means, and between the feedback means and the input terminal of the amplifying means, giving linear attenuation to the input signal and the feedback signal. It is characterized in that a damping means is connected.

[0007]

[Operation] In the present invention, when the attenuation ratio of the attenuation means is n, the input signal voltage Vi is converted to nVi and applied to the input terminal of the amplification means, and a part of the amplified output signal is passed through the feedback means. The structure is such that the signal is fed back to the damping means. here,
The gain of the negative feedback amplifier of the present invention is equal to the gain A of the amplification means in a conventional negative feedback amplifier converted to nA, so if the feedback amount by the feedback means of the present invention is B2, then nA/(1- nAB2). In order to ensure this gain is A/(1-AB1), which is the gain of the conventional negative feedback amplifier, and to reduce the distorted output voltage, the attenuation ratio n
may be set to a value smaller than 1.

That is, since a voltage n times as small as the input signal voltage is applied to the amplifying means that generates distortion, the secondary distortion factor can be reduced by n times, and the feedback amount B of negative feedback can be reduced.
By reducing 2, the gain as a negative feedback amplifier can be ensured. Furthermore, since the third-order cross-modulation distortion is subject to a two-degree nonlinear effect at its distortion rate, it can be further reduced by a factor of n2.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a block diagram showing the basic configuration of a negative feedback amplifier according to the present invention. In the figure, the negative feedback amplifier of the present invention includes an amplifier 10 and an attenuator 3 constituting the main amplification section.
0 and a feedback circuit 22 that connects its input and output. Here, the input voltage is Vi and the output voltage is Vo
, the voltage amplification degree of the amplifier 10 is A, the transfer function of the feedback circuit 22 is B2, and the attenuation ratio of the attenuator 30 is n, then Vo
=nAVi+nAB2Vo (1-nAB2)Vo=nAVi Therefore, the voltage gain G of the negative feedback amplifier is G=Vo/
Vi=nA/(1-nAB2). Here, the main amplification section (amplifier 10 and attenuator 30) input voltage Vi
If there is a second-order nonlinearity between Vo = AVi + DVi2 between the output voltage Vo and the output voltage Vo, the distorted output voltage VH
is, VH=D(nVi)2+nAB2VH (1-AB2)
Since VH = DVi2 and therefore VH = DVi2 n2/(1-nAB2), the distortion is suppressed to n2/(1-nAB2) by negative feedback. That is, the distortion is caused by the attenuator 30
The suppression amount n2 due to negative feedback and the suppression amount 1/(1-n
AB2) is suppressed by the product of

Table 1 compares the gain and distortion of the conventional negative feedback amplifier and the negative feedback amplifier of the present invention.

[0011]

[Table 1]

Here, the conditions for the gain of the conventional negative feedback amplifier and the negative feedback amplifier of the present invention to be the same are as follows.

[0013]

[Math 1]

[0014] Furthermore, the conditions under which the overall suppression ratio of second-order distortion is improved by the negative feedback amplifier of the present invention are as follows:

001
5]

[Math 2]

[0016] Therefore, the condition for the damping ratio n that satisfies these conditions is n<1. That is, the attenuation ratio n (n<
By inserting the attenuator 30 of 1), it is possible to reduce the distorted output voltage while keeping the gain constant. At this time, the condition for keeping the gain constant is given by equation (1).

00
17]

[Math 3]

Since A<0, B2-B1<0. In other words, the transfer function (feedback amount) of the feedback circuit 22
It can be seen that B2 becomes smaller and the gain can be kept constant. Here, the total suppression ratio of second-order distortion when the gains are the same is the feedback circuit transfer function B1 of the conventional configuration shown in FIG.
It can be expressed as n/(1-AB1) using . That is, when the gain is the same as that of the conventional configuration, the second-order distortion factor can be reduced by n times (n<1). Therefore, for third-order cross-modulation distortion that occurs when multiple carriers are simultaneously amplified, the second-order distortion rate is improved twice, which is n2 times (n<1) compared to the conventional configuration. can be made smaller.

FIG. 2 is a block diagram showing the configuration of a first embodiment of the negative feedback amplifier of the present invention. In the figure, this embodiment is characterized in that an attenuator 30a is configured by a voltage dividing circuit using capacitors 31 and 32. Here, the capacitance of each capacitor 31, 32 is assumed to be C1, C2. Therefore, the input voltage nVi to the amplifier 10 is reduced by the attenuator 30a.

[0020]

[Math 4]

[0021] Here, since n<1, (3
) If the transfer function B2 of the feedback circuit 22 is set based on the formula B2-B1=C2/AC1, the gain is the same as that of a conventional negative feedback amplifier, and the third-order intermodulation distortion is multiplied by n2 (n<1 ) can be suppressed. Figure 3 shows
FIG. 2 is a block diagram showing the configuration of a second embodiment of the negative feedback amplifier of the present invention.

In the figure, in this embodiment, resistors 33,
It is characterized in that the attenuator 30b is constituted by a voltage dividing circuit using 34. Here, the resistance values of the respective resistors 33 and 34 are assumed to be R1 and R2. Therefore, the input voltage nVi to the amplifier 10 is reduced by the attenuator 30b.

[0023]

[Math 5]

[0024] Here, since n<1, (3
) If the transfer function B2 of the feedback circuit 22 is set based on the formula B2-B1=R1/AR2, the gain is the same as that of a conventional negative feedback amplifier, and the third-order intermodulation distortion is multiplied by n2 (n<1 ) can be suppressed. Note that in the first embodiment and the second embodiment, the attenuators 30a and 30b
Since there is no phase rotation at , the stability of conventional negative feedback amplifiers can be maintained. In addition, if it is stable even if there is a phase rotation due to the feedback circuit 22 and the attenuator 30,
The attenuator may be configured by combining a capacitor and a resistor.

FIG. 4 is a block diagram showing the configuration of a third embodiment of the negative feedback amplifier of the present invention. In the figure, this embodiment is characterized in that an attenuator 30c is constituted by an attenuator 35 having an attenuation amount αdB. Therefore, the input voltage nVi to the amplifier 10 is reduced by the attenuator 30c.

[
0026

[Math 6]

[0027] Here, since n<1, (3
) so that the formula is satisfied.

[0028]

[Math 7]

The transfer function B2 of the feedback circuit 22 is based on
By setting , the gain is the same as that of a conventional negative feedback amplifier and
It is possible to suppress the order cross modulation distortion by n2 times (n<1). FIG. 5 is a block diagram showing the configuration of a fourth embodiment of the negative feedback amplifier of the present invention. In the figure, this embodiment is characterized in that an attenuator is constructed using an input parasitic capacitor 12 of a transistor or other amplification element 11 that constitutes an amplifier 10. That is, a capacitor 36 is connected in series to the input stage of the amplifier 10, and by this capacitor 36 and the input parasitic capacitor 12, an attenuator 30a similar to that of the first embodiment is realized. Here, the input parasitic capacitor 12
And the capacitance of the capacitor 36 is assumed to be CP, C3. Note that matching circuits (impedance converters) 37 and 38 are inserted in the input stage and output stage of the negative feedback amplifier of this embodiment.

Therefore, similarly to the first embodiment, if the transfer function B2 of the feedback circuit 22 is set based on B2-B1=CP/AC3, the gain is the same as that of the conventional negative feedback amplifier, and the third-order cross modulation is achieved. Distortion can be suppressed by n2 times (n<1). The advantage of the configuration of this embodiment is that when the attenuator 30 is formed by a voltage divider circuit using a capacitor, the input parasitic capacitor of the amplification element 11 is used, so only one additional capacitor is required, and the configuration In addition, the effects of variations in parasitic capacitance can be easily reduced by reducing
3) can be suppressed.

By the way, in the embodiment configuration shown above, an example was shown in which the attenuator was configured with a capacitor or a resistor, but the same result can be obtained even if an attenuator is configured with a transformer, transmission line, or other linear element. characteristics can be obtained. Further, as the amplifier 10, an electron tube or a tunnel diode can also be used.

[0032]

[Effects of the Invention] As explained above, the present invention reduces the signal voltage input to the amplification means through the attenuation means, and compensates for the gain reduction caused by this by reducing the amount of negative feedback. Therefore, compared to conventional negative feedback amplifiers, third-order intermodulation distortion characteristics can be significantly improved without loss of gain.

Furthermore, the configuration according to the present invention can achieve the same effect as when the gate width of the FET is effectively increased by several times, and can easily realize miniaturization of the circuit scale and reduction of power consumption. be able to. Further, when the attenuation means is realized by a capacitor, the noise figure does not deteriorate, so it can be applied to a low-noise amplifier.

Furthermore, the present invention can be applied to low-noise receiving amplifiers, variable gain amplifiers, and high-output transmitting amplifiers for digital microwave devices that require highly linear amplifiers, or that require low power consumption and low distortion characteristics. When applied to transmitting/receiving amplifiers for car phones, mobile phones, and other mobile communication devices, performance can be improved at a low cost.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the basic configuration of a negative feedback amplifier of the present invention.

FIG. 2 is a block diagram showing the configuration of a first embodiment of the negative feedback amplifier of the present invention.

FIG. 3 is a block diagram showing the configuration of a second embodiment of the negative feedback amplifier of the present invention.

FIG. 4 is a block diagram showing the configuration of a third embodiment of the negative feedback amplifier of the present invention.

FIG. 5 is a block diagram showing the configuration of a fourth embodiment of the negative feedback amplifier of the present invention.

FIG. 6 is a block diagram showing the basic configuration of a conventional feedback amplifier.

[Explanation of symbols]

10 Amplifier 11 Amplification element 12 Input parasitic capacitor 21, 22 Feedback circuit 30 Attenuator 31, 32 Capacitor 33, 34 Resistor 35 Attenuator 36 Capacitor 37, 38 Matching circuit

Claims (1)

[Claims] 1. A negative feedback amplifier comprising: amplification means for amplifying a signal input from a signal input terminal; and feedback means for feeding back a signal at an output terminal of the amplification means to an input terminal of the amplification means, A linear attenuation means for linearly attenuating the input signal and the feedback signal is connected between the signal input terminal and the input terminal of the amplification means, and between the feedback means and the input terminal of the amplification means. Negative feedback amplifier.