JP3282354B2 - Amplifier circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an amplifier circuit and, more particularly, to an amplifier circuit having a variable gain in a predetermined frequency band.

[0002]

2. Description of the Related Art FIG. 4 is a circuit diagram showing an example of a conventional amplifier circuit. The amplifier circuit 10 shown in FIG. 1 has a configuration in which the gain in the low frequency band is made variable, and is used, for example, for a reproduction amplifier circuit for a headphone stereo audio signal.

That is, the amplifier 11 shown in FIG.
₄ and low-frequency cutoff frequency f _L determined by capacitor C ₂
Thus, the frequency characteristics are as shown in FIG. 5A, in which the voltage amplification gain A _V1 decreases. Output voltage of the amplifier 11 is input to the summing amplifier 12 through a variable resistor R _5.

On the other hand, the voltage amplification gain A _V2 of the amplifier 13 is
The frequency characteristics are almost flat up to the high frequency as shown in FIG. The output voltage of the amplifier 13 is input to the addition amplifier 12 and added to the output voltage of the amplifier 11.

Therefore, the frequency characteristic of the output voltage v _OUT from the amplifier 13 obtained at the output terminal 14 is shown in FIG.
It is as shown in (C). That is, the variable resistor R ₅
, The amount of boost in the low frequency range can be variably adjusted as shown by the arrow A, and the audibility can be corrected according to the user's preference, the type of music source, and the like. .

However, in the above-described amplifier circuit, when the boost amount in the low frequency band is varied, the mid-band cutoff frequency f _M changes at the same time as shown by an arrow F in FIG. There was a fear. Further, there is a problem that the circuit becomes expensive because many amplifiers are required.

Therefore, the present applicant has previously filed Japanese Patent Application No.
No. 8521 proposed an amplifier circuit that can solve the above problem. FIG. 6 shows a circuit diagram of the amplifier circuit 21 shown in the above-mentioned prior application.

In FIG. 6, the non-inverting input of the differential amplifier 22 is shown.
The input terminal v _INHas arrived,
The inverting input terminal is a resistor R_{twenty two}Grounded. Ma
A feedback circuit 26 is provided between the inverting input terminal and the output terminal.
It is connected.

The feedback circuit 26 has a configuration in which a resistor R ₂₁ is connected in series to a parallel circuit of a resistor R ₂₃ , a capacitor C _21, and a variable impedance circuit 25 formed between terminals 23 and 24. . The variable impedance circuit 25
The impedance R _V is variable.

The voltage amplification gain _AD of the amplifier circuit 21 is shown in FIG.
Lowpass boosted the frequency band, or midrange frequency substantially flat, is the frequency characteristic of attenuation in the f _H or more high frequency as shown in the.

As is well known, the voltage amplification gain A _DL of the low frequency band is

[0012]

(Equation 1)

## EQU1 ##

The mid-band cutoff frequency f _DM is substantially determined by the time constant of the series circuit of the capacitor C ₂₁ and the resistor R ₂₁ ,

[0015]

(Equation 2)

## EQU1 ##

The low cutoff frequency f _DL is substantially determined by the time constant of the parallel circuit of the capacitor C ₂₁ , the resistor R _23, and the variable impedance circuit 25.

[0018]

(Equation 3)

## EQU1 ##

Therefore, the mid-band cutoff frequency f _DM is given by (2)
With the constant as shown in the equation, the variable impedance circuit 2
By variably adjusting the value of the impedance R _V of 5, the low-frequency cutoff frequency f _DL can be variably set as shown in FIG.

At this time, the voltage amplification gain A _DL of the low frequency band is obtained.
Is also varied according to the impedance R _V of the variable impedance circuit 25 as is clear from the equation (1), and the boost amount in the low frequency band is variably adjusted.

That is, the voltage amplification gain A _DL of the low frequency band
_Changes the value of the impedance R _V from zero to an extremely large value as compared with the value of the resistor R ₂₃ , so that the minimum voltage amplification gain A _DLmin to the maximum voltage amplification gain A _{DL
It is varied} up to _DLmax .

As described above, in the circuit shown in FIG. 7, the boost amount in the low frequency band can be varied while the middle cutoff frequency _fDM is kept constant, and the audibility is not impaired. Further, unlike the amplifier circuit of FIG. 4, many amplifiers are not required, and the cost can be reduced.

[0024]

However, in the amplifier circuit 21 shown in FIG. 6, when the voltage amplification gain A _DL of the low frequency is set large, as shown by the broken line in FIG. Around the _DM , a slight dent occurs in the voltage amplification gain _AD .

Accordingly, the present invention has been made in view of the above points, and an amplifying circuit which does not change the cut-off frequency f even when the gain in a predetermined frequency band is varied, and further improves the frequency characteristics. At low cost.

[0026]

According to the present invention, there is provided an amplifying means for amplifying and outputting an input signal, an output signal from the amplifying means being fed back to an input terminal of the amplifying means, and an output signal having a predetermined frequency. Wherein the feedback means comprises a first resistor.
The second resistor is connected in series with the parallel circuit of the capacitor and the feedback capacitor.
And a voltage between both ends of the first resistor,
The second resistor connected to the input terminal of the amplifying means.
The variable impedance circuit for feeding back current to one end of a resistor
Having a low frequency band by the variable impedance circuit.
Boosts the band so that it is almost flat above the mid-frequency band.
Frequency characteristics that attenuate at high frequencies and
Variable impedance of the variable impedance circuit
By keeping the mid-range cutoff frequency constant,
The gain is variable in several bands.

[0027]

[0028]

According to the present invention, a current source for a variable impedance circuit is provided.
By changing the current value of the
The equivalent impedance in the frequency band can be varied. others
By setting the current value of the current source,
Variable band gain and constant low frequency range and characteristics
Mid-band cutoff frequency at the boundary with the mid-band
Can be made constant.

Further, the configuration in which the current is fed back to a portion different from the portion where the voltage is detected makes it possible to further improve the characteristics near the cutoff frequency.

[0030]

[0031]

[0032]

FIG. 1 is a circuit diagram of an embodiment of the present invention, and shows an amplifier circuit 1 configured so that the gain (boost amount) of a low frequency band can be varied.

In FIG. 1, an input signal v _IN from a signal source e is input to a non-inverting input terminal of a differential amplifier 2 serving as amplifying means, and an inverting input terminal is grounded via a resistor R _2. I have. A feedback circuit 6 as feedback means is connected between the inverting input terminal and the output terminal.

The feedback circuit 6 includes a resistor R ₃ and a capacitor C _1.
The resistor R ₁ in parallel circuit to the circuit connected in series, a configuration of connecting the variable impedance circuit 5. The variable impedance circuit 5 is a circuit that detects a voltage between the terminal 3 and the terminal 4 and feeds back a current corresponding to the detected voltage to the terminal 9. The ratio between the detected voltage and the returned current can be varied, and the equivalent impedance viewed from the terminals 9 and 4 can be varied.

FIG. 2 shows an example of a specific circuit of the variable impedance circuit 5. The variable impedance circuit 5
In brief, it comprises a voltage detection unit 7 and a VI (voltage-current) conversion unit 8. The terminals 3 and 4 in FIG. 1 are connected to the input terminals of the voltage detector 7.

The voltage detecting section 7 includes transistors Q ₁ and Q ₂ whose bases are connected to both terminals 3 and 4, a voltage sensing resistor R connected between the emitters of both transistors, an emitter of both transistors and ground. A current source J ₁ and a current source J ₂ are connected between them, and a diode D having an anode connected in common and a cathode connected to the collectors of both transistors.
₁ and D _2, is composed of the connected voltage source E between the anode and the power source terminals of both diodes. Current source J
₁ and the current source J ₂ have the same configuration, respectively the current I
Supply _V1 .

The VI converter 8 has a common emitter.
Connected transistor Q_ThreeAnd Q _FourAnd both Transis
Current source connected between the common emitter of the
J_ThreeAnd the current that is the collector load of both transistors
Transistor Q in mirror configuration _FiveAnd Q₆And is composed of
I have. Current source J_ThreeIs the current I_V2Supply.

The VI converter 8 includes a transistor Q_Three
And Q_FourBoth bases are diode D _TwoAnd D₁Cassow of
Voltage ΔV supplied between both bases.
Current I according to F_RWith transistor Q_FiveFrom the collector of
It has a well-known configuration for taking out.

In the voltage detecting section 7, the transistors Q ₁ and Q
_{By making} the forward drop characteristics between the base emitter and the emitter ₂ substantially the same, assuming that the current flowing through the resistor R is iR and the voltage between the terminals 3 and 4 is V ₃ ,

[0040]

(Equation 4)

## EQU1 ##

Further, the voltage ΔVF of the difference between the forward drop voltages V _F1 and V _F2 of the diodes D ₁ and D ₂ is:

[0043]

(Equation 5)

Is as follows. Here, T is the operating temperature [K], k
Is the Boltzmann constant 1.380662 × 10 ⁻²³ [JK ⁻¹ ], and q is the electron charge amount 1.6021892 × 10 ⁻¹⁹ [C].

The transistors Q ₃ and Q ₄ and the transistors Q ₅ and Q _{6 of the VI} conversion section 8 have the same characteristics.

[0046]

(Equation 6)

Is as follows.

From equations (5) and (7),

[0049]

(Equation 7)

When this equation is expanded, I _V2 (I _V1 + iR) −I _R (I _V1 + iR) = I _V2 (I _V1 −iR) + I _R (I _V1 −iR) (9)

Therefore, the current I _R output from the VI conversion unit 8 according to ΔVF is:

[0052]

(Equation 8)

Is as follows. The current I _R is fed back to the terminal 9 in FIG.

By substituting the value of the current iR in equation (4) into equation (11),

[0055]

(Equation 9)

Is as follows.

In the circuit shown in FIG. 1, a voltage V ₃ is generated between the terminals 3 and 4 according to the voltage between the terminals 9 and 4. As shown in the above equation (12), the current I proportional to the voltage V _3
R is fed back to terminal 9. Therefore, it indicates that the equivalent impedance _RVL is connected between the terminal 9 and the terminal 4.

Since the value of the resistor R is constant, each current source J
By changing the current ratio (I _V1 / I _V2 ) between ₁ (J ₂ ) and J ₃ , the feedback current I _R can be varied and the equivalent impedance R _VL can be variably adjusted.

Here, the frequency characteristic of the circuit of FIG. 1 in which the variable impedance circuit 5 is provided in the feedback circuit 6 will be described.

As shown in FIG. 3, the voltage amplification gain A _V of the amplifier circuit 1 is boosted in the low frequency band, is substantially flat above the middle frequency band, and attenuates at the high frequency band above f _H. Is done.

Voltage amplification gain A of low frequency of amplification circuit 1
_VL is a variable impedance circuit 5 at a low frequency.
The equivalent impedance between the terminals 9,4 as R _VL of

[0062]

(Equation 10)

Can be expressed by

The middle cutoff frequency f _CM is substantially determined by the time constant of the series circuit of the capacitor C ₁ and the resistor R ₁ ,

[0065]

[Equation 11]

## EQU5 ##

The low-frequency cutoff frequency f _CL is substantially determined by the time constant of a parallel circuit of the capacitor C ₁ , the resistor R ₃ and the equivalent impedance R _VL of the variable impedance circuit 5,

[0068]

(Equation 12)

Is represented by

Therefore, the mid-band cutoff frequency f _CM is given by (14)
The variable impedance circuit 5 is kept constant as shown in the equation.
By variably adjusting the value of the equivalent impedance R _VL of
The lower cutoff frequency f _CL is represented by f _CLmax , f _CL1, f _{CL2 ,.}
Can be variably set as follows.

At this time, the voltage amplification gain A _VL in the low frequency band is also varied according to the impedance R _VL of the variable impedance circuit 5 as evident from the equation (13), and the boost amount in the low frequency band is variable. Is configured.

That is, the voltage amplification gain A _VL of the low frequency band
_VL1 for example shown _A, A _VL2, is variable as ~, maximum voltage amplification gain A _VLmax low frequency the equivalent impedance of the variable impedance circuit 5 R resistance value of _VL R
_{By making} the value extremely large compared to the value of ₃ , (1
From equation 3),

[0073]

(Equation 13)

Is set as follows.

At this time, the low frequency cutoff frequency f _CLmax is
In equation (15), the value of R _VL is infinite, and

[0076]

[Equation 14]

Is given by

The minimum voltage amplification gain A of the low frequency band
_VLmin, the resistance R _1, as parallel impedance of the equivalent impedance R _VL series circuit and the variable impedance circuit 5 of R ₃ is equal to the value of the resistor R _1, by adjusting the equivalent impedance R _VL, (13 )

[0079]

(Equation 15)

It is assumed that This value is the mid-band cutoff frequency f _CM
The above frequency characteristics are nothing but voltage amplification gain in a flat band.

At frequencies equal to or higher than the low cutoff frequency f _CM , the impedance of the capacitor C ₁ becomes small, and the voltage V ₃ between the terminals 3 and 4 becomes extremely smaller than the voltage between the terminals 9 and 4. . Therefore, the current I _R fed back to the terminal 9 by the variable impedance circuit 5 becomes extremely small, and the equivalent impedance R _V becomes substantially infinite. Therefore, the equivalent impedance R _V at the low frequency range
Is variable, but at frequencies above the low cut-off frequency f _CM ,
It does not affect the voltage amplification gain A _V shown in the equation (18).

Note that the voltage amplification gain A _V is determined by the gain of the amplifier 2 above the frequency f _H.

As described above, in the circuit of FIG. 1, the boost amount in the low frequency band can be varied while the middle cutoff frequency f _CM is kept constant, and the hearing is not impaired.
The auditory sensation can be corrected according to the user's preference, the type of music source, and the like.

Further, unlike the amplifier circuit of FIG. 4, only one amplifier is required, and the cost can be reduced.

Further, in the amplifier circuit 1 of FIG. 1, unlike the circuit of the prior application of FIG. 6, the voltage V ₃ between the terminals 3 and 4 is detected, and
The current I _R according to the detected voltage is fed back to the terminal 9. In this configuration, even if the voltage amplification gain A _VL of the low frequency is set large, it is confirmed by circuit simulation (for example, SPICE) that the voltage amplification gain A _V does not have a dent around the middle cutoff frequency f _CM. Has been verified.

Therefore, in the amplifier circuit 1 of the present embodiment, FIG.
It is possible to realize better frequency characteristics than the circuit of FIG.

By applying the configuration of the variable impedance circuit 5 of this embodiment to a filter circuit such as a high-pass filter circuit, a low-pass filter circuit, and a band-pass filter circuit, it is possible to obtain a desired overall frequency characteristic of the filter circuit. It is also conceivable to variably adjust only the gain of a predetermined frequency band without affecting.

[0088]

As described above , according to the present invention, the variable in
By changing the current value of the current source of the impedance circuit,
The equivalent impedance of the feedback means in the predetermined low frequency band
The current value of the current source must be set because the
By changing the gain of the low frequency band,
At the boundary between the frequency band and the mid-frequency band that makes the characteristics constant
The mid-range cutoff frequency can be made constant. Ma
Also, the current is fed back to a part different from the part where the voltage is detected.
The configuration further improves the characteristics near the mid-range cutoff frequency.
Can be good.

[0089]

[Brief description of the drawings]

FIG. 1 is a circuit diagram of one embodiment of the present invention.

FIG. 2 is a specific circuit diagram of the variable impedance circuit of FIG.

FIG. 3 is a diagram illustrating frequency characteristics of the amplifier circuit of FIG. 1;

FIG. 4 is a circuit diagram of an example of a conventional amplifier circuit.

FIG. 5 is a diagram illustrating frequency characteristics of the amplifier circuit of FIG. 4;

FIG. 6 is a circuit diagram of the amplifier circuit of the prior application.

FIG. 7 is a diagram illustrating frequency characteristics of the amplifier circuit of FIG. 7;

[Explanation of symbols]

First amplifier circuit 2 differential amplifiers 3, 4, 9 terminal 5 the variable impedance circuit 6 feedback circuit 7 voltage detector 8 V-I (voltage - current) conversion unit J ₁ through J ₃ current source

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H03G 3/12 H03F 1/34 H03G 5/16

Claims (1)

(57) [Claims] Amplifying means for amplifying and outputting an input signal
And an output signal from the amplifying means.
Feedback to the terminal to make the output signal a predetermined frequency characteristic
Amplifier circuit having feedback means configured as described above.
hand,The feedback means comprises a parallel connection of a first resistor and a feedback capacitor.
A circuit in which a second resistor is connected in series to the circuit, Detecting the voltage between both ends of the first resistor,
One end of the second resistor connected to the input terminal of
The variable impedance circuit for feeding back the current, The variable impedance circuit boosts the low frequency band.
And becomes almost flat above the middle frequency band,
It has a frequency characteristic that attenuates with the wave number, and
By changing the impedance of the impedance circuit,
While maintaining a constant mid-range cutoff frequency,
Variable gain Characteristic amplifier circuit.