JPH0677751A - Amplifier for active filter - Google Patents

Abstract

PURPOSE:To obtain the amplifier for active filter which improves offset between input and output signals and suppresses the degradation of filter characteristics by providing independently a power source for every differential current amplifying stage. CONSTITUTION:This amplifier is independently provided with power supply parts 4a and 4b for respective differential current amplifying stages 1a and 1b. The power supply part 4a is constituted of a current mirror circuit for obtaining a output current corresponding to an input current. Therefore, since the output current of the power supply part 4a is supplied to the collector terminal of a transistor TrQ12a, the output current at a value set to the input current of the power supply part 4a is surely impressed to the first amplifying stage 1a. Thus, the collector currents of TrQ1a and Q2a are made equal, and the offset of an input/output signal potential in the amplifying stage 1a is suppressed. On the other hand, the next differential current amplifying stage 1b is made similar to the first differential current amplifying stage 1a as well. As a result, the offset between input and output signals is improved, and the degradation of filter characteristics is suppressed.

Description

Detailed Description of the Invention

[Object of the Invention]

[0002]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an active filter amplifier with improved offset.

[0003]

2. Description of the Related Art An active filter, which is often used for voice and image processing, realizes a desired filter characteristic by using an operational amplifier together with a capacitor and a resistor.

An amplifier used in such an active filter is shown in FIG. 2, for example.

The amplifier shown in FIG. 2 has two identically constructed amplifiers.
It is composed of an amplification section 2 in which two differential current amplification stages 1a and 1b are connected in cascade, and a power supply section 3 which supplies a current to the amplification section 2.

[0006] the first stage of differential current amplifier stage 1a is a bipolar transistor Q _1a of the NPN type and a pair of emitter terminals are commonly connected, and Q _2a, these transistors Q _1a, Q _2a
Is constituted by a current source I _1a for biasing and a bias circuit for the respective transistors Q _1a and Q _2a . The bias circuit of the transistor Q _1a is an NP that receives an input signal.
It is composed of an N-type bipolar transistor Q _3a , a resistor R _1a , a diode-connected transistor Q _4a and a current source I _2a . On the other hand, the bias circuit of the transistor Q _2a comprises an NPN type bipolar transistor Q _5a for receiving the output signal of the next stage, a resistor R _2a , a diode-connected transistor Q _6a and a current source I _2a . Such a first-stage differential current amplification stage 1a uses the collector terminal of the transistor _Q2a as an output stage and supplies an output signal from this output stage to the next-stage differential current amplification stage 1b.

The differential current amplification stage 1b of the next stage is constructed in the same manner as the differential current amplification stage 1a of the first stage, and the transistor Q _1b
In receiving the first stage of the output signal at the transistor Q _3b constituting a bias circuit, an output signal output from the collector terminal of the transistor Q _2b serving as an output terminal of the current stage is a transistor Q _5b constituting a bias circuit of the transistor Q _2b It is given to the base terminal.

The power supply unit 3 includes PNP-type bipolar transistors Q _7a , Q _7b and Q ₈ , resistors R _3a , R _3b , R ₄ and R _5.
It is constituted by a current mirror circuit having and a current source I _3. The power supply unit 3 includes transistors Q _7a and Q _7b.
The same current is intended to be supplied to the collector terminals of the transistors Q _2a and Q _2b of the differential current amplification stages 1a and 1b as the collector currents of the above.

The amplifier configured as described above operates in each of the differential current amplifying stages 1a and 1b so as to obtain an output signal potential equivalent to the input signal potential, so that the differential current amplifying stage 1a of the first stage is operated. An output signal potential equivalent to the input signal potential given to the transistor Q _3a is obtained at the collector terminal of the transistor Q _{2b which} is the output end of the amplifier.

However, in the power supply section 3 described above, it becomes impossible to supply an equivalent collector current to the collector terminals of the transistors Q _2a and Q _2b in the differential current amplification stages 1a and 1b, respectively. This will be described below.

FIG. 3 is a diagram showing a circuit configuration of a current mirror circuit composed of one transistor for supplying a current.

In FIG. 3, each transistor Q
₁ , Q ₂ current amplification factor β and base-emitter voltage V _BE
Are equal, the current gain I _o / of the current mirror circuit
I _i is the base current of the transistors Q ₁ and Q ₂ , which is I ₁ ,
If I ₂ is given, it can be expressed by Equation 1.

[0013]

[Equation 1]

On the other hand, the potential V _{A at the} point a shown in FIG. 3 is expressed by the equation 2, which gives the ratio I ₁ / I ₂ of the base currents I ₁ and I _2.
I ₂ is represented by Formula 3.

[0015]

[Equation 2]

[0016]

[Equation 3]

By substituting equation _{3 into} equation ₁ , equation 4 is obtained. In equation 4, R _c / R ₂ = A, R ₁ / R ₂ = n
Then, Equation 5 is obtained.

[0018]

[Equation 4]

[0019]

[Equation 5]

## EQU3 ## In Equation 5, when the term 1 / β ² is taken into consideration and expanded, Equation 6 is obtained.

[0021]

[Equation 6]

[0022] In Equation 6, since the current gain I _o / I _i of the current mirror circuit is n times is, 1 / beta that coefficient in 1 / beta ² term to the extent ^that the ^second term can be ignored beta ²
The coefficient of the term 1 / β must be 0, and A = n (n + 1) = R _c / R ₂
Becomes As a result, the compensation resistance R _c for compensating β of the transistors Q ₁ and Q ₂ is expressed by Equation 7, and R ₂ = R ₁ / n
Then, Equation 8 is obtained.

[0023]

[Equation 7]

[0024]

[Equation 8]

Here, by substituting the equation 8 into the equation 2, the equation 9 is obtained. On the other hand, the emitter current of transistor Q ₂ (1+
β) I ₂ is expressed by Equation 10, and by substituting Equation 9 into Equation 10, Equation 11 is obtained.

[0026]

[Equation 9]

[0027]

[Equation 10]

[0028]

[Equation 11]

Next, similarly to the power supply unit 3 of FIG. 2, in the current mirror circuit, a case of supplying a current transistor is two as shown in FIG. 4, a similar transistor Q ₂ of the emitter current to that described above Ask for.

In FIG. 3, assuming that the output current I _o2 in the transistor Q ₂ is n times the input current I _{o and} the output current I _o3 in the transistor Q ₃ is m times the input current,
A compensating resistor R _c1 for compensating β of the transistors Q ₁ , Q ₂ , and Q ₃ is expressed by Expression 12 corresponding to _Expression 8.

[0031]

[Equation 12]

Further, the potential V _{B at the} point b in FIG. 4 is expressed by Expression 13, and when Expression 12 is substituted into Expression 13, Expression 14 is obtained.

[0033]

[Equation 13]

[0034]

[Equation 14]

On the other hand, the emitter current (1 + β) I ₂ of the transistor Q ₂ is represented by the equation 15, and the equation 16 is obtained by substituting the equation 14 into the equation 15.

[0036]

[Equation 15]

[0037]

[Equation 16]

Therefore, the output current I of the circuit shown in FIG.
_o and the output current of the circuit shown in FIG. 4 I _o2 is what must become inherently equal, will be different as is clear from the number 11 and number 16, the output current in the current mirror circuit shown in FIG. 4 I _o2 cannot be obtained exactly as n times the input current I _o . As a result, the pair of transistors Q in each of the differential current amplification stages 1a and 1b is
_1a, Q _2a and transistors Q _1b, occurs a difference in the collector potential of the Q _2b, each of the differential current amplifier stage 1a, the offset between the input and output signals in 1b will occur.

[0039]

As described above,
In a conventional active filter amplifier in which a plurality of differential current amplification stages having a common power supply section 3 are connected in cascade, an offset occurs between the input and output signals of each differential current amplification stage. There was an offset as a whole. Therefore, if an active filter is constructed using such an amplifier, there is a problem that the filter characteristics are deteriorated.

Therefore, the present invention has been made in view of the above, and an object thereof is to improve an offset between input and output signals and to contribute to suppressing deterioration of filter characteristics. To provide an amplifier.

[Constitution of Invention]

[0042]

In order to achieve the above object, the present invention provides a first transistor which is base-biased on the basis of an input signal or an output signal of a differential current amplifying stage of the preceding stage and a next stage or its own. Of the second transistors whose base terminals are biased based on the output signal of the differential current amplification stage of the second transistor are commonly connected to obtain an output potential equal to the input potential at the collector terminal of the second transistor. Supplying a collector current to a differential current amplifying unit in which a plurality of current amplifying stages are connected in cascade, and a second transistor of each of the differential current amplifying stages, which is provided independently of each of the differential current amplifying stages. And a power supply unit for

[0043]

In the above structure, the present invention ensures the current set in each differential current amplification stage by providing an independent power source for each differential current amplification stage constituting the differential current amplification unit. I am trying to supply it to.

[0044]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a diagram showing a circuit configuration of an active filter amplifier according to an embodiment of the present invention. Like the active filter amplifier shown in FIG. 2, the active filter amplifier of the embodiment shown in FIG. 1 has two differential current amplification stages connected in cascade, and outputs a potential equivalent to the input signal potential as an output signal. It is obtained as an electric potential. In FIG. 1, the same reference numerals as those in FIG. 2 have the same functions, and the description thereof will be omitted.

In FIG. 1, the amplifier is configured to have independent power supply units 4a and 4b for each differential current amplification stage 1a and 1b.

The power supply section 4a is the first stage differential current amplification stage 1a.
_Is a current source composed of a current mirror circuit for supplying a current to the collector terminal of the transistor Q _2a . The power supply unit 4a includes PNP-type bipolar transistors _Q11a and Q11.
_12a , resistors R _11a , R _12a and R _13a, and a current source I _11a .

The emitter terminal of the transistor Q _11a is connected to the power supply V _cc via the resistor R _11a , and the base terminal is connected to the collector terminal via the compensating resistor R _12a for compensating the current amplification factor β. , Collector terminal is current source I
It is connected to the ground potential via _11a . The transistor Q _12a has its emitter terminal connected to the power supply V _cc via the resistor R _13a , its base terminal connected to the base terminal of the transistor Q _11a via the resistor R _12a , and its collector terminal connected to the first stage differential current amplifier. Transistor Q _2a in stage 1a
It is connected to the collector terminal of.

In the power supply section 4a having such a structure,
It is a current mirror circuit that obtains one output current with respect to the input current. Therefore, such a power supply unit 4a
Is supplied to the collector terminal of the transistor Q _12a , the output current having a value set with respect to the input current of the power supply section 4a is surely given to the first differential current amplification stage 1a. . As a result, the transistor Q _1a ,
The collector currents of Q _2a become equal, and the offset of the input / output signal potential in the first stage differential current amplification stage 1a is suppressed.

On the other hand, even in the differential current amplification stage 1b of the next stage, as described above, it becomes the same as the differential current amplification stage 1a of the first stage.

Therefore, the offset of the amplifier as a whole is remarkable when the value of h _FE of the transistor is small, as is clear from the simulation result shown in Table 1, when the input signal is simulated at about 3.4 (V). Will have a significant improvement effect.

[0052]

[Table 1]

However, in the active filter to which the above-mentioned amplifier is applied, the offset of the amplifier is improved, so that the deterioration of the filter characteristic is suppressed.

[0054]

As described above, according to the present invention, since an independent power source is provided for each differential current amplification stage, the current set in each differential current amplification stage can be accurately measured. Can be supplied to the active filter amplifier, which can improve the offset between the input signal potential and the output signal potential, and can suppress the deterioration of the filter characteristic.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of an active filter amplifier according to an embodiment of the present invention.

FIG. 2 is a diagram showing a configuration of a conventional active filter amplifier.

FIG. 3 is a diagram showing a circuit configuration of a current mirror circuit.

FIG. 4 is a diagram showing a circuit configuration of a current mirror circuit.

[Explanation of symbols]

 1a, 1b Differential current amplification stage 2 Differential current amplification section 3, 4a, 4b Power supply section

Claims (1)

[Claims] 1. A first bias that is base biased based on an input signal or an output signal from a preceding differential current amplification stage.
And a second transistor whose base bias is based on the output signal of the next stage or the differential current amplifying stage of its own stage are commonly connected to each other.
A differential current amplifying section in which a plurality of differential current amplifying stages for obtaining an output potential equivalent to the input potential are cascade-connected to the collector terminal of the transistor, and each of the differential current amplifying stages is independently provided. An active filter amplifier comprising: a power supply unit that supplies a collector current to the second transistors of each differential current amplification stage.