JPH0666597B2 - Variable transconductance amplifier circuit - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a variable transconductance amplifier circuit which is suitable for the configuration of an active filter including an integrator circuit and has a good high frequency characteristic. Is.

[Conventional technology]

FIG. 6 shows an example of a conventional variable transconductance amplifier circuit. The variable transconductance amplifier circuit includes an amplifier A and a single end output circuit B. Amplifier A, the transistors Q _1, Q ₂ to input terminals 1 and 2 are connected to the base, the transistor Q _1,
A resistor R _E connected between the emitters of Q ₂ and a transistor
Current source transistors Q ₇ and Q ₈ connected to the emitters of Q ₁ and Q ₂ , respectively, and diodes (diode-connected transistors) D ₁ and D connected to the collectors of transistors Q ₁ and Q _2.
It is formed from ₂ and. The anodes of the diodes D ₁ and D ₂ are commonly connected and connected to the power supply terminal 4 via the resistor R. The current source transistors Q ₇ and Q ₈ are
_A current mirror circuit is formed together with ₉ , and a variable current source circuit 8 is connected to the collector of the transistor Q ₉ . By supplying the variable current I _n to the diode-connected transistor Q ₉ via the variable current source circuit 8, the variable current I _n flows as the collector current of the current source transistors Q ₇ and Q ₈ . Single-ended output circuit B includes transistors Q _3, Q ₄ forming a differential pair of transistors, and is formed by a current mirror circuit consisting of transistors Q _5, Q ₆ is its load. The connection point between the cathode of the diode (diode-connected transistor) D ₁ and the collector of the transistor Q ₁ is connected to the base of the transistor Q ₃ , and the connection point between the cathode of the diode D ₂ and the collector of the transistor Q ₂ is the transistor. Connected to the base of Q ₄ . The emitters of the transistors Q ₃ and Q ₄ are commonly connected to the constant current source circuit 6 and to the power supply terminal 4. The collector of the transistor Q ₃ are connected to the collector of the transistor Q ₅ is connected to the output terminal 3, the collector of the transistor Q ₄ are connected to the base and collector of the transistor Q _6,
A current mirror circuit is formed by Q ₅ and Q ₆ . The emitters of the transistors Q ₅ and Q ₆ are grounded. The collector of the transistor Q ₄ is connected to the collector-base of the transistor Q ₆ , and the emitter of Q ₆ is grounded. A differential transistor pair consisting of transistors Q _3, Q ₄ is a constant current 2I _X via a constant current source circuit 6 is supplied as an operating current.

In the figure, if an input voltage V _i is applied between the input terminals 1 and 2, a current i _a flows through the resistor R _E , so that the relational expression of i _a = V _i / R _E (1) It stands. The collector current of the transistor Q _1, Q _2, respectively _{(I n} + i _a), current flows in the _(I n -i _a). When the potential difference between the collectors of the transistors Q ₁ and Q ₂ is V ₀ , the potential difference V ₀ is expressed by the following equation.

_{V 0} = V _T ln _{[(I n + i a) /} I s ] -V _T ln _{[(I n -i a) / I} s ] = _{V T} ln _{[(I n + i a) /} (I n -i _a )] ……
(2) [However, V _T is a thermal voltage. If the base potential difference between the transistors Q ₃ and Q ₄ is V _B , the base potential difference V _B is expressed by the following equation. Note that a transistor that changes depending on the potential difference V _B between the bases
Q _3, variation in the collector current of Q ₄ are are expressed in ± i _b.
I _s is the reverse saturation current.

_{V B} = V _T ln _{[(I X + i b) /} I s ] -V _T ln _{[(I X -i b) / I} s ] = _{V T} ln _{[(I X + i b) /} (I X -i _b )] ……
(3) Since the potential difference V ₀ between the collectors of the transistors Q ₁ and Q ₂ and the potential difference V _B between the bases of the transistors Q ₃ and Q ₄ are equal to each other, the current _ib is expressed by the following equations (2) and (3). In equation (1), i _a = V _i
From / R _E , it is expressed by the following equation.

_{(I n + i a) /} (I n -i a) = (I X + i b) / (I X -i b) i b = I X · i a / I n = I X · V i / R E · I _n (4) Therefore, the conductance gm of the conventional variable transconductance amplifier circuit is expressed by the following formula because the output current is 2i _b when the formula (4) is partially differentiated.

_{gm = ∂ (2i b) /} ∂V i = 2I X / R E · I n ...... (5) trans-conductance gm of the variable transconductance amplifier circuit in the conventional example shown in FIG. 6, the formula (5) It is expressed by a relational expression.

Conventionally, when changing the transconductance gm,
By adjusting the value of the variable current I _n supplied from the variable-type current source circuit 8, it is possible to change gm as is clear from the equation (5). But the transistor Q ₁
A operating current collector current _(I n + _{i a)} and the collector current is the operating current of the transistor Q _{2 (I n} -
Due to the change of i _a ), the frequency characteristic is deteriorated as shown in FIG.

[Problems to be Solved by the Invention]

Regarding the frequency characteristic of the conventional variable transconductance amplifier circuit, as is clear from FIG. 4, when the variable current I _n is narrowed down in order to reduce the value of the transconductance gm,
The frequency characteristic is the curve (a) (b) (c) in Fig. 4.
As shown in (d), it has a drawback that it gradually deteriorates. Incidentally, the horizontal axis of FIG. 4 represents the frequency, and the vertical axis represents the amount of attenuation. As described above, in the normal transistor or the amplifier circuit, the cutoff frequency f _T is decreased by narrowing the current, and the resistance value of the emitter resistance r _e thereof is increased.
The frequency characteristic is deteriorated, and the frequency characteristic is deteriorated as shown in FIG.

The present invention has been made to remedy the above-mentioned drawbacks, and its main object is to provide a variable transconductance amplifier circuit having good frequency characteristics even when the transconductance gm is changed.

Still another object is to provide a variable transconductance amplifier circuit whose transconductance gm can be easily controlled.

[Means to solve the problem]

The present invention has been made based on the above-mentioned problems, and the collectors of the first and second transistors are connected to the cathodes of the first and second diodes (diode-connected transistors), respectively, and the emitters thereof are connected to each other. And a resistor connected to each of the emitters of the first and second current source transistors, and an input voltage is applied to the bases of the first and second transistors. In a variable transconductance amplifier circuit composed of the single-ended output circuit, the collector currents flowing in the first and second current source transistors are varied to change the operating currents of the first and second transistors. Variable transconductance comprising first means for adjusting in advance and second means for supplying a variable current to the first and second transistors. About the width circuit.

[Action]

The present invention provides a variable transconductance amplifier circuit with a variable current source circuit.By supplying or drawing a current from the variable current source circuit added to the amplifier to the variable transconductance amplifier circuit, the transconductance gm It is possible to change the value,
Moreover, the high frequency characteristic is improved by preventing the deterioration of the cutoff frequency.

〔Example〕

FIG. 1 shows an embodiment of a variable transconductance amplifier circuit according to the present invention. The variable transconductance amplifier circuit is composed of an amplifier A, a single end output circuit B and a variable current source circuit C. In the embodiment of FIG. 1, a variable current source circuit C is added to the circuit of FIG. 6, and the amplifier A and the single end output circuit B are
Since the circuit is the same as that of the conventional example in the figure, the description of the configuration is omitted. The variable current source circuit C, the transistors Q ₁₀ through Q ₁₂
And a variable current source circuit 7 is connected to the collector / base of the transistor Q ₁₀ , and the other end is connected to the ground terminal 5. The first
Unlike the variable current source circuit 8 of FIG. 6, the current source circuit 8'of the figure is a current source circuit which supplies a current value preset to a predetermined value.

In figure, the input voltage V _i is applied to input terminals 1 and 2, by connecting a current source circuit 8 'to the base and collector of the transistor Q _9, the collector current I in the transistor Q _7, Q _{8 n} have been supplied. However, collector current I
_n is a predetermined current. If the input voltage V _i is input, the current i _a flows through the resistor R _E. In addition, the current I passes through the transistors Q ₁₁ and Q ₁₂ of the variable current source circuit C.
_Y is supplied to the transistors Q ₁ and Q ₂ , respectively. However, the collector current of the transistor _{Q 1 (I n + i a} ), the collector current of the transistor Q ₂ is (I
a since _n -i _a) a is respectively constant, current I _Y is a current flowing through the diode D _1, D ₂ changes the flow. The current to the diode _{D 1 (I n -I Y +} i a), the current _(I n _-I Y -i _a) flows through the diode D _2.

The forward voltage of the diode D ₁ is V _T ln ((I _{n
-I Y + i a) / I} s), the forward voltage of the diode D ₂ is represented by _{V T ln ((I n -I} Y -i a) / I s).

Therefore, the potential difference V ₀ between the collectors of the transistors Q ₁ and Q ₂ ,
That is, the potential difference between the cathodes of the diodes D ₁ and D ₂ is expressed as follows.

If the potential difference between the bases of the transistors Q ₃ and Q ₄ is V _B , the potential difference V _B between the bases is expressed as follows. In addition,
i _b is the variation of the collector current of the transistor Q _3, Q ₄ vary by a base potential difference V _B as described above.

Since the potential difference V ₀ between the collectors of the transistors Q ₁ and Q ₂ and the potential difference V _B between the bases of the transistors Q ₃ and Q ₄ are equal, the current i _b is calculated from the equations (6) and (7) as follows. Represented by.

Further, when the relationship of i _a = V ₁ / R _{E in} the expression (1) is substituted into the expression (8), it is expressed as the following expression.

_{i b = I X · V i} / R E · (I n -I Y) ...... (9) the output current of the variable transconductance amplifier circuit of FIG. 1 is a 2i _b. Therefore, the transconductance gm of this circuit is expressed by the following equation.

gm = ∂ (2i _b ) / ∂V _i = 2I _X / R _E · (I _n + I _Y ) (10) As is apparent from the equation (10), the variable current I is fed from the variable current source circuit C to the amplifier A. It is obvious that the value of the transconductance gm of the variable transconductance amplifier circuit of FIG. 1 can be changed by adding _Y. That is,
Transistor to Q ₁ operating current _(I n _{+ i} a), the operating current _(I n _-i a) of the transistor Q _2, the operating current _{I X} of the transistor Q ₃ and the transistor Q ₄ in a state with constant, variable current source circuit It is possible to reduce the value of the transconductance gm by increasing the current value I _Y from C. FIG. 5 shows the frequency characteristic of this embodiment, and shows that the frequency characteristic is more stable than the conventional one of FIG. 4 even if the transconductance gm is changed. This is the transistor Q ₁ , Q ₂
Because the operating current of is constant.

FIG. 2 shows another embodiment of the variable transconductance amplifier circuit of the present invention.

Example of the second figure, the variable the variable current source circuit C as compared with the embodiment in Figure 1 is connected to the collector and base of current mirror circuit and the transistor Q ₁₀ comprising transistors Q ₁₀ through Q ₁₂ Formed by current source circuit 7, transistor Q ₁₁
The collector of is connected to the connection point between diode D ₁ and transistor Q _1, and the collector of transistor Q ₁₂ is connected to diode D 1.
₂ is connected to the connection point of the transistor Q ₂ . The collector currents I _Y are drawn from the amplifier circuit A into the transistors Q ₁₁ and Q ₁₂ , respectively. The collector current I _{n of the} transistors Q ₇ and Q ₈ and the operating current I _X of the transistors Q ₃ and Q ₄ are constant.

In the case of this embodiment, the transconductance gm is increased by increasing the variable current I _Y. Also in this embodiment, as shown in the frequency characteristic of FIG.
Since the operating currents of the transistors Q ₁ and Q ₂ are constant, the frequency characteristics do not deteriorate.

FIG. 3 shows another embodiment of the variable transconductance amplifier circuit of the present invention.

In the embodiment shown in FIG. 3, compared with the embodiment shown in FIG. 1, the variable current source circuit C is composed of a differential amplifier circuit D. Resistors R ₁ and R ₂ are connected between the emitters of the transistors Q ₂₀ and Q ₂₁ , and a current source circuit 11 is connected to the connection point between the resistors R ₁ and R ₂ and connected to the ground terminal 5. The collector and the collector of the transistor Q ₂₀ are connected to the base and collector of the transistor Q ₂₂ , which is diode-connected, and the base and the transistor Q ₂₃ having a common base form a first current mirror circuit, and the transistor Q ₂₁ The base and collector of a diode-connected transistor Q ₂₄ D are connected to the collector of the transistor Q ₂₄ D, and a second current mirror circuit is formed by the base and the transistor Q ₂₅ having a common base. Transistors Q ₂₆ and Q ₂₇ are connected to the collectors of transistors Q ₂₃ and Q _{25, respectively} , to form a single-ended output circuit, and the connection point between transistors Q ₂₃ and Q ₂₆ and Q ₂₅ and Q ₂₇ are connected.
The connection point with ₂₇ is connected to the connection point between the diode D ₂ and the transistor Q ₂ and the connection point between the diode D ₁ and the transistor Q ₁ , respectively.

In the case of this embodiment, the input voltage V _i is applied to input terminals 1 and 2, the transconductance is previously adjusted by the current source circuit 8 ', applied to the input terminals 11 and 12 of the differential amplifier circuit D The transconductance of the amplifier A is changed by supplying the positive and negative polar currents ± I _Y to the collectors of the transistors Q ₁ and Q ₂ of the amplifier A by the variable voltage V _IN . Moreover, the frequency characteristics are not deteriorated.

〔effect〕

The variable transconductance amplifier circuit of the present invention includes a variable current source circuit, and even if the control current is narrowed down to change the transconductance gm of the amplifier by the variable current source circuit, deterioration of the cutoff frequency of the amplifier is prevented. This is extremely effective because it can prevent the deterioration of the frequency characteristic.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a variable transconductance amplifier circuit according to the present invention, FIG. 2 is a circuit diagram showing another embodiment of a variable transconductance amplifier circuit according to the present invention, and FIG. FIG. 4 is a circuit diagram showing another embodiment of the variable transconductance amplifier circuit according to the present invention, FIG. 4 is a diagram showing frequency characteristics of a conventional variable transconductance amplifier circuit, and FIG. 5 is a variable transconductance of the present invention. Figure 6 shows the frequency characteristics of the amplifier circuit.
It is a figure which shows an example of the conventional variable transconductance amplifier circuit. A: Amplifier, B: Single-ended output circuit, C: Variable current source circuit, D: Differential amplifier circuit, 1, 2: Input terminal, 3: Output terminal, 4: Power supply terminal, 5: Ground terminal, 6: Constant Current source circuit, 7: Variable type current source circuit, 8 ': Current source circuit, 9: Current mirror circuit.

Claims (3)

[Claims] A first collector having collectors connected to cathodes of first and second diodes, respectively, an emitter connected to a current source transistor and a resistor connected between the emitters.
An amplifier including a first transistor and a second transistor, and a third node whose base is connected to a first connection point of the first transistor and the first diode and a second connection point of the second transistor and the second diode, respectively. And a single-ended output circuit formed of a current mirror circuit that is a load of the differential pair, and a first pair of inputs.
In a variable transconductance amplifier circuit added to the bases of the first and second transistors, a preset collector current is supplied to the first and second transistors through the current source transistor to supply the first and second transistors. And a variable current source circuit for supplying a variable current to the first and second connection points, and a variable transconductance amplifier circuit. 2. The variable current source circuit includes a seventh transistor having a collector connected to a first connection point between the first diode and the first transistor, the second diode and the second transistor. A ninth transistor forming a current mirror circuit having an output stage of an eighth transistor whose collector is connected to a second connection point with the collector of the transistor;
The variable transconductance amplifier circuit according to claim 1, wherein the variable transconductance amplifier circuit is formed of a variable current source circuit connected to the ninth transistor. 3. The variable current source circuit is a differential amplifier circuit, and a differential pair of transistors having a base to which a variable voltage is applied, first and second current mirror circuits forming a load circuit thereof, And a third current mirror circuit connected to the output stage of the first and second current mirror circuits, the connection point of the first and third current mirror circuits being the first transistor and the first diode of the first diode. A connection point, a connection point between the second and third current mirror circuits is connected to a second connection point of the second transistor and the second diode, respectively. The variable transconductance amplifier circuit described.