JPH0738556B2 - Integrator circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an integrator circuit which is a constituent element of an active filter, and more particularly to an integrator circuit which constitutes a filter suitable for being integrated into an IC.

[Background of the Invention]

When incorporating a circuit that includes a filter into an IC, how to incorporate the filter inside the IC and reduce the number of external parts is an important issue. Generally, when making a filter into an IC, an active filter is used. (1) The accuracy of the resistance value and the capacitance value of the capacitor is not good,
The cutoff frequency determined by the product of them varies.

(2) Since the resistance value and the capacitance value of the capacitor cannot be increased so much, it is difficult to make a low cutoff frequency.

There are problems such as.

Incidentally, as a device related to this kind of device which solves the above-mentioned problems, for example, JP-A-55-45224 and JP-A-55-452.
No. 66 publication is cited.

In these circuits, a PNP transistor is used as a load current source for a transistor forming an integrating circuit. Second, the collector-emitter voltage V _CE vs. collector current I _C characteristics with the base current of the transistor as a parameter
Shown in the figure.

When V _CE increases in FIG. 2 I _C also tends to increase, the degree is larger the larger the I _C. That is, the output resistance r _C seen from the collector of the transistor exists (ideally, r _C is infinite because it is a current source), and the larger the collector current I _C , the smaller the output resistance r _C. In an IC transistor, the output resistance of the NPN transistor can be large, but the output resistance of the PNP transistor is small and drops to several hundred kΩ to several tens kΩ.

Therefore, when the PNP transistor is used as the load current source of the integrating circuit, the characteristics deviate from the ideal current source, and a problem occurs especially in a low frequency region. That is, the region lower than the frequency determined by the product of the output resistance r _C and the integration capacitance C does not operate as an integration circuit.

[Object of the Invention]

An object of the present invention is to eliminate the above problems and provide an integrating circuit suitable for an IC filter.

[Outline of Invention]

The present invention uses a cascode-connected PNP transistor as a load current source in an integrator circuit that constitutes a filter, thereby significantly increasing the output resistance and enabling the integration action up to a low frequency region. .

Example of Invention

 An embodiment of the present invention will be described below with reference to FIG.

In FIG. ₁ , the emitters of the transistors Q ₁ and Q ₂ are connected to each other and to the current source A ₁ , and the transistors Q ₁ and Q _{2 form} a differential pair. Also transistor Q ₁
Has a base connected to the input terminal T ₁ to which an input signal voltage Vin is applied, and a base of the transistor Q _{2 to which} a bias voltage V _B1 is applied. In addition, the collector of transistor Q ₂ has PNP
The load integrating capacitor C is connected together with the collector of the transistor Q ₂₁ to generate the output voltage Vout as the output terminal T ₃ . Transistor Q ₂₁ becomes more cascode connected, the collector of the transistor Q ₅ to the emitter of Q ₂₁ is connected, based Q ₂₁ is the bias voltage V _B5 is applied. The base of the transistor Q ₅ is connected to the base and collector of the transistor Q ₂₂ and the collector of Q ₁ to form a current mirror circuit, and the collector current of the transistor Q _{1 and} the collector current of the transistor Q ₂₁ are made substantially equal. Now, assuming that the output resistance as seen from the collector of the transistor Q ₂₁ is r _C and the current amplification factor is hfe, by connecting the transistors Q ₅ and Q ₂₁ in cascode, the impedance Z _O seen from the output terminal T ₃ to the Q ₂₁ side can be obtained. Is approximately represented by Z _O = hfer _C.

This can be explained using an equivalent circuit. FIG. 9 shows the cascode section of the transistors Q ₂₁ and Q ₅ of FIG.

Further, the power supplies V _CC and V _B5 are considered to be ground because they are equivalent circuits in AC operation. B, E and C indicate the base, emitter and collector terminals of the transistor.

ib, hfe, and r _C indicate base current, current amplification factor, and collector output resistance, respectively.

Those subscripts indicate the transistors Q ₅ and Q ₂₁ , respectively.

Output impedance Z of collector of transistor Q _21
O is represented by the ratio of the change in the voltage V generated therein to the change in the current i flowing therethrough. That is Is. Below, we obtain Z _O.

From the equivalent circuit, V _E = ib ₂₁ · rb ₂₁ − V = V _E + rc ₂₁ (hfe ₂₁ · ib ₂₁ + i) −, and from ,, Therefore, the output impedance Z _O is Generally, the values of rc, rb, and rfe of the transistors in the IC are rc: several tens of KΩ or more rb: several KΩ rfe: several 10, so the formula is Z _O ≈ (1 + hfe ₂₁ ) rc ₂₁ ≈ hfe ₂₁ rc ₂₁ .

Therefore, by using a cascode-connected PNP transistor as the load current source of the integrating circuit, the output impedance can be increased up to hfe times the output resistance of the transistor itself. As a result, the low-frequency side operating region of the integrator circuit also increases to hfe times (output resistance of Q ₂ >> Q ₂₁
Of the week force resistance). Also, if the operating region on the low frequency side is the same, the capacitance value of the integrating capacitor can be reduced to 1 / hfe, which is convenient because the capacitor can be easily built in when integrated into an IC.

Further, another embodiment of the present invention is shown in FIG.

In Figure 3, the resistor R ₁ to the emitter of transistor Q _1,
A resistor R ₂ is connected to the emitter of the transistor Q _{2, the} other ends of the resistors R ₁ and R ₂ are connected to a constant current source A ₁ , respectively, and the transistors Q ₁ and Q _{2 form} a differential pair. . The base of the transistor Q ₁ is connected to the input terminal T ₁ and the input signal voltage
Vin is added, and the bias voltage V _B1 is applied to the base of the transistor Q ₂ . Further, the collector current of the transistor Q ₂ is shunted by the differential pair transistors Q ₃ and Q ₄ whose emitters are connected to the collector of Q ₂ , respectively. A base of the transistor Q ₃ is applied with a bias voltage V _B2 , a base of the transistor Q ₄ is connected to a control terminal T ₂ , and a control voltage Vc for adjusting the degree of the shunt is applied to T ₃ . The collector of the transistor Q ₄ is connected to the collector of the transistor Q ₂₁ and also connected to the load capacitor C whose other end is grounded. Furthermore, the collector is also connected to the output terminal T ₃ and produces an output voltage Vout. Transistor Q ₂₁
The base of is connected to the bias voltage V _B5 , and the emitter is connected to the collector of the transistor Q ₅ , forming a cascode connection. The base of transistor Q ₅ is transistor Q ₆
It is connected to the base and collector, and operates as a current mirror, and a current almost equal to the collector current of the transistor Q ₆ flows to the collector of the transistor Q ₅
Almost constant current flows in the collector of Q ₂₁ .

On the other hand, the respective emitters of the transistors Q ₇ and Q ₈ are connected and also connected to the constant current source A ₂ . Constant current source
The current value of A _{2 is} a value of I _O / 2 which is 1/2 of the current I _O of the constant current source A ₁ . The bases of the transistors Q ₇ and Q ₈ are connected to the bases of the transistors Q ₃ and Q ₄ , respectively, and have the same current shunt function. Therefore transistor Q ₄
Collector current and the collector current of Q ₈ of the collector current of Q _6, the collector current of Q ₅ is are substantially equal to each other.

It is obvious that the above effect is not lost even in such a configuration, the high frequency characteristic is further improved, and the integral gain adjusting function is added, which will be described below.

The input voltage of the terminal T ₁ Vin, Vout the output voltage of the terminal T _3, an alternating current by the input voltage flows to the collector of the transistor Q ₄ i _S, shunt ratio of the current i _S by the transistor Q _3, Q ₄ (Q ₄ Of the current flowing in the collector of the transistor k), the transistor Q ₁ ,
If the emitter resistance of Q ₂ is re Is established and is flows into the capacitor C, so that the output voltage Vout is obtained. If the angular frequency of the signal is w, Then, from equations (1) and (2), the transfer function H of this circuit is
_{When 3} (s) is calculated, Therefore, the gain is K / C- (R ₁ + R ₂ + 2re). Also, the emitter resistance re is determined by the emitter current I _E , k: Boltzmann's constant T: Absolute temperature q: Represented by electron charge, re = 2 at room temperature with an emitter current of 100 μA
It is 60Ω.

Generally, the transition frequency f _T, which is an index showing the high frequency performance of a transistor, changes as shown in FIG. 4 depending on the emitter current. In order to improve the high frequency performance, the emitter current should be increased and used in the region where f _T can be large.

If the integral gain is determined by the emitter resistance, it may be inconvenient because it does not match the condition of using the transistor with the high frequency performance improved. Therefore, in this embodiment, the resistors R ₁ and R ₂ and the shunt ratio K are introduced so that the transistor is operated by the emitter current which always increases f _T , and the integral gain is determined by the resistors R ₁ and R ₂ and the shunt ratio K. There is. Furthermore, if the resistors R ₁ and R ₂ are made equal to R _E and are made sufficiently larger than the emitter resistance re, then equation (3) becomes approximately It is represented by.

Further, when the integrated capacitor changes in the values of the capacitor capacitance C and the resistance R _E and the integral gain changes, the shunt ratio of the signal current flowing through the collector of the transistor Q ₄ is controlled by controlling the control voltage V _C of the control terminal T _2. It is possible to control K to obtain a desired integral gain. Further, by using the resistor R _E , the input dynamic range can be expanded and the distortion and noise characteristics can be improved.

FIG. 5 shows another embodiment of the present invention. In FIG.
The same components as those in FIG. 3 are designated by the same reference numerals. The transistors Q ₁₀ , Q ₁₁ , Q ₁₂ , and Q ₁₃ constant current source A ₃ constitute a DC level shift type emitter follower. The integrated output of the capacitor C is applied to the base of the transistor Q ₁₀ , and is taken out from the emitter via the terminal T ₄ as the output voltage Vout. On the other hand, it is also added to the base of the transistor Q ₂ via the transistors Q ₁₁ , Q ₁₂ , and Q ₁₃ to form a negative feedback loop.

Therefore, the transfer function H ₅ (s) of FIG. And the cutoff frequency w _C becomes Is a low-pass filter.

Then, in this case, the flapping of the capacitor C and the resistance R _E can be accurately set by controlling the diversion ratio K. Further, even if the values of the capacitor C and the resistance R _E are small, the cutoff frequency w _C can be lowered by reducing the shunt ratio K. Therefore, even if the filter having a relatively low cutoff frequency is used, the capacitor C and the resistance can be changed to the IC. Since it can be built in, the effect of making it IC becomes large.

FIG. 6 shows still another embodiment in which the integrating circuit of the present invention is a high pass filter. In FIG. 6, the same components as those in FIGS. 3 and 5 are designated by the same reference numerals. Input voltage Vi
n is applied to the load capacitor C via terminal T ₅ . The other end of the capacitor C is connected to the collector of the transistor Q ₄ . The base of transistor Q ₁ is bias voltage V _B3
It is connected to the. Therefore, the transfer function H of FIG.
₆ (s) is , Which represents a high-pass filter. It is obvious that the same effect as the embodiment of FIG. 5 can be obtained.

A more general application of the integrating circuit is a biquad filter shown in FIG. In FIG. 7, G ₁ and G ₂ are integrating circuits, and the integrating gains are G ₁ and G ₂ , respectively. Also G ₃ ,
G ₄ and G ₅ represent coefficient circuits that give constants c, b, and a that determine the magnitude of the input voltage Vin, respectively. The input voltage Vin is applied to the terminal T ₆ and the output voltage Vout is applied to the terminal T ₇ .
The transfer function H ₇ (s) of the circuit of FIG. ₇ is The various filters for low pass, high pass, band pass, etc. can be configured by the values of a, b, and c.

FIG. 8 shows the coefficients a, b, and c in FIG. 7 as a =
An embodiment of the present invention that constitutes a low-pass filter with 0, b = 0, and c = 1 will be shown. In FIG. 8, the transistors in the circuit corresponding to the integrating circuit G ₁ in FIG. 7 are numbered in the 100s, and the transistors in G ₂ are numbered in the 200s. The same symbols as those in have the same symbols. The control voltage V _C for controlling the diversion ratio and the cascode bias voltage V _B5 are common to both integration circuits, and the diversion ratio K is equal.
There is. Therefore, the transfer function H of the circuit of FIG.
₈ (s) is from equation (9) And shows a second-order low-pass characteristic.

Here, R ₁₀₁ = R ₁₀₂ = R _E1 and R ₂₀₁ = R ₂₀₂ = R _E2 . Further, the transistor Q ₉ is a base current compensation transistor for making the current mirror operation more accurate. It is obvious that the effect of the present invention is the same as that in the embodiment shown in FIG.

In the above embodiment, the output impedance is increased by using the PNP load transistor side as the cascode, but it is clear that the same effect can be obtained by using the NPN side transistor forming the differential pair as the cascode.

〔The invention's effect〕

According to the present invention, since the impedance of the load current source of the integrating circuit can be made large, the low frequency side operating region of the integrating circuit can be widened. Also, if the operating regions are the same, the capacitance value of the integrating capacitor can be reduced, which is convenient when integrated into an IC.

[Brief description of drawings]

1 is a circuit diagram showing an embodiment of the present invention, FIG. 2 is a graph showing static characteristics of a transistor, FIG. 3 is a circuit diagram showing another embodiment, and FIG. 4 is a transition frequency f _T and an emitter. FIG. 5 is a graph showing a relation of electric currents, FIG. 5 is a circuit diagram showing still another embodiment, and FIG. 6 is a circuit diagram showing another embodiment.
FIG. 7 is a block diagram showing a general biquad filter, FIG. 8 is a circuit diagram showing another embodiment of the present invention, and FIG.
The figure is a view for explaining the cascode section in FIG. T ₁ …… Input terminal, T ₃ …… Output terminal, Q ₅ , Q ₂₁ …… Load transistor.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masanori Kamiya 471 Kamono-cho, Minokamo City, Gifu Prefecture Gifu Factory, Hitachi Ltd. (56) References JP-A-55-45224 (JP, A) JP-A-SHO 55-45266 (JP, A) Actually opened 61-9222 (JP, U)

Claims (1)

[Claims] 1. A first and a second transistor forming a differential pair, a collector and a collector of the second transistor are connected, and a base is connected to a bias voltage.
And a load capacitor connected to the collector of the second transistor, and a fourth capacitor having a collector connected to the emitter of the third transistor and a load capacitor connected to the collector of the second transistor. An integrating circuit, wherein an integrated output of a signal voltage input via a pair is obtained as a terminal voltage of the capacitor.