JPS5816206B2 - constant current circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION This invention provides that when the current flowing into (or flowing out of) an input terminal changes, the current flowing into (or flowing out of) an output terminal accurately maintains a proportional relationship with the input current, and the proportionality constant thereof also changes. This relates to a constant current circuit that can be set freely.

Figure 1 shows an example of a conventional constant current circuit, where 1 is an input terminal, 2 is an output terminal, 3 is a ground terminal; 4 is a drive transistor, 5 is a constant current output transistor, and 6 and 7 are resistors. .

In this circuit, the relationship between input current (1) and output current (2) is as shown in the following equation.

Here, q: electronic charge, K: Boltzmann constant, T: absolute temperature, s1: saturation current of transistor 4, saturation current of transistor 5, R1: resistance value of resistor 6, R2: resistance value of resistor 7. be.

Note that in equation (1), the base current IB of transistors 4 and 5
The effect of is ignored.

In general, the DC current amplification rate (hereinafter h) of an N'PN transistor is
(referred to as FE) is high at 100 or more, so even if 1B=o, there will not be a large error.

To summarize the above equation (1), equation (2) has 12 inside and outside the logarithm, so 12 cannot be determined easily, but now R
Assuming that the voltage drop across R1 and R2 is large, the left side becomes <<the right side, and 12 is determined by the resistance ratio of R1 and R2 as shown in the following equation.

Therefore, as shown in FIG. 5, the proportionality coefficient of the relationship between 11 and I2 can be freely changed by changing R2.

However, if 11 changes (for example, 1mA to 1μ8)
), it is difficult to maintain the same proportionality coefficient in any current range.

The reason for this is that in order to derive the above-mentioned equation (3), the left side of equation (2)
(Although we assumed that the right-hand side was the right-hand side, this assumption no longer holds true when 11 becomes small.

In other words, when 11 becomes very small, in equation (2), the left side>>the right side, so the relationship between 11 and 12 is not determined by the area of the transistor, but is fixed at the proportional coefficient Is, which is S.

Therefore, the relationship between 11 and I2 is determined by R1 and R2 only in the region where 11 is large, as shown in FIG. 8 (shown by a broken line).

), and as 11 becomes smaller, it approaches the fixed ratio determined by the area ratio device.

For this reason, the constant current circuit shown in FIG. 1 cannot obtain a constant current ratio over a wide range of input current 11.

In addition to FIG. 1, conventional circuits such as those shown in FIGS. 2 to 4 are often used, but they are basically the same as FIG. 1.

That is, FIG. 2 uses PNP transistors 4.5 instead of NPN transistors 4 and 5 shown in FIG. 1, and is completely the same except that the direction of the current is changed.

Also, Fig. 3 is for reducing the error caused by the influence of the base current IB of transistor 4゜5 in Fig. 1.
By adding transistor 9, transistors 4 and 5
By supplying the base current from the power supply terminal 8 through the transistor 9, the influence on the transistor 11 is reduced to "hFE" corresponding to the hFE of the transistor 9 compared to FIG.

FIG. 4 is used when determining the setting ratio of 11 and 12 more precisely, and the stability of the area ratio 182 is increased by adding a transistor 10.

As mentioned above, conventional circuits have various shapes as shown in Figures 1 to 4, but Figure 1 is the basic one, and in all of them, the voltage drop across resistors R, R2 is It is determined by the values of 1 and 12, so 11. As I2 becomes smaller, the voltage drop also becomes smaller, and there is a drawback that if 11 changes over a wide range, it is not possible to obtain an output current I2 with a constant current ratio.

In view of the above-mentioned points, the present invention provides a constant current circuit that can obtain an output current having a predetermined proportional relationship even when the input current changes over a wide range, and can freely set the proportionality constant. It is something.

FIG. 7 shows a basic embodiment of the present invention, in which the resistor of the conventional circuit is removed and a voltage power source 11 is inserted between the emitter of the output transistor 5 and the ground terminal 3.

However, in this lever circuit, the input current from input terminal 1 is
The relationship with the output current I2 is expressed by the following equation.

Here, ■ is the voltage of the voltage source 11.

Rearranging equation (5), if V is a voltage expressed by -KTtnA (where A is a positive constant), then substitute it into equation (6).

Here, IS2/ISt corresponds to the emitter area ratio of transistors 4 and 5, and is a value that is determined by that transistor once a transistor is selected.

In other words, it cannot be used as a means to freely adjust the proportionality coefficients of 11 and I2.

What remains is a constant called A. By changing this, the proportional coefficient can be adjusted based on the force axis.

Moreover, looking at equation (8), it is a simple linear function,
Since this method can be applied to the entire practical range of 11 and I2, the constant linear relationship between 11 and I2 shown in FIG. 5 can be maintained over a wide current range.

In other words, in a wide range such as 1 pJy to 177 LA, the proportional relationship of, for example, I2-3×■1 holds true, with good accuracy. A proportional coefficient of =3 can be set and adjusted.

That is, according to the constant current circuit of the present invention shown in FIG. 7, by adjusting the output of the variable voltage source 11, the ratio between 11 and I2 can be arbitrarily and accurately set over a wide current range.

Further, FIGS. 8 to 12 show specific embodiments of the present invention, which will be described in detail below.

However, although FIG. 7 shows the case where transistors 4 and 5 are NPN transistors, FIG. 7 is a basic circuit of various types of constant current circuits, and FIGS. 2 to 4 and their combination circuits Therefore, the following explanation will be made using the NPN transistor circuit shown in FIG. 7 as a representative example.

Now, FIG. 8 shows a configuration in which the voltage source 11 of FIG. 7 is constructed using a differential amplifier 12.

The output voltage vo becomes a value obtained by amplifying the input offset voltage Vio, and it becomes Vio! ' An It uses the fact that it is expressed in the form of A.

A specific circuit example of the differential amplifier 12 is shown in FIG.

In FIG. 9, the input stage is PNP transistors 22 and 23.
It is composed of an amplifying transistor, NPN transistors 25 and 26 constituting a load, and a constant current source 24.

Resistors 27 and 28 and variable resistor 15 are resistors for adjusting the input offset voltage.

The emitters of transistors 22 and 23 in the input stage are connected in common, and a current is supplied from a constant current source 24 to perform a differential amplification operation as described later.

The amplified output of the input stage is taken out from the collector of transistor 23 and transmitted to the base of NPN transistor 29.

Transistor 29 forms a common emitter amplifier with resistor 30 as a load.

The output is taken out from the collector of the transistor 29 and transmitted to an emitter follower circuit composed of an NPN transistor 31 and a load 32.

The output as a differential amplifier is taken out from the emitter of the NPN transistor 31.

Since this circuit is a general circuit, only a brief explanation will be provided.

The input terminals are the base terminals of the PNP transistors 22 and 23, and are inverted inputs (-human power) in phase relation with the output signal.

) and non-inverting input (10 inputs).

If the voltages of the three input terminals are equal, the transistor 22
The currents 11 and I2 flowing through the circuits 11 and 23 are equal, and in this case, the output becomes zero potential.

It is assumed that a positive voltage is applied to the power supply terminal 13 and a negative voltage is applied to the power supply terminal 14.

When voltage is applied between the input terminals, that is, when the voltage is higher for 10 inputs than for one input, transistor 2
When the base potential of 3 is high, 12 decreases and 11 increases by that amount.

This relationship is expressed by the following equation.

Input voltage difference ΔV■ = (10 input voltage) - (- input terminal) I
When 2 decreases, the collector potential of the transistor 23 decreases,
Since the base voltage of transistor 29 decreases, the collector potential increases and the output voltage increases.

The relationship between input voltage and output voltage is determined by voltage gain.

Here, as shown in FIG. 8, if negative feedback is applied from the output terminal to the power terminal using resistors 42 and 41, and the resistance values are respectively R and R8, the voltage gain is determined by AV=5.

In FIG. 9, a difference in I2 occurs between 8 which provides the input voltage and <■ as determined by equation (9), but conversely, it can also be said that if 11 and I2 are changed, this will appear as the input voltage.

This technique is commonly used to adjust the input offset voltage.

This method is carried out by adjusting the semi-fixed resistor 15 shown in FIG.

When the slider of the resistor 15 is moved, the resistance that is applied in parallel to the resistors 2T and 28 changes, so that the resistances 11 and 12 change for the reason explained in FIG. 1.

Therefore, a voltage expressed by equation (9) is generated between the input terminals.

This is Vio. In FIG. 8, the output voltage Vo of the differential amplifier 12 is expressed by the following equation.

KT Looking at the above equation, we can see that V=-tnA mentioned in Figure 7 has Vo
It can be seen that they match.

Of course, ■ is multiplied by 58, but this is the magnification degree of the amplifier, and v=”
! - Regardless of the essential form of tnA, the brush t0q
KT I (If you change equation (10), vo = -t.

(□2R8 and nariba) It becomes the form of tnA.

) As described above, by using a differential amplifier as shown in FIG. 9, the input stage current I2. In the method of adjusting the offset by changing I2, the offset adjustment means adjusting the current ratio between the currents 11 and I2 of the transistors 4 and 5 in FIG. 8.

FIG. 10 is an example of another method using a differential amplifier.

Diodes 18 and 19 each have I3. I4 is flowing, and ■3+14 is supplied by the constant current source 40.

The output terminal of the differential amplifier 12 and the output terminal are connected through the impedance of a diode 19, and negative feedback is applied at the same time.

Due to the negative feedback effect, the single power of the amplifier 12 and the potential of the ten inputs become equal.

(However, it is assumed that the input offset voltage is negligibly small.

) Resistors 16 and 17 are connected together at one end.

The other end is connected to both inputs of the differential amplifier 12 and to a diode 18,19.

Since the potentials of both inputs of the differential amplifier 12 are equal, the resistor 16
The voltages applied across 17 and 17 are equal.

Assuming that the input current of the amplifier 12 is negligibly small, the current flowing through the resistor 16°17 3. (2) 4 directly flows through the diodes 18 and 19.

The cathode terminal of the diode 18 is connected to the ground terminal 3, and the anode potentials of the diodes 18 and 19 are necessarily equal since both input potentials of the amplifier 12 are equal, and the diode 19 is connected to the output of the differential amplifier 12. Since the cathode terminals of diodes 18 and 19 are connected, the output voltage Vo is
This is the difference in forward voltage of , and is expressed by the following equation.

Here, VD (18): forward voltage of diode 18, V
D wholesale: diode 19 forward voltage, ■3. ■4: Current flowing through diode 18.19, ■s18. Is1. :
This is the saturation current of the diode 18,19.

(11) Expression number 3. ■4 C resistor 16, 17 (7) Resistance value R5° Since it is determined by R4, ■3/ is replaced with R443, and it becomes like ■4th order formula.

The α1 formula also agrees with V-=7nA in FIG. 7, and the ratio of 11 and 12 can be changed by changing VO.

As a means for changing Vo, the horse (or ya) may be used as a variable resistor, as shown in FIG.

・Figure 11 is a circuit in which the voltage corresponding to the voltage source 11 is obtained from the bias voltage source 40 of another circuit, in which the differential amplifier 12 is used as a buffer amplifier, and the voltage gain is determined by changing the resistor 34. A■-■(However, R5
: the resistance value of the resistor 34, R6: the resistance value of the feedback resistor 35) to change the output voltage and adjust the current ratio of (1) and (2).

FIG. 12 shows a similar example in which the differential amplifier 12 is used as a buffer amplifier with a fixed voltage gain of AV7, and the voltage of the bias voltage source 40 of another circuit is divided by the variable resistor 36. and 12 current ratios are set.

As described above, in the constant current circuit of the present invention, the current ratio between the input current and the output current can be adjusted with high precision over a wide current range.

Therefore, it is extremely effective as a current amplifier (or attenuator).

[Brief explanation of the drawing]

Figure 1 is a circuit diagram showing a conventional basic constant current circuit;
Figures 4 to 4 are circuit diagrams showing other conventional constant current circuits, Figures 5 and 6 are characteristic diagrams showing the relationship between the input current and output current of the constant current circuit, and Figure 5 is an ideal Indicates the case where
FIG. 6 shows the case in a conventional circuit. FIG. 7 is a circuit diagram showing a basic embodiment of this invention;
is a circuit diagram showing a specific embodiment of this invention, FIG. 9 is a circuit diagram showing an example of the differential amplifier in FIG. 8, and FIGS. FIG. 2 is a circuit diagram showing an example. Note that the same reference numerals in the figures indicate the same or corresponding parts. 1...Input terminal, 2...Output terminal, 3
...Ground terminal, 4...Drive transistor, 5...Output transistor, 11...
・Voltage source, 12...Differential amplifier, 40...
...Bias power supply for other circuits.

Claims (1)

[Scope of Claims] 1 A transistor with a collector as an input terminal, a collector and a base connected to each other, and an emitter as grounded;
a second transistor connected to the base of the first transistor; and a voltage source having one end connected to the emitter of the second transistor and the other end grounded, and adjusting the output voltage of the voltage source. A constant current circuit characterized in that the ratio of currents flowing through the input terminal and the output terminal is changed by changing the ratio of the current flowing through the input terminal and the output terminal. 2. Claim 1, wherein the voltage source has an output voltage corresponding to -1nk (where q: electronic charge, K: Boltzmann's constant, T: absolute temperature, A: positive constant)
Constant current circuit described in section. 3. The constant current circuit according to claim 1 or 2, wherein the voltage source is constituted by a differential amplifier. 4. The definition according to claims 1 to 3, characterized in that the voltage source is constituted by a person or a differential amplifier to which a bias voltage from another input is applied to one of the power terminals. current circuit. □