JPS5922969B2 - constant current circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a constant current circuit with a relatively simple configuration and low voltage loss.

FIG. 1 shows a simple constant current circuit that has been widely used in the past.One feed line 1 is connected to the collector of a control transistor 2.
The emitter of is connected to the other feed line 4 via a detection resistor 3, and a detection transistor 5 is connected to both ends of the detection resistor 3.
The collector of the detection transistor 5 is connected to the base of the control transistor 2 and is also connected to the power supply line 1 via a resistor 6.

Further, the feed line 1 is connected to the feed terminal 1a, the feed line 4 is connected to the feed terminal 4a, and the feed line 4 is connected to the feed terminal 1a, and the feed line 4 is connected to the feed terminal 4a.
A and a load resistor 8 are connected between the power supply terminal 4a and the power supply terminal 4a.
are connected in series.

Incidentally, a capacitor 9 for preventing oscillation is connected in parallel with the detection resistor 3. Now, in the constant current circuit shown in FIG. 1, the output current 10 is controlled so that the voltage across the detection resistor 3 becomes equal to the base-emitter forward voltage EBE5 of the detection transistor 5. EI the power supply voltage
, where the resistance value of resistor 3 is RA and the resistance value of load resistor 8 is RL, when E1>10 (RA+RL), the output current is 10
becomes as follows.

10=---1-(1) In other words, when the output current 10 changes due to changes in the load, the voltage across the detection resistor 3 also changes, and the collector current of the detection transistor 5 changes rapidly. Therefore, the emitter of the control transistor 2 and the feed line 4
The voltage between them, that is, the voltage across the detection resistor 3, is returned to its original value, and the stability of the output current 10 is maintained.

Now, Figure 2 shows the circuit shown in Figure 1, with the resistance value RA of resistor 3 set to 1.
80Ω, resistance value RB of resistor 6 is 10KΩ, resistance RL of load resistor 8 is 820Ω, transistor 2 and transistor 5 are silicon NPN transistors 2SC-828.
(manufactured by Matsushita Electric), which shows the change in the output current 10 with respect to the change in the power supply voltage EI when the capacitor of the capacitor 9 is 1 μF.

(Actual measurement value) From Fig. 2, in the circuit shown in Fig. 1, when the power supply voltage EI becomes 5 or less, the output current 10 is almost stabilized, and when the minimum power supply voltage is 5V, the output current at this time is 3.68 mA. It can be seen that the output current is 4.16 mA when the power supply voltage is 20V.

By the way, the characteristics shown by the broken line in Figure 2 are as follows: the resistance value RA of the detection resistor is 180Ω, and the resistance value RL of the load resistor is 820Ω.
, This is the output current characteristic of an ideal constant current circuit with respect to the power supply voltage when the output current is 4 mA. While the value is zero,
The actual characteristics of the constant current circuit shown in Figure 1 are that the output current is 3.6
The value obtained by subtracting the voltage drop across the detection resistor and load resistor due to the output current from the power supply voltage at 8 mA is 1.32. This causes a combined voltage drop of 3.68V across the sense resistor and load resistor, for example.

This means that a power supply voltage of 5V or more is required to obtain a stabilized output current of 3.68mA. This is a big problem with portable devices. In the circuit shown in Figure 1, the value obtained by subtracting the voltage drop across the detection resistor and load resistor due to the output current from the minimum power supply voltage is large because the potential of the control transistor 2 is lower than the potential of the feed line 1 at the base of the transistor. - This is due to the fact that the voltage does not increase beyond the value obtained by subtracting the forward voltage between the emitters. It would be better to connect it to a power supply point with a higher potential, but in this case, an additional power supply was required, which caused many problems.

The constant current circuit of the present invention solves the above problems.

FIG. 3 shows a circuit connection diagram of a constant current circuit according to an embodiment of the present invention, and the same parts or parts as in FIG. 1 are indicated by the same figure numbers, and the explanation thereof will be omitted.

In FIG. 3, the collector of the detection transistor 5 is connected to the base of an amplification transistor 10 which is complementary to the detection transistor 5, and is also connected to the positive feed line 1 via a resistor 6.
The collector of the control transistor 11 is connected to the base of a control transistor 11 which is complementary to the detection transistor 5, and is also connected to the negative power supply line 4 via a resistor 12, and the collector of the control transistor 11 is connected to one end of the detection resistor 3. It is connected to the.

Further, the emitter of the amplification transistor 10 and the emitter of the control transistor 11 are both connected to the positive power supply line 1.

Now, also in the constant current circuit shown in FIG. 3, the output current 10 is as shown in equation (1), and when the output current 10 changes due to changes in the load, the collector current of the detection transistor 5 changes suddenly. , amplification transistor 10
The collector current of the control transistor 11 also changes rapidly, which causes the base current of the control transistor 11 to change, and the output current 10 is controlled to return to its original value.

By the way, in the circuit shown in FIG. 3, the base bias resistor of the control transistor 11, that is, one end of the resistor 12 is connected to the negative side feed line 4, which has a lower potential than the collector of the control transistor 11, so that the power supply voltage EI is
When it becomes smaller than O(RA+RL), the base current of the detection transistor 5I stops flowing and its collector current also becomes zero, so the base current and collector current of the amplification transistor 10 also become zero, and the control transistor 11
The base current of the control transistor 1 flows through the resistor 12 to a potential point lower than its collector potential.
The voltage loss between the collector and emitter of transistor 1 is completely saturated, and the voltage loss in the control transistor 11 at this time becomes extremely small.

Furthermore, while the constant current circuit in Figure 1 is a voltage-driven circuit that changes the emitter potential by changing the base potential of the control transistor, the constant current circuit in Figure 3 uses the base voltage of the control transistor. It is a current 1 drive type circuit that stabilizes the output current by changing the collector current according to the change in the current.In addition, in the constant current circuit shown in FIG. Since the potential does not change much, the change in the collector-base voltage of control transistor 2 due to a change in power supply voltage is
Output current 1 in the form of change in base potential, change in leakage current
In contrast, in the constant current circuit shown in FIG. 3, the collector potential of the detection transistor 5 changes in accordance with the change in the power supply voltage, and the collector current of the detection transistor 5 due to the change in collector potential changes. The change acts in a direction to offset the change in the collector current of the control transistor 11 due to the change in the power supply voltage.

Therefore, there is almost no change in the output voltage due to a change in the power supply voltage.

By the way, Figure 4 shows the resistance value R of resistor 3 in the circuit of Figure 3.
A is 180Ω, resistance value RB of resistor 6 is 10KΩ, resistor 1
The resistance value Rc of the load resistor 8 is 10KΩ, the resistance value RL of the load resistor 8 is 820Ω, the capacitance of the capacitor 9 is 1μF, and the transistor 5 is a silicon NPN type transistor 2SC-828 (
Transistors 10 and 11 are silicon PNP transistors 2SA-564 (manufactured by Matsushita Electric).
Output current 10 with respect to change in power supply voltage EI when
This shows the changes in

(Actual measurement value) From Fig. 4, in the constant current circuit shown in Fig. 3, the output current is 3.65 mA when the minimum power supply voltage is 3.8V, and the output current l depends on the detection resistor from the minimum power supply voltage. It can be seen that the value obtained by subtracting the voltage drop across the load resistance is 0.15, which is much smaller than the circuit shown in FIG.

In addition, in the circuit shown in Fig. 1, when the power supply voltage changes from 5V to 20V, the output current changes by 0.48mA, but in the circuit shown in Fig. 3 of the present invention, when the power supply voltage changes from 3.8V to 20V, the output current changes by 0.48mA. The change in output current is only 0.03mA
The performance has improved in this respect as well.

Incidentally, although FIG. 3 shows an example in which the constant current circuit of the present invention is constructed using bipolar transistors, it can also be constructed using field effect transistors as shown in FIG.

In the constant current circuit of FIG. 5, a P-channel enhancement type MOS transistor 13 is used as a control transistor, a P-channel enhancement type MOS transistor 14 is used as an amplification transistor, and an N-channel enhancement type MOS transistor 15 is used as a detection transistor. There is.

It is also possible to use a bipolar transistor and a field effect transistor together for the control transistor, amplification transistor, and detection transistor. In this case, if the control transistor and the amplification transistor are complementary transistors with respect to the detection transistor and the bias voltage, respectively. good.

That is, in the constant current circuit of the present invention, an input electrode of a detection transistor is connected between a common electrode and a detection resistor, and an output electrode of the detection transistor is connected to an input electrode of an amplification transistor complementary to the detection transistor in terms of bias voltage, A common electrode of the amplification transistor is connected to a power supply line on the output electrode side of the detection transistor, the output electrode is connected to an input electrode of a control transistor complementary to the detection transistor with respect to bias voltage, and the common electrode of the control transistor is connected to a power supply line on the output electrode side of the detection transistor, and the output electrode is connected to one end of the detection resistor, and in the bipolar transistor, the input electrode is the base and the common electrode is the emitter. , the output electrode becomes a collector, the input electrode becomes a gate, the common electrode becomes a source, and the output electrode becomes a drain in a field effect transistor. Note that the detection transistor, amplification transistor, and control transistor do not need to be a single transistor, but may be transistors connected in a composite manner. In particular, when obtaining a large current output, the control transistor is connected in a Darlington manner.

As described above, in the constant current circuit of the present invention, the input electrode and common electrode of the detection transistor are connected to both ends of the detection resistor, and the output electrode is connected to the input electrode of the amplification transistor complementary to the detection transistor. and an output electrode of the amplification transistor is connected to an input electrode of the control transistor,
By connecting the output electrode of the control transistor to one end of the detection resistor, the control transistor can be completely saturated when the power supply voltage decreases, resulting in a circuit with less voltage loss at low power supply voltages. is obtained,
It has a great effect.

[Brief explanation of drawings]

Fig. 1 is a circuit wiring diagram showing a conventional example, Fig. 2 is a characteristic diagram for explaining its operation, Fig. 3 is a circuit wiring diagram of a constant current circuit according to an embodiment of the present invention, and Fig. 4 is a circuit wiring diagram showing a conventional example. 3 is a characteristic diagram for explaining the operation of the circuit, and FIG. 5 is a circuit connection diagram of a constant current circuit in another embodiment of the present invention. 1...Positive side feed line, 3...Detection resistor, 4...Minus side feed line, 5...
...Detection transistor (bipolar transistor), 1
0... Amplification transistor (bipolar transistor), 11... Control transistor (bipolar transistor), 13... Control transistor (field effect transistor), 14... Amplification transistor (field effect transistor), 15...
Detection transistor fuyu (field effect transistor).

Claims (1)

[Claims] 1. An input electrode and a common electrode of a detection transistor are connected between a detection resistor, an output electrode of the detection transistor is connected to an input electrode of an amplification transistor that is complementary with respect to a bias voltage, and A common electrode is connected to a power supply line on the output electrode side of the detection transistor, the output electrode is connected to an input electrode of a control transistor complementary to the detection transistor in terms of bias voltage, and the common electrode of the control transistor is connected to the feed line on the output electrode side of the detection transistor. a second resistor is connected between the output electrode of the detection transistor and the common electrode of the amplification transistor; A constant current circuit characterized in that a third resistor is connected between the output electrode of the amplification transistor and the common electrode of the detection transistor. 2. The detection transistor, the amplification transistor, and the control transistor are each made up of bipolar transistors, and the input electrode, output electrode, and common electrode are the base, collector, and emitter of the bipolar transistors, respectively. Constant current circuit described in item 1. 3. The detection transistor, the amplification transistor, and the control transistor are each constituted by a field effect transistor, and the input electrode, output electrode, and common electrode are the gate, drain, and source of the field effect transistor, respectively. Constant current circuit according to range 1.