JPH0612872B2 - Current switching circuit - Google Patents

Description

TECHNICAL FIELD The present invention relates to a current switching circuit using a transistor switch.

(Prior Art) Conventionally, in this type of current switching circuit, the changeover switch unit has four transistors T _{r1 to} T _{r as} shown in the circuit diagram of FIG. 3 so as to reduce the error of the input / output current. It was composed of Darlington differential connection by _r4 . In the figure, X ₁ is the input terminal of the switched current, A ₁ and A ₂ are the output terminals of the switched current, and C
₁ and C ₂ are control terminals of the changeover switch. The error of the input / output current is caused by the base current of the transistor of the changeover switch section when the changed current supplied from the current source I is output via the changeover switch section.

For example, when the changeover switch section formed by the transistors T _r3 and T _r4 is in the OFF state and the changeover switch section formed by the transistors T _r1 and _r2 is in the ON state, the current source current I ₀ of the output current I ₀ obtained at the output terminal A ₁ is obtained. The error current ΔI with respect to is as follows.

Here, β is the current amplification factor when the emitter of the transistor that serves as a changeover switch is grounded.

This is the fourth switch for this switch with Darlington connection.
There is also a current switching circuit using a single transistor changeover switch as shown in the figure. In this figure, the transistor T _r6 is off and the transistor T _r5 is on, and the output current I ₀ ′ is obtained at the collector terminal A ₃ of the transistor T _r5 .
In this circuit, the error current ΔI ′ of the output current I ₀ ′ with respect to the current I of the current source is as follows.

Assuming that β = 100, the error of the output current with respect to the current source current is 0.00098% in the case of FIG. 3 from the equation (1), whereas in the case of FIG. 4, it is calculated by the equation (2). 0.99%
Becomes That is, the changeover switch by the Darlington connection as shown in FIG. 3 has a significantly improved error as compared with the changeover switch by one transistor shown in FIG. 4 and becomes a highly accurate current changeover circuit.

However, the current switching circuit by the Darlington connection has a drawback that the switching speed of switching is slow.

Now, in FIG. 3, the control signal of the changeover switch by the transistors T _r1 and T _r2 is at high level, the changeover switch is in the ON state, and the control signal of the changeover switch by the transistors T _r3 and T _r4 is at low level. And the changeover switch is in the off state. Here, the high level and the low level of the control signal are the potentials such that the former is in the ON state with respect to the potential of the input terminal X ₁ of the current changeover switch, and the latter is similarly the input terminal X 1. _{This is} a potential with which the switch is turned off with respect to the potential of 1. The potential difference between the high level and the low level is about 400 to 600 mV according to the principle of the differential circuit.

Next, when the control signal of the changeover switch by the transistors T _r3 and T _r4 changes to high level in synchronization with the control signal of the changeover switch by the transistors T _r1 and T _r2 becoming low level, the output current I ₀ is output. The terminal A ₁ is transferred to the output terminal A ₂ . Change in the output current at this time produces a fifth becomes time t _s than the change of the control signal as shown in FIG delay.

This delay is due to the charges accumulated in the base region of the transistor T _r2 and can be considered as follows.
That is, when the changeover switch formed by the transistors T _r1 and T _r2 is in the ON state, the characteristic of each transistor such as the emitter junction capacitance and the charge Q which is determined by the driving condition are accumulated in the base region of the transistor T _r2 . Conceivable.

Then, the base potential of the transistor T _r1 becomes low level, the transistor T _r1 is turned off, the charge Q of the base region of the transistor T _r2 is discharged through a high impedance between the emitter and base of Tonranjisuta T _r1 However, the transistor T _r2 remains in the ON state until the charge Q is fully discharged, and the output current flows to the output terminal A ₁ during this period, and it takes time until the current is switched to 100%. become. That is, the switching speed of current switching is reduced.

(Object of the Invention) An object of the present invention is to provide a current switching circuit in which the switching speed of such current switching is improved.

(Structure of the Invention) According to the current switching circuit of the present invention, the input current supplied to the input terminal is supplied to the first and second control terminals according to the control signal and the inverted control signal. A current source circuit connected between a reference voltage terminal and the input terminal, a first conductivity type in which an emitter is connected to the input terminal and a collector is connected to the first output terminal. Second transistor of the first conductivity type, the emitter of which is connected to the input terminal and the collector of which is connected to the second output terminal, the base of which is the first control terminal and the emitter of which is the first control terminal. The third transistor of the first conductivity type, the collector of which is connected to the first output terminal, and the base of which is the second controller. The roll terminal has the emitter connected to the base of the second transistor, and the collector connected to the second output terminal. The fourth transistor of the first conductivity type has the base connected to the first control terminal and the emitter connected to the second control terminal. A second transistor of the second conductivity type, the collector of which is connected to the base of the first transistor, and the base of which is the second control terminal, the emitter of which is the base of the second transistor which is the collector of which is described above; It is characterized by having the sixth transistor of the second conductivity type connected to the reference voltage terminal, respectively.

According to the current switching circuit of the present invention, when the potential for turning off the third transistor is applied to the first control terminal, the fifth transistor is turned on to discharge the accumulated charge in the base of the first transistor, Fifth after discharging
Since the transistor (1) turns off by itself due to the decrease in the emitter potential due to discharge, saturation does not occur and the turn-off time of the fifth transistor does not matter, so that the turn-on of the next first transistor can be performed at high speed. Furthermore, the second
When a potential for turning off the fourth transistor is applied to the control terminal of, the sixth transistor turns on and the second transistor turns on.
The stored charge in the base of the transistor is discharged, and when this discharge ends, the sixth transistor turns off by itself due to the decrease in the emitter potential due to the discharge, so that saturation does not occur and the turn-off time of the sixth transistor does not matter. Also, the turn-off of the next second transistor can be performed at high speed. Further, since the current source is provided between the input terminal and the reference voltage terminal, the fifth and sixth transistors are different from those in which the fifth and sixth transistors are directly connected to the input terminal.
The discharge capacity of the transistor can be improved.

(Example) Next, the present invention will be described in detail with reference to the drawings.

FIG. 1 is a circuit diagram of an embodiment of the present invention. In the figure, the same symbols as in FIG. 3 indicate the same components, and T _r10 ,
Reference _{numeral 11} is a transistor having a polarity (pnp) different from that of the transistors Tr1 to _Tr4 added according to the present invention. Now, _assuming that a high-level signal is applied to the base terminal of the transistor T _r1 and a low-level signal is applied to the base terminal of the transistor T _r3 , forward bias is applied between the base and emitter of the transistors T _r1 and T _r2. Transistor T _r1 ,
The changeover switch by T _r2 is in the ON state, the base-emitter of the other transistors T _r3 and T _r4 are reverse biased, and the changeover switch by T _r3 , T _r4 is in the OFF state, and the output current I ₀ ″. Is output to the output terminal A _1. Here, the high level is a potential at which the changeover switch can be sufficiently turned on with respect to the input terminal X ₁ , and the low level is at least 1.2 to 1. Next, when the base terminal of the transistor T _r3 changes to a high level signal in synchronization with the low level signal of the base terminal of the transistor T _r1 , the bases of the transistors T _r3 and T _r4 are changed. A forward bias is applied between the emitters, and the changeover switch formed by the transistors T _r3 and T _r4 is turned on, and the base of the transistor T _r1 is
Emitter is at the same time turned off in reverse biased, the base-emitter of the transistor T _r10 is forward biased at a potential retention by charges accumulated in the base region of the transistor T _r2, transistor T _r10 is turned on As a result, the electric charge accumulated in the base of the transistor T _r2 is absorbed and discharged. When the discharge of the accumulated charge is completed, the transistor T _r10 is turned off, but at the same time, the transistor T _r2 is also completely turned off, so that the changeover switch by the transistors T _r1 and T _r2 is completely turned off. . Therefore, the output current is the transistor T _r3 ,
Only the changeover switch by T _r4 flows, and it means that the output terminal A ₂ is safely switched. Moreover, the discharge of the accumulated charge in the base region of the transistor T _r2 at this time is
Since the control signal is started at the same time as the low level, the switching is performed in an extremely short time.

FIG. 2 is a waveform diagram showing how the output current is switched by the circuit of FIG. In this case, the switching speed is greatly improved as compared with FIG. For example, when the load impedance connected to the output terminal is about 10 kΩ, the time required for switching is 4 to 6 μsec in the case of FIG. 5, whereas that time is 0.5 to 6 μsec in the case of FIG. 1 μsec
Shortened to a degree.

(Effects of the Invention) As described above, according to the present invention, the changeover switch can be operated at high speed while ensuring high accuracy by the Darlington connection of the current changeover switch.

[Brief description of drawings]

FIG. 1 is a circuit diagram of an embodiment of the present invention, FIG. 2 is an operation waveform diagram of FIG. 1, and FIGS. 3 and 4 are conventional Darlington differential and current switching circuits by differential of one transistor. FIG. 5 is an operation waveform diagram of FIG. In the figure, T _r1 to ₄ , T _10,11 ... Transistor, I ... Current source current, I _{0 to} I ₀ ″ ... Output current, C ₁ , C ₂ ... Control terminal, A ₁ , A ₂ ... ... output terminal, X ₁ ... input terminal.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Norio Terada 5-7-15 Shiba, Minato-ku, Tokyo NEC Electric Microcomputer System Co., Ltd. (56) References Japanese Patent Publication No. 57-11172 (JP, B2)

Claims (1)

[Claims] 1. A current switching circuit for outputting an input current supplied to an input terminal from a first or second output terminal according to a control signal and an inverted control signal supplied to the first and second control terminals. A current source connected between the reference voltage terminal and the input terminal, a first conductive type first transistor having an emitter connected to the input terminal and a collector connected to the first output terminal, and an emitter A second transistor of the first conductivity type having a collector connected to the input terminal and the second output terminal, a base connected to the first control terminal, an emitter connected to the base of the first transistor, and a collector connected to the base. First
Of the first conductivity type connected to the output terminals of
Of the first transistor, the base of which is connected to the second control terminal, the emitter of which is connected to the base of the second transistor and the collector of which is connected to the second output terminal.
A conductive fourth transistor, a base connected to the first control terminal, an emitter connected to the first transistor base, and a collector connected to the reference voltage terminal; and a base. Has a second transistor of the second conductivity type in which the emitter is connected to the second control terminal, the collector is connected to the base of the second transistor, and the collector is connected to the reference voltage terminal.