JPS6160617B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an analog current switch circuit suitable for integrated circuits.

Conventionally, it has been difficult to integrate logic circuits and analog circuits on the same semiconductor substrate because their manufacturing processes are significantly different. Therefore,
In particular, it is inevitable that digital-to-analog converters and the like that require analog current switch circuits including logic circuits will become larger. Furthermore, since the analog current to be switched changes by the base current of the transistor in the switching circuit, a highly accurate analog current switch circuit cannot be expected.

An object of the present invention is to provide an analog current switch circuit that can be easily formed on the same semiconductor substrate and has high accuracy.

Next, a description will be given with reference to the drawings.

1 and 2 are diagrams showing analog current switch circuits. In FIG. 1, the current of an analog current source 3 is formed by a pair of differential transistors based on the output of a logical AND NOT (hereinafter referred to as NAND) circuit 1. The current switch circuit 2 outputs a NAND output from a terminal 4 and an AND output from a terminal 5.
On the other hand, in FIG. 2, as in the case of FIG. 1, the output of the logical sum (hereinafter referred to as OR) circuit 6 is switched by the current switch circuit 2 formed by a transistor pair, and the OR output from the terminal 4 is A logical sum negation (hereinafter referred to as NOR) output is taken out from terminal 5. 7 is a level shift circuit for biasing the current switch circuit 2 appropriately. In the above two analog current switch circuits, if each logic circuit is composed of MOS transistors, etc., the manufacturing process will be complicated if they are formed on the same substrate together with the bipolar truss transistor that constitutes current switch circuit 2. Even if the above logic circuit is TTL, it is difficult to miniaturize and simplify the circuit. Furthermore, the current output from the current switch circuit 2 becomes smaller than the current value of the analog current source 3 by the amount of the base current flowing through the transistors constituting the current switch circuit 2, and the current value of the analog current source 3 always decreases due to humidity or other conditions. It may not be the same value.

FIG. 3 is a diagram explaining the principle of the present invention, in which transistor 8 _-o+1 and transistors 8 _{-1 .} . . 8 _-o form a differential type current switch circuit. Both transistors operate in the active region and have highly accurate characteristics. Analog current is provided from analog current source 14 and is passed through transistor 17 to current source 2.
3. The same base bias is applied to the transistor 17 and the transistor 8 _-o+1 from the level shift circuit 11. A current is supplied to the current switch circuit from a current source 24, and the output currents of the current sources 23 and 24 are made to be the same.

Now, when a logic signal is input to any or all of the logic input terminals _11-1 to 11 _-o , any or all of the transistors _8-1 to 8 _-o become conductive, and the transistor 8 _-o+1 becomes conductive. is non-conductive, and the current of the current source 24 is taken out from the output terminal 12 as an OR output. At this time, the current of the current source 24 is
Since the current is made to be the same as the current of the analog current source 14, the base current of the transistor 17 is added to the current of the analog current source 14. Therefore, the current applied to the emitters of the conducting transistors 8 _-1 to 8 _-o is the sum of the current of the analog current source 14 and the base current of the transistor 17, and the current taken out to the output terminal 12 is From the current given to the emitters of the conductive transistors _8-1 to 8 _-o , the transistors _8-1 to 8-o
8 minus the base current of _-o . The conductive transistors 8 _-1 to 8 _-o equivalently form one transistor.

The current amplification factor of the Tr is usually constant over a collector current operating range of about two digits. Therefore, even if the output current of the current source 24 is passed through each ONTr and the collector current per one transistor decreases, the current amplification factor of the transistor remains unchanged, and equivalently the current as one transistor decreases. The amplification factor also remains unchanged. Therefore, transistors 8 _-1 to 8
If _-o and transistor 8 _-o+1 have the same current amplification factor, then the sum of the base current of transistor 8 _-o+1 and the base current of the conducting transistors 8 _-1 to 8 _-o are becoming equal. Therefore, the current obtained at the output terminal 12 is equal to the current value of the analog current source 14.

In addition, if no logic signal is input, transistor 8 _-o+1 becomes conductive and outputs from output terminal 13.
Current is extracted as a NOR output. The current of the NOR output from the output terminal 13 is also equal to the current of the analog current source 14 for the same reason as described above.

In this way, whether it is OR output or NOR output,
A constant current from the analog constant current source 14 can be extracted at all times. Therefore, since the current output from the output terminal is equal to the current of the analog current source in any case, a highly accurate analog current switch circuit can be achieved. Furthermore, since the circuit configuration is simple, it can be easily integrated, which in turn makes it possible to lower costs and improve reliability.

FIG. 4 is a diagram showing an embodiment of the present invention, in which current from a high-precision analog current source 14 is supplied to a base current compensation transistor 17 and a constant current source 23. Constant current source 23 includes transistor 18 and resistor 19
It is composed of. Further, the output of the amplifier 16 biases the transistors 18 and 20 constituting the constant current sources 23 and 24, respectively, so that the constant current source 24 serves as a power mirror of the constant current source 23.

On the other hand, the output of the level shift circuit 15 biased by the output of the amplifier 16 biases the base of the base current compensation transistor 8 _-o+1 . The level shift circuit 15 is provided with the output of the analog current source 14 and a reference potential which are compared and amplified by an amplifier 16. A constant DC voltage is superimposed on the output of the amplifier 16 in the level shift circuit, and the output of the transistor 17 and A base bias voltage of 8 _-o+1 is applied. The amplifier 16 is connected to the analog current source 1
They are connected to form a negative feedback circuit that virtually grounds the output point of No. 4. In this negative feedback circuit,
The Tr 18 and the analog current source 14 form an inverting amplification stage. With this configuration, the transistors 18,
Since the base potential of transistor 20 is also increased, transistors 18 and 20 always operate in the active region regardless of the output current from analog current source 14, and always generate the current from analog current source 14 plus the base current of transistor 17. Output accurately. Also,
Similarly, the base bias of the transistor 17 is applied according to the current from the analog current source 14, so the sum of the current from the analog current source 14 plus the base current of the transistor 17 is always accurately applied to the collector of the transistor 18. I can do it. Therefore, normal operation can be ensured over a wide range of current from the analog current source 14. It was explained with reference to Fig. 3 that the OR output from the output terminal 12 and the NOR output from the output terminal 13 are always equal to the current from the analog current source 14, and do not include the error due to the base current. Explain in detail.

It is assumed that a logic signal is applied to the bases of any or all of _Tr8-1 to Tr8 _-o and biased to a "HIGH" level. This "HIGH" level voltage is set higher than the output voltage of the level shift circuit 15 that biases the bases of Tr17 and Tr8 _-o+1 . Among Tr8 _-1 to _{Tr8 -o} , the Tr to which no logic signal is applied is biased to the "LOW" level. This "LOW" level voltage is set lower than the output voltage of the level shift circuit. Since the base potential of the "ON" Tr is biased to a higher level than the base potential of the "OFF" Tr and the base potential of Tr8 _-o+1 , that is, the output level of the level shift circuit, the "OFF" Tr and Tr8 _{- o+1} is in the cutoff state. Assuming that the input impedance of the amplifier 16 is sufficiently high and can be ignored compared to the current (I ₁₄ ) of the current source 14, I ₁₄ = I _c17 (collector current of Tr 17) I _c18 = I _E17 = I _C17 + I _B17 = I _C17 (1+1/β ₁₇ ) I _E18 = I _C18 + I _B18 = I _C18 (1+1/β ₁₈ ) Now, assuming R ₁₉ = R ₂₁ and Tr ₁₈ = Tr ₂₀ , I _E18 = I _E20 , β ₁₈ = β ₂₀ (β is the current amplification factor) Also, assuming that the entire ONTr forms one transistor, I _E (ONTr) = I _c (ONTr) + I _B (ONTr) = I _c (ONTr) (1+1/βONTr) I _c20 = I _E (ONTr) I _E20 = I _c20 + I _B20 = I _c20 (1+1/β ₂₀ ) From the above, I _c (ONTr) = βONTr/βONTr+1 ・I _E (ONTr) = (βONTr/βONTr+1)・ (β ₁₇ +1/ β ₁₇ )・(β ₂₀ /β ₂₀ +1) (β ₁₈ +1/β ₁₈ )・I ₁₄ =βONTr/βONTr+1)・(β ₁₇ /β ₁₇ +1)・I ₁₄ In other words, the ON transistor of the switch The error between the output current flowing through the terminal 12 and the current I ₁₄ of the current source 14 is determined by the current amplification factor β ₁₇ of the Tr ₁₇ and the entire turned-on Tr considered as one Tr. It is determined by the current amplification factor βONTr.

If βONTr= _β17 , then _Ic (ONTr)= _I14 . That is, Tr ₁₇ operates to compensate for the base current error even in the output state to terminal 12 via the transistor whose switch is ON.

In this way, according to the present invention, a current equal to the analog current source current is output in any case, so a high-precision analog current switch circuit is constructed, and since the circuit configuration is simple, integration is extremely easy. Become.

[Brief explanation of the drawing]

1 and 2 are diagrams showing a conventional analog current switch circuit, and FIG. 3 is a diagram explaining the principle of the present invention.
FIG. 4 is a circuit diagram showing one embodiment of the present invention. 1...NAND circuit, 2...Current switch circuit,
3, 9, 14...Analog current source, 4, 5, 1
2, 13... Output terminal, 6... OR circuit, 7, 1
1, 15...Level shift circuit, 8 _-1 to 8 _-o+
1 , 17, 18, 20...transistor, 11 _-1
~11 _-o ...Logic input terminal, 16...Amplifier, 2
3, 24...constant current source.

Claims (1)

[Claims] 1 The emitter and collector are commonly connected,
A group of transistors each receiving a logic input signal at their bases, a first transistor differentially connected to the transistor group, current supply means for supplying a current to be controlled, and a collector for the current from the current supply means. a second transistor received by the
a first current source that receives current from the emitter of the second transistor; a second current source that receives current from a differential connection point between the transistor group and the first transistor; Second
means for applying the same base bias having an intermediate value between the high level and the low level of the logic input signal to each base of the transistors of the transistors; An analog current switch circuit comprising an amplifier commonly connected to the bases of transistors constituting a current source.