JPS59122120A - Digital-analog converter - Google Patents

Abstract

PURPOSE:To prevent the fluctuation in a D-A conversion output current due to the variance of beta by adding sum of base currents of transistors (TRs) constituting a current source and TRs constituting a switch to the current source. CONSTITUTION:Let a current of the current source 32 be I, then almost the same current I flows also to diodes 30, 26, TRs 28, 1 and resistors 27, 5. A VBE31 of a TR31 is given by VBE=VBE28+VBE29-VBE30. Since VBE=VBE30, the relation of VBE31=VBE29 is obtained. Further, a current flowing to a TR29 is I/beta and a current flowing to a TR31 is 2 (I/beta), when the emitter area of the TR31 is twice that of the TR29. On the other hand, when a TR31 is conductive by an input signal, 2 (I/beta) is added to the emitter current, resulting in being (1+I/beta) I, and an output current IOUT at a terminal 23 is IOUT=(1+1/beta) (1-1/beta). This equation indicates that the stability is improved by nearly 1/beta times that of a conventional circuit.

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to a digital-to-analog converter, and more particularly, to a method for reducing deterioration in precision of a digital-to-analog converter due to variations in flying gain of transistors. Regarding obtaining high precision digital-to-analog converters.

[Technical background of the invention and its problems]

As an example of a conventional digital-to-analog converter (hereinafter referred to as DAC), a 4-pin DAC using an R-2R ladder resistor network is shown in No. 111J. npn bipolar transistor l and resistor] '15 constitutes a current source, and similarly transistor 2 and resistor 6, transistor 3 and resistor 7
．． The transistor 4 and the resistor 8 each constitute a current source having the same current value, and the current value is determined by the voltage VB of the bias terminal 9. On the other hand, the data applied to the 4-bit digital data input terminal group 18 switches the differential switch constituted by the npn bipolar transistors 10, 11, 12, 13, 14, 15, 16, and 17, and It is selected whether to flow the current to the Fi wire resistor network 19 or to the power supply 21, and the output terminal 20 receives a weighted voltage corresponding to the digital data, that is, the digital data is converted to analog data. The voltage VOut is output.

The accuracy in AC is largely dependent on the accuracy of the current source. This is a single semiconductor substrate (hereinafter referred to as IC)
The following three items can be cited as error factors when forming on the top of ■Variation in transistor baseφemitter voltage VBE △VBB, ■Variation in transistor current gain β △β, ■Variation in resistance value R△
There is R. Regarding ■, increase the bias 1 pressure VB (and
This can be reduced by increasing the pressure applied to the resistor RE, and (2) can be reduced by increasing the resistance S.

(2) will be explained with reference to the drawings. D shown in Figure 1
In AC. The 1-bit current source and switch circuit are connected to the second
As shown in the figure. Here, the current flowing through the output terminal 23 is determined.

Assuming that the current IU flowing through the resistors RE and 5 is equal for each bit, the collector current of the transistor 1 is (1-1/β).
■The collector current when the switch transistor 11 is conductive is (1-1/β)・(1-1/β)・I, and the output current I out of the terminal 23 is I out = (1-2/ 79+'/79N)"I
Here, assuming that β75 and △β vary, the output current difference between the two pits is △■U(2/β)・(Al7β piece...
(11) This is an error. This is due to the difference in base current when the emitter current is the same.

It is only necessary to reduce the current drawn by the bias VB 9, and for this purpose, a Darlington connection with a transistor 25 connected as shown in FIG. 3 can be used. In this case, the output current difference is ΔE (1/β)・(Al7β piece...
(21 and f)Al1 is improved, but there is a drawback that ΔVBE of transistors 25 and 1 increases.

Another drawback is that the base current of the switching transistor 11 is not compensated for.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide a digital-to-analog converter in which deterioration in accuracy of the converter due to variations in current gain β of transistors is improved.

[Summary of the invention]

The third aspect of the present invention is to reduce the influence of β on the output current by compensating the base currents of the current source transistor and the switch transistor, thereby improving accuracy deterioration due to variations in β.

[Embodiments of the invention]

The present invention will be described in detail below using one drawing.

FIG. 4 shows a base current compensation circuit used in the present invention. In its operation, if the current of the m current source 32 is I, the diodes 30, 26 . h range metal 28゜1, resistance 27.5
Almost all I flows through it. Here, VBE of transistor 31
31U, VBE31 = VBE28+VBK29-VBE30
Here, from VBE28=VBE30.

Vl (E31 = VBE29. Furthermore, the current HI/β flowing through the transistor 29
, and if the emitter area is twice that of the transistor 29 (transistor 31H) (that is, the reverse saturation current Is
is doubled) The current flowing through the transistor 31 is 2
・(I/β).

On the other hand, the collector current of transistor 1 is (1-1//)・
■ Transistor 11
When is conducting, its emitter current is.

2・(■/β) is added and becomes (1+1/β)・■ Therefore, the output current of the terminal 23 is.

Iout = (1+1/β01-1/7), 2
The difference current △I between the two pits is.

△any (1/β)・(2/β)・(△β/β), which is compared to Figure 2, that is, equation (1), and 1/β, and Figure 3, that is, compared to equation (2). It is improved by 2/β.

An embodiment of the DAC of the present invention is shown in FIG. This is an application of the base compensation circuit shown in FIG. 4 to a 4-pin DAC using a ladder-shaped resistor network.

Another embodiment of the invention is shown in FIG. This is a transistor 3 which is a DAC with binary weighting applied to the current source.
The emitter areas of 7.38 and 39.40 are the ratios of A: 2 A: 4 A: 8 A (where A is the emitter area of the transistor 37), respectively, and the accuracy is expressed as #). Similarly, the base current compensation transistor according to the present invention 31, 45.46. and the emitter area ratio of 47 and 48Fi, respectively, or
The base current is compensated by the number of transistors. Figure 5.6 Fruit 7II! Although all focus compensation was performed in example i, only the upper bits may be compensated if necessary. Furthermore, it is clear that the present invention can be applied regardless of the number of bits, and can be applied to all digital-to-analog converters that require precision of the current source.

〔Effect of the invention〕

As described above, according to the present invention, by adding the sum of the base currents of the transistors constituting the current source and the transistors constituting the switch to the current source, the current of the transistor that affects the output current of the DAC is calculated as ff. Can reduce the impact of gain. That is, the influence of current gain variations on accuracy can be reduced, and.

- Since the base upstream compensation circuit is implemented in the part that operates with direct current, it does not cause speed deterioration and can operate at high speed, making it possible to provide a highly accurate digital-to-analog converter.

[Brief explanation of drawings]

Figure 1 shows a conventional DAC using a ladder network. FIG. 2 shows an excerpt of one bit of the circuit shown in FIG. FIG. 3 is a diagram illustrating a conventional DAC and an embodiment of the present invention in which the base current of the current source is compensated. 9...Bias ridge pressure application terminal, 18.24...Digital data input terminal group %19...Ladder-shaped resistance network. 20.23...Output terminal% 21,22...Power supply r
9 voltage application terminals, 32... current source. Representative Patent Attorney Kensuke Chika (and 1 other person) Figure 1 Figure 21 Figure 3 Figure 4

Claims (1)

[Claims] A current having a current value obtained by adding the sum of the base current of the transistor configuring the current source of the digital-to-analog converter and the base current of the transistor configuring the switch connected to the current source to the current of the current source. A digital-to-analog converter, comprising: a source.