JPH04109715A - Digital input circuit - Google Patents

Abstract

PURPOSE:To easily connect the circuit to a circuit separated from a power supply of the former circuit by connecting emitters of a couple of complementary transistors (TRs) in series and using bases of the TRs for input terminals of a differential amplifier circuit. CONSTITUTION:Bases of a couple of complementary TRs Q1, Q2 whose emitters are connected in series are used for input terminals 3,4. It is possible to limit a collector current of the TRs Q1, Q2 by a signal voltage fed between the input terminals 3 and 4. Since only a base current flows to the input terminals 3 and 4, the circuit is driven by a signal source whose impedance is high and since an output impedance of the collectors of the TRs Q1, Q2 is high and the output follows to an input common mode signal, the circuit is immune to noise as a result.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a digital input circuit, and more particularly to a digital signal input circuit that detects only difference components of input voltages and does not detect in-phase components.

[Conventional technology]

An example of a conventional digital input circuit of this type is shown in FIG. This is transistor Q5. This is a general differential circuit consisting of Q6, loads 9 and 10, and current source 2. One end of the digital input signal is directly applied from input terminal 3 to one input terminal of the differential circuit, and the digital input signal The other end is connected to the input terminal 4 with a level shift of two diodes by diodes D, , D2. Therefore, the output of this differential circuit is determined depending on whether the input terminal 3 has increased by two forward voltages of the diode with respect to the input terminal 4.

In this circuit, a logic input can generally be detected with a threshold value of about 1.4 square meters, and it can be made insensitive to the common mode component of the logic input.

Furthermore, another example of this type of digital input circuit is shown in the fourth example.
As shown in the figure. Darlington connected transistor Q7 with load 9. One end of the base of Q8 serves as input terminal 3, and the other input terminal 4 serves as the emitter of transistor Q8. When a digital input signal is applied between the input terminals 3 and 4, it is generally possible to control the current of the load 9 by a threshold value of 1.4, that is, to detect the digital input signal.

[Problem to be solved by the invention]

In the conventional digital input circuit described above in FIG.
The disadvantage is that the impedance of the input signal source must be low, which limits the use of the circuit.

Furthermore, in FIG. 4, because the power supply terminal 2, which is a low potential power supply, and the input terminal 4, which is one end of the input terminal, are common due to the circuit configuration, the impedance is low with respect to the common mode component of the input signal. , which has the drawback of being restricted in its use, and also having the same restrictions as shown in FIG. 3.

An object of the present invention is to eliminate these drawbacks and provide a digital input circuit that can be driven by a high impedance signal source and can be separated from a power supply circuit.

[Means to solve the problem]

The configuration of the digital input circuit of the present invention is such that each emitter or source of a pair of complementary transistors is connected in series,
The device is characterized in that the bases or gates of a pair of these transistors are used as input terminals, and the collectors or drains of these transistors are respectively connected to a load as output terminals.

〔Example〕

Next, the present invention will be explained with reference to the drawings.

FIG. 1 is a circuit diagram of an embodiment of the present invention. A pair of complementary transistors Q3 whose emitters are connected in series. The base of each Q2 is used as an input terminal 3.4. Input terminal 3.
By the signal voltage applied between the transistors Ql and 4,
It is possible to control the collector current of Q2. That is, digitally, transistor Q1. Q2 VBE
(base-emitter voltage), it can be detected with a threshold of about 1.4V. Furthermore, since only the base current flows through the input terminal 3.4, it can be driven by a signal source with high impedance, and the transistors Qs and Q
Since the output impedance of the collector of z is high and follows the input in-phase signal, it becomes insensitive as a result.

FIG. 2 is a circuit diagram of a specific example of FIG. 1. The bases of the transistors Ql and Q2, whose emitters are connected in series, are connected to the signal source S through current limiting resistors R,,R,. Here, the current limiting resistors R2, R, limit excessive current when a low impedance signal source is input with a large amplitude. Considering digitally, when the voltage amplitude of the signal source S exceeds approximately 1.4■, the transistor 7.8 is turned on and the transistor Q! , Q2 are determined by the current mirror source side transistor Q3, which is the load 9, and the resistor 10 (R1), and the collector current of the current mirror transistors QS and Q4 is determined by the output terminal 6.
is output to.

Conversely, when the signal source S drops below approximately 1.4V, the transistor Q
1. Q2 is in the OFF state, and the collector current no longer flows and is not output to the output terminal 6. In other words, this circuit depends only on the voltage amplitude of the signal source S, and there is no in-phase component.

Although this embodiment has been described using bipolar transistors, they may be replaced with complementary field effect transistors.

〔Effect of the invention〕

As explained above, the present invention connects the emitters of a pair of complementary transistors in series, and uses the bases of each as differential input terminals, so that it can be sufficiently driven by a signal source with high impedance, and the in-phase input signal can be Because components can be removed,
This has the advantage that it can be easily connected to a separate circuit from the power supply of this circuit.

If the mutual conductance of transistors Ql and Q2 is significantly gm, the mutual conductance Gm in this circuit will be Gm~gm/2, and from an analog perspective, it can also be used as an amplifier circuit, and the above-mentioned effect can be obtained. .

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing one embodiment of the present invention, and Fig. 2 is a circuit diagram showing an embodiment of the present invention.
3 and 4 are circuit diagrams showing a conventional two-side input circuit. 1.2...Power terminal, 3.4...Digital input terminal, 5,6...Output terminal, 9,10...Load, D. D2...Diode for level silt, ■,...Current source for diode bias, D2...Current source for differential circuit, Ql, Q2...Directly connected transistor, Q3
．． Q4...Current mirror transistor, Q5. Q6
...Transistor for differential circuit, Q. Q8... Darlington connection transistor, R1...
Resistor, R2, R3... Current limiting resistor, S... Input signal source.

Claims (1)

[Claims] A digital device characterized in that the emitters or sources of a pair of complementary transistors are connected in series, the bases or gates of the pair of transistors are used as input terminals, and the collectors or drains of these transistors are respectively connected as output terminals to a load. input circuit.