Semiconductor integrated circuit device

Details Semiconductor integrated circuit device Semiconductor integrated circuit device

2001-02-14

2003-05-06

Mottola, Steven J. (Department: 2817)

Amplifiers

With semiconductor amplifying device

Including differential amplifier

C330S306000

Reexamination Certificate

active

06559718

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to a semiconductor integrated circuit device. More specifically, the invention relates to technology for preventing malfunction caused by high-frequency noise that enters through input terminals of a differential amplifier circuit, such as technology effective in coping with electromagnetic wave noise in the operational amplifier IC.
A variety of circuit forms have heretofore been proposed using a differential amplifier circuit as an operational amplifier or a comparator IC for detecting the levels of analog input signals. However, systems using the semiconductor integrated circuit device are accompanied by a problem of malfunction caused by electromagnetic interference waves. On the other hand, the operational amplifier IC and the comparator IC have generally been considered to be free from the problem of malfunction that stems from the electromagnetic wave noise owing to the employment of a differential amplifier circuit that is less affected by noise of the same phase.
In recent years, however, it has been pointed out that even the operational amplifier IC and the comparator IC are subject to malfunction due to the infiltration of electromagnetic wave noise through input terminals. In order to prevent malfunction caused by electromagnetic wave noise, therefore, there has been proposed an invention according to which a filter unit which is a capacitor using an insulating film as a dielectric is formed between an input pin and a differential amplifier circuit (Japanese Patent Laid-Open No. 167827/1997).
The above proposed invention, however, simply discloses forming the filter unit which is a capacitor using an insulating film as a dielectric between the input pin and the differential amplifier circuit, but teaches none of a concrete capacity of the capacitor or a cut-off frequency of the filter unit constituted by the capacitor.
SUMMARY OF THE INVENTION
The present inventors have analyzed the causes of malfunction of the operational amplifier IC due to electromagnetic wave noise, quite independently of the above proposed invention. As a result, the inventors have discovered the occurrence of malfunction due to the mechanism described below.
First, the inventors have speculated that the cause is due to the input of different noises to an inverted input terminal and to a noninverted input terminal, since the differential amplifier circuit is immune to the noises of the same phase that enter through the inverted input terminal and the noninverted input terminal and does not malfunction. The operational amplifier IC is used in a state where an analog signal is input to one input terminal and a reference voltage is applied to the other input terminal. In this case, the lengths of wirings are different up to the inverted input terminal and up to the noninverted input terminal, and the electromagnetic wave noises do not enter under quite the same condition; i.e., noises are often out of the same phase.
As shown in
FIG. 1
, therefore, a circuit is formed in a manner that ground potential is applied to an inverted input terminal (−) of the operational amplifier OP through a resistor r
1
, an end of a feedback resistor r
2
is connected thereto, and ground potential is applied to a noninverted input terminal (+) through resistors r
3
and r
4
connected in parallel. The resistors r
1
and r
3
are 51&OHgr;, and the resistors r
2
and r
4
are 5.1 k&OHgr;. The resistors r
3
and r
4
are connected in parallel to the noninverted input terminal (+), from such a standpoint that an input offset will not occur in the circuit under the same condition as the inverted input terminal (−) to which the two resistors r
1
and r
2
are connected. The cut-off frequency fc of the operational amplifier that is used is about 300 kHz.
In this circuit, first, a high-frequency noise source RF is connected to the noninverted input terminal (+) to give it false electromagnetic wave noise, high-frequency waves are input to the noninverted input terminal (+) from the high-frequency noise source RF, and an output voltage is observed while changing the frequency. Next, referring to
FIG. 2
, a high-frequency noise source RF is connected to the inverted input terminal (−) of the operational amplifier OP to give it false electromagnetic wave noise, high-frequency waves are input to the inverted input terminal (−), and the output voltage is observed while changing the frequency.
FIG. 3
illustrates a change in the output voltage that is observed when high-frequency noise is input to the noninverted input terminal (+), and
FIG. 4
illustrates a change in the output voltage that is observed when high-frequency noise is input to the inverted input terminal (−). It is learned from
FIG. 3
that when high-frequency noise is input to the noninverted input terminal (+), the output voltage Vout starts decreasing from around 1 MHz which is slightly higher than the cut-off frequency fc of the operational amplifier, becomes the lowest around 100 MHz, rises thereafter and returns to the initial level around 1 GHz. It is similarly learned from
FIG. 4
that when high-frequency noise is input to the inverted input terminal (−), the output voltage Vout starts increasing from around 1 MHz which is slightly higher than the cut-off frequency fc of the operational amplifier, becomes the highest around 100 MHz, decreases thereafter and returns to the initial level around 1 GHz.
The present inventors have studied the cause of temporary increase or decrease of the output voltage Vout over a given frequency band, and have reached the conclusion that the phenomenon mentioned below is a cause.
FIG. 5
illustrates a circuit constitution of the operational amplifier OP used in the above experiment. In this operational amplifier, level shift circuits
12
and
13
constituted by emitter followers for broadening the lower-limit level of a dynamic range of input signals toward the lower side of the ground potential, are inserted in a stage preceding an active load-type differential amplifier stage
11
.
FIG. 6
shows measurement of changes in a potential V
1
at an input node and in a potential V
2
at an output node n
2
of the level shift circuit
12
at the time when a signal of a frequency lower than the cut-off frequency fc is input to the inverted input terminal (−) of the operational amplifier. In this case, as will be obvious from
FIG. 6
, the two potentials V
1
and V
2
change in the same manner being deviated by a forward voltage Vbe (about 0.7 V) across base and emitter of an input transistor Q
1
.
FIG. 7
illustrates changes in the potential V
1
at the input node and in the potential V
2
at the output node n
2
of the level shift circuit
12
at the time when a signal of a frequency of about 100 MHz which is higher than the cut-off frequency fc is input to the inverted input terminal (−) of the operational amplifier. In this case, the input potential V
1
varies depending upon the input, but the potential V
2
at the node n
2
assumes a saw-tooth wave form of a small amplitude as shown in
FIG. 7
, and an average DC level is considerably lower than that of FIG.
6
.
As described above, the potential V
2
at the node n
2
assumes the saw-tooth wave form probably because a parasitic capacity Cjs between the base and the substrate of a differential transistor Q
3
in a differential amplifier stage is connected to the output node n
2
of the level shift circuit and, hence, a current of a current source I
1
in the level shift circuit is consumed for charging the parasitic capacity Cjs when V
2
increases, and the electric charge in the parasitic capacity Cjs is quickly extracted by a collector current of the input transistor Q
1
when V
2
decreases. It was found that when the high-frequency wave is input to the inverted input terminal (−) of the operational amplifier of FIG.
5
and the DC level of the potential V
2
at the node n
2
decreases, the DC level varies depending upon the frequency an

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