Amplifiers – With semiconductor amplifying device – Including gain control means
Reexamination Certificate
1999-10-18
2001-08-28
Pascal, Robert (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including gain control means
C330S302000, C330S307000
Reexamination Certificate
active
06281754
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an amplifier circuit.
A semiconductor integrated circuit (hereafter referred to as an IC) that processes analog signals is often provided with an amplifier circuit. Various types of amplifier circuits have been developed to meet requirements corresponding to different purposes of use, and among them, the grounded emitter-type amplifier circuit is one of those that can be achieved through the simplest circuit structure.
FIG. 8
illustrates the grounded emitter-type amplifier circuit
101
in the prior art.
An input terminal “in” of the amplifier circuit
101
is connected to a base terminal of an amplifying transistor Tr. Its output terminal “out” is connected to both a collector terminal of the amplifying transistor Tr and the source Vcc via a load resistor RLD. An emitter terminal of the amplifying transistor Tr is connected to the ground GND via a feedback resistor RE.
The amplifier circuit
101
structured as described above multiplies a base current ib that is caused to flow into the base terminal by an input voltage ei by a factor of hfe (the current amplification factor of the amplifying transistor Tr) and supplies a collector current ic (=hfe×ib) to the collector terminal. In addition, it outputs an output voltage eo at a value achieved by subtracting the product of the collector current ic and the load resistance RLD from the voltage supplied by the source Vcc. It is to be noted that the feedback resistor RE fulfills a function of stabilizing the collector current ic when a load fluctuation occurs at a circuit connected to the output terminal “out” or when a fluctuation occurs in the ambient temperature.
Now, the IC manufacturing process includes an inspection step implemented to improve production throughput, to assure product reliability, to screen good/defective chip products and the like. During the inspection step, a probe test is normally conducted on ICs to measure their electrical characteristics. In this probe test, the electrical characteristics of the ICs are measured by placing probes on the bonding pads of a wafer where the ICs are formed.
However, during such a probe test on ICs, noise attributable to the parasitic capacity component present in the test apparatus and the like may enter the ICs. In particular, in an IC provided with an amplifier circuit such as the amplifier circuit
101
, such noise may cause the amplifier circuit to oscillate. Such oscillation of the amplifier circuit, which constitutes an erroneous operation of the IC, often results in difficulty in obtaining accurate electrical characteristics of the IC having the amplifier circuit through a probe test.
SUMMARY OF THE INVENTION
An object of the present invention, which has been completed by addressing the problem of the prior art discussed above, is to provide an amplifier circuit that does not allow oscillation attributable to noise, i.e., that achieves an improvement in the noise-resisting characteristics.
In order to achieve the object described above, in a first aspect of the present invention, an amplifier circuit formed on a wafer is provided. This amplifier circuit is characterized in that it is provided with an amplifying transistor that outputs an amplified signal based upon an input signal, a gain control unit that is capable of lowering the gain of the amplifying transistor with respect to the input signal in a specific frequency band and a gain control unit switcher that is capable of switching the gain control unit to either an enabled state or a disabled state.
By adopting this structure, it becomes possible to set the gain of the amplifiying transistor to 0 dB or lower relative to the input signal having a frequency equal to or higher than a specific frequency level. For instance, even when a positive feedback occurs in the amplifying transistor due to high frequency external noise, it is possible to prevent oscillation of the amplifier circuit by setting the gain of the amplifying transistor at 0 dB. In addition, since the gain control unit that controls the gain of the amplifying transistor is switched to the enabled state or the disabled state by the gain control unit switcher, it is possible to reduce the gain of the amplifier circuit only during the period of time over which a positive feedback tends to occur, e.g., during the period of time over which a probe test is conducted on the amplifier circuit.
Furthermore, it is desirable to constitute the gain control unit as a capacitor having one of its terminals connected to a control terminal of the amplifying transistor and another terminal connected to the gain control unit switcher. By adopting this structure, the gain control unit is realized as a simple circuit.
The gain control unit switcher may be achieved in the form of a wiring. It is desirable to connect this wiring, which is preferably formed so that it can be cut when the wafer is diced into chips, to the other terminal of the capacitor at one end and to a first source terminal of the amplifying transistor at the other end. As explained earlier, oscillation may occur at the amplifier circuit during a probe test. Since the probe test is conducted before the wafer is made into chips through dicing under normal circumstances, the gain control for the amplifying transistor performed by the capacitor constituting the gain control unit is often not required after the wafer is cut into chips. According to the present invention, since the wiring constituting the gain control unit switcher is cut through dicing, the capacitor constituting the gain control unit is electrically cut off from the amplifying transistor. Thus, during a normal operation of the amplifier circuit after chip production, erroneous operation of the amplifier circuit such as a reduction in the gain of the amplifying transistor caused by the capacitor and the like are prevented.
Alternatively, the gain control unit switcher may be constituted of a wiring with one end of the wiring connected to the other terminal of the capacitor and the other end of the wiring connected to a pad. In this structure, the capacitor constituting the gain control unit can be set in an enabled state by inputting a specific voltage signal through the pad. As a result, the gain of the amplifying transistor can be reduced during a target period of time.
In addition, it is desirable to constitute the gain control unit as a capacitor having one of its terminals connected to a first source terminal of the amplifying transistor and another terminal connected to the gain control unit switcher. By adopting this structure, the gain control unit is realized as a simple circuit.
The gain control unit switcher may be achieved in the form of wiring. It is desirable to connect this wiring, which is preferably formed so that it can be cut when the wafer is diced into chips, to the other terminal of the capacitor at one end thereof and to a first source at the other end. As explained earlier, oscillation may occur at the amplifier circuit during a probe test. Since the probe test is conducted before the wafer is made into chips through dicing under normal circumstances, the gain control for the amplifying transistor performed by the capacitor constituting the gain control unit is often not required after the wafer is cut into chips. According to the present invention, since the wiring constituting the gain control unit switcher is cut through dicing, the capacitor constituting the gain control unit is electrically cut off from the first source and the capacitor is set in a disabled state. Thus, during a normal operation of the amplifier circuit after chip production, erroneous operation of the amplifier circuit such as a reduction in the gain of the amplifying transistor caused by the capacitor and the like are prevented.
Alternatively, the gain control unit switcher may be constituted of a wiring with one end of the wiring connected to the other terminal of the capacitor and the other end of the wiring connected to a pad. In this structure, the capacitor cons
Jones Volentine PLLC
Nguyen Patricia T.
OKI Electric Industry Co., Ltd.
Pascal Robert
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