Amplifiers – With semiconductor amplifying device – Including atomic particle or radiant energy impinging on a...
Reexamination Certificate
2002-06-20
2004-03-30
Mottola, Steven J. (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including atomic particle or radiant energy impinging on a...
C330S086000
Reexamination Certificate
active
06714082
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor amplifier circuit. In particular, the present invention relates to a semiconductor amplifier circuit used as a component of a receiver for optical communication and having a high gain and a wide dynamic range.
2. Description of the Related Art
A receiver for optical communication has a function for converting an optical signal transmitted through a relay transmission path, such as an optical fiber, to an electrical signal by a light-receiving element and for amplifying the electrical signal by an amplifier circuit. The amplifier should have a high gain in order to amplify a weak signal which has been attenuated due to a long-distance transmission. Further, since the optical signal intensity widely varies depending on the optical transmission distance, a wide dynamic range suitable for this variation in the optical signal intensity is required.
A transimpedance amplifier
2
, in which a feedback resistor
27
is connected in parallel to an operational amplifier
28
, as shown in
FIG. 2
, is widely used as an amplifier circuit for optical communication. The transimpedance amplifier
2
converts a current signal input from a light-receiving element
1
to a voltage signal so as to amplify the signal. The gain of the transimpedance amplifier
2
depends on the resistance of the feedback resistor
27
. Therefore, a high gain can be obtained by increasing the resistance of the feedback resistor
27
. However, by increasing the resistance, the output voltage is likely to saturate and thus the dynamic range becomes narrow.
As described above, the gain and the dynamic range of the transimpedance amplifier depend on the value of the feedback resistor and are in a trade-off relationship. Thus, a high gain and a wide dynamic range cannot be realized at the same time.
FIG. 3
shows an example of a known amplifier circuit for optical communication including an improved transimpedance amplifier. As shown in
FIG. 3
, a DC voltage generating circuit
4
including a resistor
41
and a capacitor
42
connected in series is connected to the output terminal of the transimpedance amplifier
2
. Also, a by-pass transistor
7
is connected to the input terminal of the transimpedance amplifier
2
. The DC voltage generated in the DC voltage generating circuit
4
is applied to the gate of the by-pass transistor
7
via an inverter
5
. Herein, the DC voltage generating circuit
4
generates a DC voltage whose value is substantially the same as the time-average value of the voltage signal output from the transimpedance amplifier
2
.
According to the above-described configuration, the impedance of the by-pass transistor
7
varies according to the intensity of the current signal input from the light-receiving element
1
, and thus the gain of the transimpedance amplifier
2
is adjusted. Therefore, the amplifier circuit shown in
FIG. 3
has excellent characteristics in a gain and a dynamic range, compared to the transimpedance amplifier
2
shown in FIG.
2
.
Further, the amplifier circuit shown in
FIG. 3
amplifies the voltage signal output from the transimpedance amplifier
2
by a differential amplifier
3
and externally outputs the signal. As can be seen in
FIG. 3
, the DC voltage generated in the DC voltage generating circuit
4
is applied to a reference-voltage input terminal
36
of the differential amplifier
3
. Accordingly, the voltage signal applied to a signal input terminal
35
of the differential amplifier
3
swings, with the center being a DC voltage level which is substantially equal to the time-average value, in accordance with the change of the optical signal intensity. As a result, the voltage amplitude output to the output terminal
37
and that output to the output terminal
38
are inverted with respect to each other. Therefore, the amplifier circuit shown in
FIG. 3
is capable of externally outputting the voltage signal, which has an amplitude twice as much as a single-phase amplitude and which does not have distortion.
In general, an amplifier circuit for optical communication is integrated by using a compound semiconductor having an excellent high-frequency characteristic, such as gallium arsenide (GaAs). As a transistor for constituting the circuit, Schottky gate FET is used.
The Schottky gate FET is used as an input device for the differential amplifier
3
and the inverter
5
of the amplifier circuit shown in FIG.
3
. The DC voltage generated by the DC voltage generating circuit
4
is applied to the gate of each of the FETs. Accordingly, a large gate leakage current is generated when the DC voltage exceeds the threshold voltage of the Schottky gate FETs.
In particular, when the gate leakage current of the FET used as the input device for the inverter
5
becomes large, the current applied to the resistor
41
of the DC voltage generating circuit
4
becomes large accordingly. As a result, a large potential difference occurs between both terminals of the resistor
41
, and thus the DC voltage input to the reference-voltage input terminal
36
of the differential amplifier
3
is reduced. Accordingly, the DC voltage applied to the reference-voltage input terminal
36
of the differential amplifier
3
is lower than the time-average voltage signal applied to the signal input terminal
35
. This causes a problem that the voltage signal input to the differential amplifier
3
is not normally amplified.
The above-described problem occurs because both of the DC voltage to be applied to the inverter
5
and the DC voltage to be applied to the differential amplifier
3
are generated by the DC voltage generating circuit
4
. Accordingly, when the DC voltage to be applied to the reference-voltage input terminal
36
of the differential amplifier
3
is generated by another DC voltage generating circuit, the influence of the gate leakage current of the inverter
5
can be suppressed. In order to achieve this, however, a resistor and a capacitor for forming another DC voltage generating circuit must be fabricated on an amplifier circuit chip.
Generally, in this type of DC voltage generating circuit, an external microchip capacitor is used because a large capacitance is required to smooth an input voltage. Thus, as described above, the amplifier circuit chip becomes large when another DC voltage generating circuit is provided.
Alternatively, a DC voltage can be supplied from a DC voltage generating circuit provided outside the amplifier circuit chip to the reference-voltage input terminal of the differential amplifier in the amplifier circuit chip. In this method, however, a terminal for external connection must be provided in the amplifier circuit chip, which results in an increase in the size of the chip and high cost.
Further, in the amplifier circuit shown in
FIG. 3
, the by-pass transistor
7
is connected in parallel to the input terminal of the transimpedance-amplifier
2
. Accordingly, the input capacitance of the transimpedance amplifier
2
increases by the gate capacitance of the by-pass transistor
7
. This causes further deterioration in the frequency characteristic of the transimpedance amplifier
2
in a high-frequency region. The input capacitance of the transimpedance amplifier
2
can be effectively reduced when the gate width of the by-pass transistor
7
is made to be narrow. However, this causes the increase of the impedance of the by-pass transistor
7
. As a result, the gain characteristic and the dynamic range characteristic of the amplifier circuit deteriorate.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to improve the amplification characteristic of an amplifier circuit without causing an increase in the size of the amplifier circuit and an increase in the number of external terminals.
In order to overcome the above-described problems, a semiconductor amplifier circuit according to the present invention comprises a transimpedance amplifier for amplifying an input signal; a by-pass transistor connected between a
Armstrong Westerman & Hattori, LLP
Fujitsu Quantum Devices Limited
Mottola Steven J.
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