Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Amplitude control
Reexamination Certificate
2002-01-10
2003-04-15
Tran, Toan (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Amplitude control
C327S363000, C327S563000
Reexamination Certificate
active
06549054
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a DC offset cancellation circuit that cancels a DC offset voltage occurring between a pair of complementary differential output signals-outputted from a differential amplification circuit, a differential amplification circuit with a DC offset cancellation circuit, and a photo-electric pulse conversion circuit that uses the differential amplification circuit capable of DC offset cancellation to convert an optical pulse signal to a corresponding electrical pulse.
Alternatively, it relates to a pulse shaping circuit that generates a shaped pulse signal whose logic changes in a manner similar to a rise and a fall of a base square-wave pulse signal, a pulse generation circuit that uses this pulse shaping circuit, and a photo-electric pulse conversion circuit that uses the pulse shaping circuit to convert an optical pulse signal to a corresponding electrical pulse.
2. Description of Related Art
(Related Art
1
)
In a differential amplification circuit that amplifies an input signal and outputs a pair of differential output signals, a difference in reference voltages (hereinafter referred to also as a DC offset voltage) occurring between a pair of complementary differential output signals outputted from the differential amplification circuit, namely, between a non-inversion output signal and an inversion output signal presents at times a problem. Therefore, a differential amplification circuit provided with a DC offset cancellation circuit that cancels the DC offset voltage is proposed.
A photo-electric pulse conversion circuit
10
shown in
FIG. 35
will be explained as an example. The photo-electric pulse conversion circuit
10
converts an optical pulse signal LT to an electrical pulse signal xRX. For example, it is used as a receiving circuit in IrDA communications and transmits an inversion electrical pulse signal xRX to a demodulator circuit at a later stage.
When it is used in such optical communications, the distance from a transmitter circuit (a light source) to a receiving circuit (photodiode PD) is not constant, and therefore there are various conditions. In some cases, the received optical pulse signal LT is very feeble due to a long distance, and in other cases, the received signal LT is extremely strong due to a short distance. As a result, a current input signal fluctuates from scores of nA to several mA which is several hundred times as large as the scores of nA. Even in such cases, it is necessary to reliably receive the optical pulse signal, to shape the waveform while accurately maintaining a pulse width thereof, and to send the resultant signal to the demodulator circuit at a later stage.
In the photo-electric pulse conversion circuit
10
, when the photodiode PD receives the optical pulse signal LT that rises at a second timing t
2
and falls at a first timing t
1
, a pulsating current signal I in flows according to the intensity of the light. An I-V conversion circuit IV converts this current signal Iin to a pair of complementary differential voltage signals, namely, a non-inversion voltage signal V
1
P that is in the same phase as the optical pulse signal LT and the current signal Iin and an inversion voltage signal V
1
M that is complementary thereto and outputs these signals. The waveforms of the differential voltage signals V
1
P, V
1
M when a large signal is inputted are slightly different from those when a small signal is inputted as shown in FIG.
36
. When a small signal is inputted, the current signal Iin of the photodiode PD having a pulse width tpw that nearly corresponds to the optical pulse signal LT is obtained. When a large signal is inputted, however, the waveform has a dull rising edge and a dull falling edge though it has a generally square shape. This is because the electrical signal fails to accurately follow changes in the optical input. Particularly, since the falling edge after the first timing t
1
falls slowly, the non-inversion voltage signal V
1
P also falls slowly as shown in FIG.
36
.
The differential voltage signals V
1
P, V
1
M are then amplified by a first differential amplification circuit AMP
1
provided with a DC offset cancellation circuit OFC indicated by dashed lines in
FIG. 35 and a
second differential amplification circuit AMP
2
. Then, as shown in
FIG. 37
, a reference voltage VREF according to an output VO of the amplifier is generated by a reference voltage generation circuit REFG and both signals are compared with each other by a comparison circuit CMP to obtain an inversion electrical pulse signal xRX which has the pulse width tpw corresponding to the optical pulse signal LT and which falls at the second timing t
2
and rises at the first timing t
1
.
More specifically, an offset adding circuit (mixing circuit) OFP is used to mix the offset cancellation voltage VOC into the differential voltage signals V
1
P, V
1
M such that a negative feedback is performed, thereby generating second differential signals V
2
P, V
2
M which are amplified by the first differential amplification circuit AMP
1
to output third differential signals V
3
P, V
3
M. In the DC offset cancellation circuit OFC, the third differential signals V
3
P, V
3
M are filtered by a low-pass filter LPF having characteristics of a cutoff frequency fc
1
and a through rate SR
1
to obtain the offset cancellation voltage VOC. Since the DC offset voltage occurring between the third differential signals V
3
P and V
3
M is negatively fed back in this manner, the DC offset voltage between the differential output terminals of the differential amplification circuit AMP
1
can be canceled. If a DC offset voltage exists, an output VO from the second differential amplification circuit AMP
2
fluctuates causing the pulse width obtained in the comparison circuit CMP to fluctuate. Thus, the pulse width of the inversion electrical pulse signal xRX obtained may become different from the optical pulse signal. By canceling the DC offset voltage, however, the inversion electrical pulse signal xRX having the pulse width tpw which accurately corresponds to the optical pulse signal can be obtained.
To obtain the inversion electrical pulse signal xRX having the accurate pulse width tpw, it is necessary to give the reference voltage VREF an appropriate time constant according to the magnitude of the output VO.
(Related Art
2
)
It is possible to employ a differential amplification circuit provided with a DC offset cancellation circuit in the same manner also in a photo-electric pulse conversion circuit
20
with another configuration (see FIG.
39
).
The photodiode PD receives the optical pulse signal LT that rises at the second timing t
2
and falls at the first timing t
1
to provide the current signal Iin also in this photo-electric pulse conversion circuit
20
. However, the photo-electric pulse conversion circuit
20
uses, instead of the I-V conversion circuit IV, a differentiating I-V conversion circuit DIV to convert a waveform of the current signal Iin to a pair of complementary differential voltage signals VD
1
P, VD
1
M whose waveforms are similar to a differentiated waveform of the current signal Iin. The differential voltage signals VD
1
P, VD
1
M are then amplified by the differential amplification circuit AMP provided with the offset cancellation circuit OFC to output third differential signals VD
3
P, VD
3
M. The third differential signals VD
3
P and VD
3
M are compared with each other by the comparison circuit CMP and the inversion electrical pulse signal xRX is obtained.
In the photo-electric pulse conversion circuit
20
, the differential voltage signals VD
1
P, VD
1
M whose waveforms are similar to a differentiated waveform of the current signal Iin are obtained, and are then amplified. A third differential signal VD
3
P and a third differential signal VD
3
M that sharply fall or rise at the first or the second timing t
1
, t
2
are compared. It is therefore possible to accurately reproduce the pulse width tpw of the optical pulse signal LT in the obtained inversion elec
Arent Fox Kintner & Plotkin & Kahn, PLLC
Fujitsu Limited
Tran Toan
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