Broad-band amplifier circuit and high-speed communication element using the same

A broad-band amplifier circuit is furnished with a plurality of amplifiers with a coupling capacitor being interposed between every two adjacent amplifiers. Each coupling capacitor has a capacity large enough to prevent adverse effect of external noise. Accordingly, not only the passing of a signal having a low frequency is allowed, but also an element operable in a high frequency, such as a Schottky transistor, can be provided in each amplifier as a transistor. At least the transistor of the amplifier in the last stage is connected to a diode, so that the charges accumulated between its collector and base while the amplifier stays on are fed back and eliminated. Accordingly, the broad-band amplifier circuit can carry out a flat amplifying operation over a wide range from low to high frequency bands, and therefore, can be used for all the ASK method, IrDA1.0 method, and IrDA1.1 method. Consequently, not only the manufacturing cost, but also the size of the broad-band amplifier circuit can be reduced.

FIELD OF THE INVENTION 
The present invention relates to a broad-band amplifier circuit serving as 
a preferable infrared communication element used for mutual data 
transmission between computers, or between the computers and a peripheral 
device, such as a printer, or a portable terminal, and to a high-speed 
communication element using the same. 
BACKGROUND OF THE INVENTION 
It has become popular to transmit data through infrared rays among a 
personal computer 1 of FIG. 7(a), another unillustrated personal computer, 
a printer 2 of FIG. 7(b) serving as a peripheral device, a notebook 
personal computer 3 of FIG. 7(c), a portable terminal 4 of FIG. 7(d) 
referred to as a PDA (Personal Digital Assistant), etc. Thus, each of the 
above devices includes a communication element 5 composed of a 
photodetector (photo-receiving/light-emitting) element, a circuit driving 
the photodetector element, etc. The communication element 5 is provided to 
each device in such a manner that its photodetecting surface positions at 
the device's side surface. This arrangement realizes concurrent, multiple 
transmission while omitting time-consuming cable connecting job. 
Today, the IrDA (Infrared Data Association) adopts three standard 
communication methods specified below as a standard method for the 
infrared data communication. 
1 ASK (Amplitude Shift Keying) Method 
In this communication method, the modulation technique adopted by a remote 
controller of some kinds of electronic devices is sped up. To be more 
specific, as shown in FIG. 8(a), a sub-carrier wave is modulated by a 
certain amplitude when the transmission data exhibit "0" and by a 
modulation factor of 0 (zero) when the transmission data exhibit "1" per 
cycle period T. The sub-carrier wave has a frequency of 500 kHz and a data 
transmission rate of 2.4-57.6 kbps. The infrared signals can be 
transmitted over distances of up to 300 cm. 
Using the sub-carrier wave makes this communication method advantageous 
because satisfactory noise immunity can be attained by extracting the band 
of the sub-carrier wave using a bandpass filter at the receiver's end. 
2 IrDA1.0 Method 
This communication method is based on a so-called SIR (Serial Infrared) 
method. As shown in FIG. 8(b), when the data exhibit "0" in a cycle period 
T, a pulse is outputted for a period of 3T/16 from the starting edge of 
the cycle period T. On the other hand, when the data exhibit "1", no pulse 
is outputted as is in the ASK method above. This communication method has 
a higher data transmission rate than the ASK method, for example, 
2.4-115.2 kbps. The infrared signals can be transmitted over distances of 
up to 100 cm. Compared with the ASK method, the pulse is outputted for a 
shorter period per cycle period T. Thus, this communication method is 
advantageous in terms of saving power consumption. 
3 IrDA1.1 (so-called FIR: Fast Infrared) Method 
This communication method is a pulse position modulating method in which 
each cycle period T is divided into four segments and a pulse of T/4 wide 
is outputted in one segment out of these four segments to represent the 
data. More precisely, this communication method includes two techniques: 
one is a technique shown in FIG. 8(c) which has a relatively low data 
transmission rate of up to 1.152 Mbps; the other is a technique shown in 
FIG. 8(d) which has a relatively high data transmission rate of up to 4 
Mbps. In both the techniques, infrared signals can be transmitted over 
distances of up to 100 cm. 
In the technique of FIG. 8(c), a pulse is outputted in the first of the 
four segments when the data exhibit "0", and no pulse is outputted when 
the data exhibit "1". 
In the other technique of FIG. 8(d), in response to 2-bit data in a cycle 
period T, "00", "01", "10", and "11", a pulse is outputted at the first 
through fourth of the four segments in each cycle period, respectively. As 
can be understood from its data transmission rate, this technique is 
advantageous in that it can transmit a great volume of data, and hence, it 
can transmit color image data as well. 
The frequency spectrum of each transmission method is illustrated in FIG. 
9. More specifically, in the ASK method, the frequency band width denoted 
as .alpha.1 covers a range of some hundreds kHz having its center 
sub-carrier frequency at 500 kHz. In the IrDA1.0 method, the frequency 
band width denoted as .alpha.2 covers a range of frequency from a low band 
to some hundreds kHz. In contrast, in the IrDA1.1 method, the frequency 
band width denoted as .alpha.3 covers a wide range of frequency between 60 
kHz and 20 MHz. 
Thus, as shown in FIG. 10, a typical conventional communication element 11 
comprises a transmitting section 12 applicable for all the communication 
methods and two receiving sections 13 and 14. The transmitting section 12 
comprises two light-emitting diodes 15 and 16 cascaded to each other and a 
driving circuit 17. The anode of the light-emitting diode 15 is connected 
to a high-level voltage +Vcc, while the cathode of the light-emitting 
diode 16 is connected to the output of the driving circuit 17. The input 
of the driving circuit 17 receives a driving signal of the light-emitting 
diodes 15 and 16 sent from an unillustrated modulation/demodulation 
section. 
The receiving section 13 is used for both the ASK method and IrDA1.0 
method, and comprises a photodiode 21, an amplifier 22, and a comparator 
23. The photodiode 21 outputs a current corresponding to the 
photo-receiving level. The output current is converted into a voltage by 
the amplifier 22, and the same is also amplified and outputted to the 
comparator 23. The comparator 23 outputs a receiving signal to the 
modulation/demodulation section. The receiving signal shifts to the high 
level when the output from the amplifier 22 is equal to or higher than a 
predetermined level and shifts to the low level, otherwise. 
In contrast, the receiving section 14 is a receiving device used for the 
IrDA1.1 method in a high frequency band. Like the receiving section 13, 
the receiving section 14 comprises a photodiode 24, an amplifier 25, and a 
comparator 26. 
As has been explained, if the conventional communication element 11 is to 
be used for all the communication methods explained in FIGS. 8(a) through 
8(d), the communication element 11 has to include at least two types of 
receiving devices: the receiving section 13 used for the ASK method or 
IrDA1.0 method in a relatively low frequency band; the receiving section 
14 used for the IrDA1.1 method in a relatively high frequency band. This 
arrangement poses a problem that the manufacturing cost is increased and 
the communication element 11 can not be downsized. 
Thus, using the conventional communication element 11 as the communication 
element 5 is disadvantageous in terms of further downsizing, particularly, 
the compact devices, such as the notebook personal computer 3 and portable 
terminal 4. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a downsized, 
inexpensive broad-band amplifier circuit, and a high-speed communication 
element using such a broad-band amplifier circuit. 
The above object is fulfilled by a broad-band amplifier circuit furnished 
with: 
multi-stage amplifiers, each amplifier including a transistor, the 
transistor having a feedback capacitor for allowing passing of high 
frequency components; and 
at least one coupling capacitor, the coupling capacitor being interposed 
between every two adjacent amplifiers, the coupling capacitor having a 
capacity for allowing passing of a signal of tens of kHz or higher. 
According to the above arrangement, the coupling capacitor is provided with 
the capacity allowing the passing of a signal of tens of kHz or higher. 
Thus, the broad-band amplifier circuit of the present invention can 
amplify a signal of a relatively low frequencies, which is generally used 
for data communication. Also, since the coupling capacitor allows the 
passing of a signal in a frequency band of tens of kHz or higher, adverse 
effects of external noise caused by, for example, a fluorescent light, can 
be suppressed. In addition, since the transistor in each amplifier is 
provided with the capacitor allowing the passing of high frequency 
components, the broad-band amplifier circuit of the present invention can 
amplify a signal of high frequencies as well. 
Thus, the broad-band amplifier circuit of the present invention can amplify 
a signal over a wide range of frequency from a low frequency band to a 
high frequency band with a flat frequency response. Consequently, the 
broad-band amplifier circuit of the present invention can be used for all 
the known communication methods, and can be reduced in costs and size at 
the same time. 
It is preferable that the above broad-band amplifier circuit is arranged in 
such a manner that: 
the capacity of the coupling capacitor allows passing of a signal of 60 kHz 
or higher; and 
the feedback capacity of the transistor allows passing of a signal of 20 
MHz or lower. 
Arranged in this manner, the broad-band amplifier circuit of the present 
invention can be used for the infrared communication methods including the 
ASK method, IrDA1.0 method, and IrDA1.1 method. Therefore, it has become 
possible to provide a practical broad-band amplifier circuit which is 
suitably used for a typical infrared communication method. 
It is further preferable that the above broad-band amplifier circuit is 
arranged to be further furnished with a diode for eliminating charges, 
accumulated between a collector and a base of the transistor while the 
transistor stays on, by feeding back the accumulated charges, the diode 
being provided at least to the amplifier in a last stage. 
According to the above arrangement, the diode is provided at least to the 
amplifier in the last stage, so that the charges accumulated between its 
collector and base while the transistor stays on are fed back when the 
transistor goes off. In short, the accumulated charges are consumed by the 
diode and eliminated. This enables the transistor to operate at high 
speeds, thereby realizing a highly reliable broad-band amplifier circuit. 
The above object is also fulfilled by a high-speed communication element 
furnished with: 
a receiving section for receiving a sending signal; and 
a broad-band amplifier circuit for amplifying a receiving signal outputted 
from the receiving section, 
wherein, 
the broad-band amplifier circuit includes: 
multi-stage amplifiers, each of the amplifier including a transistor, the 
transistor having a feedback capacity for allowing passing of high 
frequency components; and 
at least one coupling capacitor having a capacity for allowing passing of a 
signal of at least tens of kHz, the coupling capacitor being interposed 
between every two adjacent amplifiers. 
According to the above arrangement, the broad-band amplifier circuit of the 
present invention can also amplify a signal over a wide range of frequency 
from a low frequency band to a high frequency band with a flat frequency 
response. Thus, it has become possible to provide an inexpensive, compact, 
high-speed communication element which can be used for all the known 
communication methods. 
It is preferable that the above high-speed communication element is 
arranged in such a manner that: 
the capacity of the coupling capacitor in the broad-band amplifier circuit 
allows passing of a signal of 60 kHz or higher; and 
the feedback capacity of the transistor in the broad-band amplifier circuit 
allows passing of a signal of 20 MHz or lower. 
Arranged in this manner, the high-speed communication element of the 
present invention can be used for the infrared communication methods 
including the ASK method, IrDA1.0 method, and IrDA1.1 method. Therefore, 
it has become possible to provide a practical high-speed communication 
element which is suitably used for a typical infrared communication 
method. 
It is further preferable to arrange the above high-speed communication 
element to be further furnished with: 
a light-emitting element for emitting the above infrared rays; and 
a driving circuit for driving the light-emitting element, 
wherein, 
the driving circuit and broad-band amplifier circuit are integrated into a 
single chip to form an integrated circuit, the integrated circuit being 
mounted on a flexible print substrate in the form of a bare chip together 
with the light-emitting element and photo-receiving element. 
According to the above arrangement, the integrated circuit is mounted on 
the flexible print substrate through bonding or the like together with the 
ligh-temitting element and photo-receiving element. Components, such as 
the integrated circuit, can be readily attached to the flexible print 
substrate, and the integrated circuit in the form of a bare chip can be 
downsized or installed in a device through an automatic assembly line. 
Thus, the high-speed communication element of the present invention can be 
readily mounted on a device at its marginal space by a minimum 
time-consuming manual job, such as soldering. 
It is furthermore preferable to arrange the above high-speed communication 
element to be further furnished with a metal casing for covering the 
integrated circuit, the metal casing having an opening in a position 
opposing both a light-emitting surface of the light-emitting element and a 
photo-receiving surface of the photo-receiving element. 
According to the above arrangement, the integrated circuit is covered with 
the metal casing. Thus, even when the high-speed transmission element of 
the present invention is installed in the hardware of a personal computer 
or peripheral devices, the high-speed transmission element is, in a 
reliable manner, shielded from noises caused by an output clock of the 
processor, or switching noise caused by a switching element of a power 
source circuit. In addition, the high-speed transmission element is also 
shielded from external noise caused by, for example, a fluorescent light. 
Consequently, since the high-speed transmission element of the present 
invention does not allow the entry of any noise, a signal can be 
transmitted (communicated) over a longer distance. 
For a fuller understanding of the nature and advantages of the invention, 
reference should be made to the ensuing detailed description taken in 
conjunction with the accompanying drawings.

DESCRIPTION OF THE EMBODIMENTS 
Referring to FIGS. 1 through 6, the following description will describe an 
example embodiment of the present invention. 
FIG. 1 is an electric circuit diagram showing a communication element 32 
using a broad-band amplifier circuit 31 in accordance with the present 
embodiment. 
The communication element 32 mainly comprises a pair of light-emitting 
diodes D4 and D5 which are in effect light-emitting elements connected in 
series, a driving circuit 33 for driving the light-emitting diodes D4 and 
D5, a photodiode D1 serving as a photo-receiving element, the broad-band 
amplifier circuit 31, and a comparing circuit 34. 
The driving circuit 33 comprises a transistor Q0, three resistors R20, R23, 
and R24, and a capacitor C10. An input high-level voltage +Vcc from a 
power source input terminal 35 is smoothed by a smoothing capacitor C12, 
and further supplied to the anode of the light-emitting diode D4 through 
the current limiting resistor R20. The cathode of the light-emitting diode 
D4 is connected to the anode of the other light-emitting diode D5. The 
cathode of the light-emitting diode D5 is connected to the collector of 
the transistor Q0. On the other hand, a sending signal entering from an 
input terminal 36 is divided by the resistors R23 and R24 and inputted 
into the base of the transistor Q0. The emitter of the transistor Q0 is 
grounded, and the capacitor C10 for bypassing a high-frequency is provided 
in parallel to the resistor 23. 
Arranged in this manner, the light-emitting diodes D4 and D5 respectively 
output an infrared pulse having an amplitude determined by a current 
limited by the current limiting resistor R20 and corresponding to the 
above sending signal. 
The cathode of the photodiode D1 is impressed with the voltage +Vcc through 
resistors R27 and R6 and a smoothing capacitor C2. The output current from 
the photodiode D1 is converted into a voltage by a current-to-voltage 
converting circuit 37 serving as a pre-amplifier, and inputted into the 
comparing circuit 34 through amplifiers 38 and 39, which altogether serve 
as a 2-stage power amplifier. The comparator circuit 34 compares the 
output voltage from the amplifier 39 with a predetermined threshold level 
to shape a rectangular pulse, which is outputted as a receiving signal 
from an output terminal 40. 
The current-to-voltage converting circuit 37 comprises a pair of 
transistors Q1 and Q2, three resistors R1, R2, and R3, a coupling 
capacitor C1, a capacitor C4, and a diode D2. An output current from the 
anode of the photodiode D1 is supplied to the base of the transistor Q1 
and the same is also inputted into the collector of the transistor Q2. The 
output voltage from the smoothing capacitor C2 is impressed on the 
collector of the transistor Q1 through the resistor R1. The emitter of the 
transistor Q1 is grounded through a parallel circuit composed of the 
resistor R3 and capacitor C4, and the same is also connected to the base 
of the transistor Q2. The emitter of the transistor Q2 is also grounded. 
The bias resistor R2 and diode D2 are provided in parallel to each other 
between the base and collector of the transistor Q1, and detailed 
explanation of the diode D2 will be given below. 
The transistor Q1 of the current-to-voltage converting circuit 37 starts to 
conduct when a current, produced by the photodiode D1 upon receipt of an 
infrared pulse, flows into its base. At the same time, the transistor Q2 
also starts to conduct by a terminal voltage across the capacitor C4 and 
resistor R3 produced by an emitter current from the transistor Q1. The 
output current from the photodiode D1 flows into the collector of the 
transistor Q2, by which the voltage is produced across the resistor R1 and 
outputted to the amplifier 38 in the next stage through the coupling 
capacitor C1. 
The amplifier 38 comprises a transistor Q3, a resistor R7, and two gain 
setting resistors R4 and R5. The output voltage from the 
current-to-voltage converting circuit 37, which enters through the 
coupling capacitor C1, is supplied to the base of the transistor Q3 
through the resistor R4. The base and collector of the transistor Q3 are 
connected to each other through the resistor R5, and an output voltage 
from the capacitor C2 is impressed on the collector of the transistor Q3 
through the resistor R7. Further, the emitter of the transistor Q3 is 
grounded. 
For example, the resistance of the resistor R4 is 1 k.OMEGA., and the 
resistance of the resistor R5 is 10 k.OMEGA.. Thus, it is understood that 
the output voltage from the current-to-voltage converting circuit 37 is 
amplified by approximately 10 times by the amplifier 38 before it is 
outputted. The output from the amplifier 38 is outputted to the amplifier 
39 in the next stage through a coupling capacitor C3. 
The amplifier 39 comprises a transistor Q4, four resistors R25, R26, R11, 
R13, and a photodiode D6 which will be explained below, and the first 
through fourth components correspond to the transistor Q3, and resistors 
R4, R5, and R7 of the amplifier 38, respectively. The output from the 
amplifier 38 is inputted into the base of the transistor Q4 through the 
resistor R25. The resistor R26 and diode D6 are provided in parallel to 
each other between the base and collector of the transistor Q4. The output 
voltage from the capacitor C12 is impressed on the collector of the 
transistor Q4 through the resistors R27, R11, and R13. The output of the 
amplifier 39 is outputted from the collector of the transistor Q4, and 
supplied to the comparing circuit 34 through a coupling capacitor C7. 
For example, the resistance of the resistor R25 is 1 k.OMEGA. and the 
resistance of the resistor 26 is 22 k.OMEGA.. Thus, it is understood that 
the output from the amplifier 38 is amplified by approximately 22 times by 
the amplifier 39 before it is outputted. 
The comparing circuit 34 comprises a comparator 41, a capacitor C9 for 
smoothing a power source voltage, three dividing resistors R16, R17, and 
R18 for producing a reference voltage, an input resistor R15, two 
capacitors C5 and C8, and a diode D3. An output from the amplifier 39 is 
inputted into an inverting input terminal of the comparator 41, and the 
same is also inputted into the other non-inverting input terminal through 
the input resistor R15 and dividing resistors R17 and R18. Also, the 
terminal voltage of the capacitor C12 inputted through the resistors R27 
and R13 is divided by the resistors R16, R17, and R18 and inputted into 
the non-inverting input terminal of the comparator 41. Note that the 
capacitor C5 provided to stabilize the impressed voltage is connected in 
parallel to the series circuit composed of the resistors R16, R17, and 
R18. The capacitor C8 provided in parallel to the resistor R18 averages an 
output from the amplifier 39 to stabilize the reference voltage. 
The comparator 41 outputs a high-level signal when the input level of the 
inverting input terminal becomes equal to or greater than the reference 
voltage inputted into the other non-inverting input terminal, and outputs 
a low-level signal otherwise. The output from the comparator 41 is 
outputted to the output terminal 40 as a receiving signal. 
The communication element 32 arranged as above is used in the infrared data 
communication, and can be used for all the ASK method, IrDA1.0 method, and 
IrDA1.1 method. 
Thus, a capacity of the coupling capacitor C1 can be computed using a 
relationship established between a gain A of the amplifier 38 serving as a 
high-pass filter and the capacity of the coupling capacitor C1. To be more 
specific, let the base diffused resistor of the transistor Q3 be rb (about 
50 .OMEGA.), and the emitter's internal resistance be re (about 
25.7/Ie.OMEGA.! at room temperature, where Ie is an emitter current), 
then the gain A of the amplifier 38 can be computed by the following 
Equation: 
EQU A(f).apprxeq.{(R5+R7).parallel.hie}/{(1/2.pi.fC1)+R4+(R5+R7).parallel.hie}( 
1) 
where hie is an input impedance of the transistor Q3 and can be expressed 
as: 
EQU hie=rb+(1+hfe).times.re (2) 
where hfe is a current amplification factor of the transistor Q3. Also, 
(R5+R7).parallel.hie can be re-written as: 
EQU (R5+R7).parallel.hie=1/{(1/hie)+1/(R5+R7)}. (3) 
Herein, a low-band cut-off frequency fL is set to 60 kHz, so that the 
communication element 32 can be used for both the IrDA1.0 method and 
IrDA1.1 method and adverse effect of external noise caused by, for 
example, a fluorescent light, can be suppressed. Also, a capacity as large 
as 1.8 nF is given to the coupling capacitor C1 from Equation (1) above, 
thereby allowing the passing of components of having a frequency of 60 kHz 
or higher. 
In addition, a large capacity is given to both the coupling capacitor C7 
between the amplifier 39 and comparing circuit 34, and the coupling 
capacitor C3 between the amplifiers 38 and 39, so that the communication 
element 32 can be used for both the IrDA1.0 method and IrDA1.1 method 
while allowing the passing of the components having a frequency of 60 kHz 
or higher. For example, a capacity of a few .mu.F is given to the former, 
and a capacity of 0.1 .mu.F is given to the latter. 
A Schottky transistor of FIG. 2(a) with a small feedback capacity Cob is 
used as the transistors Q1, Q2, Q3, and Q4 (which are referred to as 
transistor Q when the description applies to each) to accelerate the 
overall operation speed. As shown in FIG. 2(b), the transistor Q is 
assumed to comprise an amplifier circuit having a gain A, and a feedback 
circuit Cob having a gain G. Thus, when A&gt;&gt;1, then, A.apprxeq.1/G, 
therefore, 
EQU A=hie/{1/2.pi.fCob)+hie}. (4) 
Equation (4) reveals that as the frequency f increases, the gain A 
approximates to 1, thereby enabling the transistors Q to operate as a 
low-pass filter. 
For the above reason, reducing the feedback capacity Cob smaller allows the 
passing of the high frequency components. For example, in the present 
embodiment, Cob=0.8 pF, to have the high-band cut-off frequency fH of 20 
MHz, so that the communication element 32 can be used for the IrDA1.1 
method. 
Further, in the present embodiment, the collector and base of at least the 
transistor Q4 in the last stage is connected through the diode D6. The 
transistor, including both a typical one and the transistor Q4, 
accumulates charges denoted by a small letter q between the collector and 
base while it stays on as is illustrated in FIG. 3. Thus, in response to a 
pulse of FIG. 4(a) supplied to the base, the base potential of the 
transistor varies moderately as shown in FIG. 4(b). For this reason, 
although the output from the collector should rise abruptly in response to 
the falling edge of the above pulse as denoted by .gamma.1 in FIG. 4(c) in 
theory, it rises moderately as denoted by .gamma.2 in practice. Thus, the 
output pulse does not respond sharply to an abrupt change in the input 
pulse, which makes a high-speed operation impossible. 
To eliminate the above problem, the charges q accumulated while the 
transistor stays on are fed back to the diode D6 when the transistor goes 
off as illustrated by a small letter i in FIG. 3 so as to be consumed by 
the diode D6 and eliminated from the transistor. This arrangement enables 
the transistor Q4 to operate at high speeds. Also, in the circuit of FIG. 
1, the diode D2 is provided to accompany with the transistor Q1 for the 
same purpose. Providing the diode of this kind to the amplifier (amplifier 
39 in FIG. 1) in the last stage alone is sufficient to attain satisfactory 
effects. The diodes may be provided to the other amplifiers if the costs, 
occupied space, etc. allow. 
FIG. 5 is a block diagram showing an electrical arrangement of a 
transmission device 51 using the above-arranged communication element 32. 
In the drawing, like components are labeled with like reference numerals 
with respect to FIG. 1, and the description of these components is not 
repeated for the explanation's convenience. 
As has been explained, the communication element 32 can be used for the ASK 
method, IrDA1.0 method, and IrDA1.1 method of FIGS. 8(a) through 8(d) by 
adopting the broad-band amplifier circuit 31. Thus, if the communication 
element 32 is installed in the personal computer 1 of FIG. 7(a), 
modulation/demodulation circuits 53-55 corresponding to the respective 
three communication methods are also installed in a data converting 
circuit 52 for converting and restoring the data transmitted between the 
personal computer 1 and communication element 32. The data converting 
circuit 52 is, in practice, a so-called ASIC (Application Specific 
Integrated Circuit) or the like. 
Transmission data from the personal computer 1, outputted from a 
transferring register or the like, are inputted into an interface circuit 
58 through buses 56 and 57, and subject to parallel-to-serial conversion. 
Then, the converted data are inputted into the modulation/demodulation 
circuits 53 and 54 of the data converting circuit 52. The transmission 
data are also inputted into a register 60 of the data converting circuit 
52 through the buses 56 and 59. A control circuit 61 of the data 
converting circuit 52 selectively energizes one of the 
modulation/demodulation circuits 53-55 in response to a selection output 
informing the adopted communication method, which is inputted from the 
personal computer 1 through the buses 56 and 59. 
Accordingly, when one of the ASK method and IrDA1.0 method is selected, the 
serial data inputted from the interface circuit 58 are modulated by the 
modulation/demodulation circuit 53 or 54 as shown in FIGS. 8(a) and 8(b), 
respectively. The communication element 32 drives the light-emitting 
diodes D4 and D5 by means of the driving circuit 33 based on the pulse 
thus modulated. On the other hand, if the IrDA1.1 method is selected, the 
transmission data transmitted through the buses 56 and 59 are stored into 
the register 60 first, and thence retrieved per cycle period T and 
modulated by the modulation/demodulation circuit 55 as shown in FIGS. 8(c) 
or 8(d). The pulse thus modulated is outputted to the driving circuit 33. 
In contrast, the signal entering into the photodiode D1 is amplified by the 
broad-band amplifier circuit 31, discriminated in level by the comparing 
circuit 34, and supplied to each of the modulation/demodulation circuit 
53-55. The control circuit 61 judges the adopted communication method 
based on the receiving signal, and selectively energizes one of the 
modulation/demodulation circuit 53-55 for the judged communication method 
to demodulate and output the received data. The output from the 
modulation/demodulation circuit 53 or 54 is subject to serial-to-parallel 
conversion by the interface circuit 58 first, and thence sent to the 
personal computer 1. 
FIG. 6 is a perspective view explaining the structure of the communication 
element 32. The broad-band amplifier circuit 31, driving circuit 33 and 
comparing circuit 34 are integrated into an integrated circuit 65 in the 
form of a bare chip. The integrated circuit 65 is mounted on a mounting 
section 66a of a flexible print substrate 66 through bonding to an 
unillustrated pattern. The light-emitting diodes D4, D5 and photodiode D1 
are attached and soldered to the mounting section 66a through the reflow 
soldering after the integrated circuit 65 is bonded thereon. 
The flexible print substrate 66 comprises the mounting section 66a, a 
linking section 66b, and a connecting section 66c. The connecting section 
66c includes a number of connector terminals, which are electrically 
connected to corresponding terminals when inserted into a socket of the 
personal computer 1 or the like. The linking section 66b links the 
mounting section 66a and connecting section 66c, and although it is not 
illustrated, a number of wiring patterns are printed thereon. In the 
present embodiment, the linking section 66b has a narrow width to occupy 
less space when mounted. 
Both the main and back surfaces of the mounting section 66a are covered 
with a metal casing 67 made of a pair of covering sections 67a and 67b 
assembled in a vertical direction. The covering section 67a facing the 
main surface of the mounting section 66a, on which the integrated circuit 
65 and the like are mounted, has an opening 68 at a position opposing the 
light-emitting diodes D4 and D5 and photodiode D1. The coating sections 
67a and 67b are soldered at the peripheral edges while enclosing the 
mounting section 66a. Providing the mounting section 66a inside the casing 
67 having the opening 68 as above makes it possible to shield the circuits 
31, 33, and 34 from the external noise while allowing the light-emitting 
diodes D4 and D5 to emit light and the photodiode D1 to receive light. 
Consequently, it has become possible to prevent the entry of noise into the 
communication element 32 having the broad-band amplifier circuit 31 with a 
high gain, thereby extending the communication distance. 
As has been explained, the communication element 32 of the present 
invention uses the broad-band amplifier circuit 31 having a flat frequency 
response over a wide frequency band ranging from 60 kHz to 20 MHz. Thus, 
the communication element 32 can be used for all the ASK method, IrDA1.0 
method, and IrDA1.1 method of FIGS. 8(a) through 8(d). As a result, not 
only the costs, but also the occupied space of the transmission element 32 
can be saved. The broad-band amplifier circuit 31 is particularly suitable 
for downsized devices, such as the notebook personal computer 3 and 
portable terminal 4 of FIGS. 7(c) and 7(d), respectively. 
In the communication element 32, the broad-band amplifier circuit 31, 
driving circuit 33, and comparing circuit 34, which are peripheral 
circuits for the light-emitting diodes D4 and D5 and photodiode D1, are 
integrated into the single-chip integrated circuit 65. Further, the 
integrated circuit 65 is mounted, on the flexible print substrate 66 in 
the form of a bare chip. This arrangement can reduce the time-consuming 
manual job, such as soldering, and makes it easy to install the 
communication element 32 in the notebook personal computer 3 or portable 
terminal 4 at the marginal space. 
In addition, since the mounting section 66a is covered with the casing 67, 
each circuit contained in the integrated circuit 65 can operate in a 
stable manner despite of noise caused by the clock in the processing 
circuits or switching noise of the switching element of the power source 
circuit of the notebook personal computer 3 or portable terminal 4, and 
external noise caused by, for example, fluorescent light. 
Note that the use of the broad-band amplifier circuit 31 of the present 
invention is not limited to the infrared communication, and the broad-band 
amplifier circuit 31 can be suitably used for other purposes. In addition, 
the infrared communication may use wires made of photoconductive material, 
such as optical fibers. 
The invention being thus described, it will be obvious that the same may be 
varied in many ways. Such variations are not to be regarded as a departure 
from the spirit and scope of the invention, and all such modifications as 
would be obvious to one skilled in the art are intended to be included 
within the scope of the following claims.