Amplifier circuit

An amplifier circuit for amplifying an input signal and outputting the amplified signal includes a first amplifier for outputting from a first output terminal a current proportional to a difference voltage between voltages at a first input terminal and a second input terminal, and a second amplifier for outputting from a second output terminal a current proportional to a difference voltage between voltages at the first input terminal and the second input terminal and for feeding back a current outputted from the second output terminal to the second input terminal, so that the amplifier circuit is operable at a supply voltage of 3 V.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an amplifier circuit for outputting an 
input signal at a specific output impedance, and in particular, to an 
amplifier circuit that is built in a video camera and the like and is used 
for amplifying a video signal and the like. 
2. Description of Related Art 
Conventionally, since an output impedance is specified at 75 ohms in an 
amplifier circuit for outputting a video signal, the amplifier circuit 
outputs a signal through a resistor of 75 ohms by driving the signal in a 
low-impedance output amplifier circuit, the signal being twice as large as 
a signal at a 75-ohm terminating resistor. 
Nevertheless, it is necessary for the conventional amplifier circuit to 
output a signal of 2 Vpp in the low-impedance output amplifier circuit so 
as to output a video signal having an amplitude of 1 Vpp. Therefore, a 
supply voltage of at least 4 V is required if the low-impedance output 
amplifier circuit is to be constructed of a push-pull circuit with bipolar 
transistors. 
In addition, if the low-impedance output amplifier circuit is constructed 
in a collector-output type, it is possible to lower the supply voltage to 
a degree. Nevertheless, it is difficult to operate the low-impedance 
output amplifier circuit at the supply voltage of 3 V. 
Recently, supply voltages have been reduced so as to get longer battery 
lives in portable apparatuses, with the operation of 3 volt power supplies 
also being requested in video cameras. Nevertheless, it is not possible 
for the conventional art to correspond to these requirements. 
BRIEF SUMMARY OF THE INVENTION 
An object of the present invention is to provide an amplifier circuit which 
is operable at a supply voltage of 3 V. 
To attain the above objects, in accordance with an aspect of the present 
invention, there is provided an amplifier circuit for amplifying an input 
signal and outputting the amplified signal, which comprises a first 
amplifier for outputting from a first output terminal a current 
proportional to a difference voltage between voltages at a first input 
terminal and a second input terminal, and a second amplifier for 
outputting from a second output terminal a current proportional to a 
difference voltage between voltages at the first input terminal and the 
second input terminal and for feeding back a current outputted from the 
second output terminal to the second input terminal.

DETAILED DESCRIPTION OF THE INVENTION 
Hereinafter, a preferred embodiment of the present invention will be 
described in detail with reference to the drawings. 
FIG. 1 is a diagram showing the fundamental construction of an amplifier 
circuit according to the embodiment. In FIG. 1, reference numeral 1 
denotes a first input terminal, reference numeral 2 denotes an second 
input terminal, reference numeral 3 denotes a first output terminal, 
reference numeral 4 denotes a second output terminal, reference numeral 5 
denotes a first Gm amplifier for outputting a current to the first output 
terminal 3, and reference numeral 6 denotes a second Gm amplifier for 
outputting a current to the second output terminal 4. 
FIG. 2 is a diagram showing the structure for measuring the input-output 
gain of the amplified circuit used in this embodiment, in which the signal 
source 7 is connected to the first input terminal 1. 
In FIG. 2, reference numeral 7 denotes an input signal source, reference 
numeral 8 denotes a terminating resistor, and reference numerals 9 and 10 
denote feedback resistors. FIG. 2 also shows conductances (Gm's) of a 
first Gm amplifier 5 and a second Gm amplifier 6, Gm1 and Gm2, 
respectively, and resistances of a terminating resistor 8 and feedback 
resistors 9 and 10, R, R1 and R2, respectively. In addition, an input 
signal is vin, and voltages of first output terminal 3 and second output 
terminal 4 are v1 and v2, respectively. The following formulas (1) and (2) 
can be obtained according to Kirchhoff's first law at the first output 
terminal 3 and the second output terminal 4: 
EQU Gm1(vin-v2)-v1/R-(v1-v2)/R1=0 (1) 
EQU Gm2(vin-v2)-v2/R+2(v1-v2)/R1=0 (2) 
By expressing the voltage v1 with the input signal vin from the formulas 
(1) and (2), the following formula (3) can be obtained: 
EQU v1=R(R1Gm1+R2Gm1+R2Gm2)/(R(1+R2Gm1+R2Gm2)+R1+R2+R1R2Gm2).multidot.vin(3) 
By means of this construction, the input-output gain is determined by 
deciding the respective values of the resistances R, R1, and R2 and the 
conductances Gm1 and Gm2. 
FIG. 3 is a diagram showing the structure for measuring the output 
impedance of the amplified circuit used in this embodiment, in which the 
signal source 7 is connected to the first output terminal 3. The following 
formulas (4) and (5) are obtained when the Kirchhoff's first law is 
applied to the first output terminal 3 and the second output terminal 4, 
similar to the above: 
EQU (v1-vin)/R+Gm1v2+(v1-v2)/R1=0 (4) 
EQU v2/R2+Gm2v2-(v1-v2)/R1=0 (5) 
By expressing the voltage v1 with the input signal vin from the formulas 
(4) and (5), the following formula (6) can be obtained: 
EQU v1=(R1+R2+R1R2Gm2)/(R1+R2+R1R2Gm2+R(1+R2Gm1+R2Gm2)).multidot.vin(6) 
Letting an output impedance at the time of viewing the circuit side from 
the first output terminal 3 be Z, the formula (6) can be expressed as the 
following formula (7), and further the output impedance Z is expressed by 
the following formula (8): 
EQU v1=(Z/(Z+R)).multidot.vin (7) 
EQU Z=(R1+R2+R1R2Gm2)/(1+R2(Gm1+Gm2)) (8) 
As described above, by using accurate external resistors as the resistances 
R1, R2 and R and improving the accuracy of the conductances Gm1 and Gm2, 
it becomes possible to realize the amplifier circuit having a constant 
output impedance and a constant input-output gain at the time of the 
terminating resistor being resistance R. 
Therefore, by adjusting an output level at the time of termination while 
performing the feedback so that the output impedance becomes 75.OMEGA., it 
becomes possible to construct the amplifier circuit which is operable at a 
supply voltage of 3 V. 
Next, sag correction in an amplifier circuit will be described. Since a 
video signal is outputted in alternating current and is terminated in the 
receiving side, the video signal is usually outputted through capacitive 
coupling (a capacitor). FIG. 4 is a diagram showing the structure of an 
amplifier circuit for outputting a signal through the capacitive coupling. 
Since a frequency characteristic decreases in the lower frequency domain of 
a video signal due to a time constant of RC in case of termination through 
the capacitive coupling 11, a sag arises in the video signal, resulting in 
uneven luminance on a picture. 
In order to prevent this, it is necessary to correct the reduction of the 
frequency characteristic at the output terminal by performing feedback of 
the video signal after the capacitive coupling to the second input 
terminal. 
FIG. 5 is a diagram showing the construction of the amplifier circuit 
having a feedback circuit for performing the feedback of the video signal 
after the capacitive coupling to the second input terminal. FIG. 6, 
similarly to FIG. 5, is a diagram showing the construction of the 
amplifier circuit having a feedback circuit for performing the feedback of 
the video signal after the capacitive coupling to the second input 
terminal. The feedback circuit shown in FIG. 5 is stable when there is not 
a terminating resistor, and the feedback circuit shown in FIG. 6 has a 
good frequency characteristic. FIG. 7 is a timing chart showing signal 
waveforms of the respective portions in the amplifier circuit shown in 
FIG. 5 or FIG. 6. 
Incidentally, although a resistor and a capacitor are connected in series 
between the terminal of the output side of the capacitive coupling 11 and 
the second output terminal in FIG. 5, only the capacitor can be also 
connected. Although a resistor and a capacitor are connected in series 
between the connecting point of two resistors, connected in series between 
the terminal of the output side of the capacitive coupling 11 and the 
ground, and the second output terminal in FIG. 6, only the capacitor can 
be also connected. 
FIG. 8 is a diagram showing the concrete construction of the amplifier 
circuit shown in FIG. 1. The amplifier circuit has a bandgap circuit 12. A 
bandgap voltage generated in the bandgap circuit 12 is inputted to the 
base of an NPN transistor (Tr) 13, and an external resistor 14 is 
connected between the emitter of the NPN transistor 13 and the ground 
(GND). 
Here, the details of the bandgap circuit 12 will be described. FIG. 9 is a 
diagram showing the principle of operation of the bandgap circuit 12. A 
bandgap circuit is a circuit for outputting a bandgap voltage (about 1.2 
V) for silicon, and has a characteristic that is not fluctuated by 
temperature. Concretely, a resistor Ra is connected to the emitter of a 
transistor having N times the size between two transistors having 
different sizes (1:N), and the same base potential is given to the two 
transistors so that the same collector currents may flow. The value of the 
collector current i flowing at this time is shown in the following formula 
(9), where T represents absolute temperature, k represents the Boltzman's 
constant, and q represents an electric charge. 
EQU i=kT.multidot.ln N/(Ra.multidot.q) (9) 
Here, by making the collector current i flow through a resistor Rb and 
adding a voltage between both ends of the resistor Rb to a base-emitter 
voltage Vbe, a voltage Vbandgap that is not fluctuated by temperature can 
be obtained as shown in the following formula (10): 
EQU Vbandgap=Vbe+2.multidot.(Rb/Ra).multidot.(k.multidot.ln N/q).multidot.T(10) 
According to the formula (10), it can be seen that a voltage obtained by 
subtracting the base-emitter voltage Vbe from the bandgap voltage Vbandgap 
is proportional to the absolute temperature T. By performing current 
transformation of this voltage with the resistance R, the obtained current 
is proportional to the absolute temperature T. 
In addition, conductance gm at the time of making a current I flow into a 
differential input circuit is shown in the following formula (11): 
EQU gm=q.multidot.I/(k.multidot.T) (11) 
Therefore, by making a current proportional to the absolute temperature T 
flow into the differential input circuit, it becomes possible to generate 
the conductance gm not depending on temperature. 
In this manner, the collector current of the NPN transistor (Tr) 13 shown 
in FIG. 8 becomes proportional to the absolute temperature. Then, by 
making currents, mirrored from the collector current of the NPN transistor 
13, flow into emitter-coupled differential input circuits 15 and 16, the 
emitter-coupled differential input circuits 15 and 16 can have 
conductances (Gm's) of a characteristic not depending on temperature. 
Moreover, output currents from the emitter-coupled differential input 
circuits 15 and 16 are mirrored by transistors 17 and 18, and are 
outputted to the second output terminal 4. Furthermore, the currents are 
mirrored by transistors 19 and 20, and are outputted to the first output 
terminal 3. 
In an amplifier circuit capable of correcting the sag, the less the 
capacitor in the feedback circuit is, the better it is from the viewpoint 
of part cost and size, so that the larger the resistor used in the 
feedback circuit is, the better it is. On the other hand, it becomes 
possible to make the output impedance obtained from FIG. 3 small by 
putting conductances in Gm1&gt;&gt;Gm2, and, at this time, the output impedance 
becomes Z.apprxeq.R1 (Gm2/Gm1). 
For example, in a case where the resistance R1 is several-ten k.OMEGA., it 
is good to set the ratio of the conductances Gm1 and Gm2 at about 100 so 
that the output impedance Z may become 75.OMEGA.. On the other hand, in 
consideration of mirror accuracy and consumption current, it is adequate 
to set the ratio of the conductances Gm1 and Gm2 at from several-ten to 
several-hundred. In this case, the current outputted from the first output 
terminal is several-ten to several-hundred times larger than the current 
outputted from the second output terminal. 
In this manner, in the amplifier circuit according to the embodiment, since 
it is possible to adjust the output level at the time of termination by 
applying the feedback so that the output impedance may become 75.OMEGA., 
the amplifier circuit can operate at a supply voltage of 3 V if the signal 
amplitude of the first output terminal is 1 Vpp. 
According to the amplifier circuit of the present invention, since it is 
possible to adjust the output level at the time of termination by applying 
the feedback so that the output impedance may become 75.OMEGA., the 
amplifier circuit can operate at a supply voltage of 3 V if the signal 
amplitude of the first output terminal is 1 Vpp, which is different from 
the output amplitude of 2 Vpp from a conventional video output amplifier 
circuit. Owing to this, it is possible to provide an amplifier circuit for 
video output that is operable at a supply voltage of 3 V. In this manner, 
since it is possible to use a supply voltage of 3 V in a portable 
apparatus such as a video camera, it is possible to achieve cost reduction 
by simplification of the construction of a power supply and performance 
improvement by lengthening continuous duty time of a battery. 
In addition, it is possible to provide an amplifier circuit having 
excellent temperature stability since it is possible to construct the 
amplifier circuit so that its conductances do not change with temperature. 
Furthermore, it is possible to make an output impedance and an input-output 
gain constant. 
Moreover, it is possible to prevent a sag from arising in a video signal 
and to make stability good at the time of a terminating resistor not being 
there since it is possible to correct the reduction of a frequency 
characteristic.