Amplifiers – Sum and difference amplifiers
Reexamination Certificate
1999-11-02
2001-07-17
Pascal, Robert (Department: 2817)
Amplifiers
Sum and difference amplifiers
C330S252000
Reexamination Certificate
active
06262628
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a differential amplifier circuit. More particularly, this invention relates to a differential amplifier circuit for amplifying an AC signal when a signal source to be amplified is a signal source in which an AC voltage signal is superimposed on a DC voltage.
BACKGROUND OF THE INVENTION
FIG. 13
is a circuit diagram showing an example of a conventional differential amplifier. In
FIG. 13
, reference numeral
1
denotes a signal source in which an AC voltage signal is superimposed on a DC voltage source, and reference numerals
2
and
3
denote transistors constituting an emitter follower type amplifier, respectively. Reference numerals
4
and
5
denote constant current sources for supplying a current to the respective emitters of the transistors
2
and
3
. Furthermore, reference numeral
6
denotes a resistor for supplying a current to the respective bases of the transistors
2
and
3
, and serves to determine the electric potential of the signal source
1
together with a resistor
7
.
According to such a structure, signals converted to have low impedances are obtained from the respective emitters of the transistors
2
and
3
. These two signals have an AC voltage signal superimposed on a DC voltage which is obtained by level-shifting a DC voltage part of the signal source
1
by a voltage (Vbe) part between the base and emitter of each of the transistors
2
and
3
. The signals obtained from the respective emitters of the transistors
2
and
3
are connected to input terminals
15
and
16
of a differential amplifier
14
through coupling capacitors
8
and
9
. Reference numerals
10
,
11
,
12
and
13
denote resistors for biasing a transistor in the differential amplifier
14
, reference numeral
17
denotes a first voltage source, that is, a positive electrode power terminal in this case, and reference numeral
18
denotes a second voltage source, that is, a negative electrode power terminal in this case.
Next, the action and operation of the conventional art will be described.
Although the signal source
1
has been represented by a basic equivalent circuit in which a signal source resistor, an AC voltage signal source and a DC voltage source are connected in series in order to easily understand the description as shown in
FIG. 13
, a signal is actually taken out by a magnetoresistive head (MRH) or the like in a more complicated equivalent circuit, for example. For this reason, the resistors
6
and
7
are serially connected to the signal source
1
in order to determine the circuit potential of the signal source
1
, the other terminal of the resistor
6
is connected to the positive electrode power terminal
17
and the other terminal of the resistor
7
is connected to the negative electrode power terminal
18
. The resistor
6
also serves to produce a bias current to be supplied to the respective bases of the transistors
2
and
3
.
The collectors of transistors
2
and
3
are connected to the positive electrode power terminal
17
respectively, and emitters are connected to the constant current sources
4
and
5
, thereby supplying an emitter current. Thus, two independent emitter follower type amplifiers are formed. The constant current sources
4
and
5
cause currents having almost the same values to flow. Consequently, voltages between the bases and emitters of the transistors
2
and
3
are almost equal to each other. Thus, a signal in which an AC voltage signal is superimposed on a DC voltage can be obtained as a low impedance signal in each of the emitters.
Furthermore, one of the terminals of each of the coupling capacitors
8
and
9
is connected to the emitter of each of the transistors
2
and
3
, and the other terminal thereof is connected to each of the input terminals
15
and
16
of the differential amplifier
14
in the subsequent stage. The resistors
10
,
11
, and
12
,
13
for supplying bias voltages are connected to the input terminals
15
and
16
of the differential amplifier
14
, respectively. The bias voltages of the input terminals
15
and
16
are set to DC voltages which are almost equal to each other, and act such that the differential amplifier
14
can stably perform an amplifying operation. An output is given as a differential output from the differential amplifier
14
to the terminals
19
and
20
.
FIGS. 14A and 14B
show examples of the differential amplifier
14
.
FIG. 14A
shows the differential amplifier
14
in
FIG. 13
, and
FIG. 14B
shows the structure of the differential amplifier
14
in more detail. In
FIG. 14B
, reference numerals
21
and
22
denote transistors constituting a differential pair, and have respective emitters connected in common and furthermore connected to a constant current source
23
, thereby supplying a bias current. Each of the collectors of the transistors
21
and
22
constituting a differential pair is connected to each of the emitters of the transistors
23
and
24
in order to reduce a time constant of a capacity parasitic to the collector section. The transistors
23
and
24
have bases connected in common and connected to a DC power source
25
. A differential current signal obtained on each of the collectors of the transistors
21
and
22
constituting the differential pair passes from each of the emitters of the transistors
23
and
24
through each of the collectors thereof, and is supplied to load resistors
26
and
27
and is then converted into a differential voltage signal. The differential voltage signal is converted to have a low impedance in two emitter follower type amplifiers constituted by transistors
28
and
29
and constant current sources
30
and
31
, and a differential output is obtained on each of the output terminals
19
and
20
. Reference numeral
32
denotes a positive electrode power terminal, and reference numeral
33
denotes a negative electrode power terminal.
Since the conventional differential amplifier has the above-mentioned structure, AC voltage signals superimposed on two different DC voltages are taken out for the 2-terminal signal source
1
by using the two coupling capacitors
8
and
9
, and are superimposed on two independent DC voltages which are newly produced from two sets of resistance dividing circuits
10
,
11
, and
12
,
13
and are almost equal to each other, thereby generating two voltage signals obtained by superposing the AC voltage signals on the DC voltages which are almost equal to each other, and the coupling capacitors
8
and
9
are connected to the two input terminals
15
and
16
of the differential amplifier
14
in the subsequent stage to perform a differential amplification so that two differential outputs are taken out. Therefore, the two coupling capacitors
8
and
9
have been required.
For example, in the case where a signal having a high frequency of 1 MHz or more is to be amplified, two capacitors are built in a semiconductor chip. Since the capacitor is constituted as a semiconductor integrated circuit, the area of 0.57 mm square is required even if it is assumed that the capacitance is 0.001 uF. Therefore, it can easily be understood that an area of 0.81 mm square is required for two capacitors. The area of 0.81 mm square is enormous on the semiconductor chip so that the semiconductor chip becomes very large. Accordingly, the manufacturing cost of the semiconductor integrated circuit is increased. In reality, it is necessary to make a pattern layout for decreasing a loss angle (tan &dgr;) of the capacitor. Therefore, a chip area which is much greater than 0.81 mm square is disadvantageously required.
SUMMARY OF THE INVENTION
In order to eliminate the above-mentioned drawbacks, it is an object of the present invention to provide a differential amplifier which does not require two coupling capacitors. Thus, a chip area corresponding to two coupling capacitors built in a semiconductor integrated circuit can be reduced, and more particularly, the influence of unnecessary parasitic elements (a capacitance and a series resistanc
Choe Henry
Leydig , Voit & Mayer, Ltd.
Mitsubishi Denki & Kabushiki Kaisha
Pascal Robert
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