Amplifiers – With semiconductor amplifying device – Including differential amplifier
Reexamination Certificate
2001-08-01
2002-10-08
Nguyen, Patricia (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including differential amplifier
C330S258000, C330S259000
Reexamination Certificate
active
06462618
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an amplifying circuit.
The invention is particularly concerned with a circuit usable for a measuring instrument such as oscilloscopes.
In spite of large amplitude inputs applied or resistance irregularities of resistors employed therein, the amplifying circuit can deliver distortionless outputs.
Regardless of fluctuations of the environmental temperature or changes of voltages or currents of power supplies, the amplifying circuit can output distortionless waveforms.
2. Description of the Prior Art
In an oscilloscope, waveforms to be observed are large frequency bandwidth signals from DC to high frequency. In the current oscilloscope, it is required to observe signals of several hundred MHz to several GHz or high repetition rate pulses.
An amplifying circuit employed in such an oscilloscope is usually included in an integrated circuit. The integrated circuit satisfies requirements to observe of large bandwidth and high repetition rate signals.
The amplifying circuit being in the integrated circuit includes, generally, differential amplifiers. In order to amplify large bandwidth and high repetition rate signals, npn transistors having excellent high frequency characteristics are employed in the differential amplifier. Many stages of the differential amplifiers including npn transistors are connected in series so as to constitute the amplifying unit. In the amplifying unit, there is a problem that the output potential is shifted up to positive in potential by the connection in series.
Shown in
FIG. 1
is a circuit diagram of a prior art amplifying unit. The unit is constituted of two stages of differential amplifiers cascadedly connected. In
FIG. 1
, elements
101
and
102
are npn transistors to form a common emitter differential amplifier
100
. Elements
201
and
202
are npn transistors to form a common emitter differential amplifier
200
L.
Elements
103
and
104
are negative feedback resistors in the differential amplifier
100
. Each of feedback resistors
103
and
104
is connected in series between emitters of the transistors
101
and
102
.
Elements
203
and
204
are negative feedback resistors in the differential amplifier
200
L. Each of feedback resistors
203
and
204
is connected in series between emitters of the transistors
201
and
202
.
Elements
105
and
106
are load resistors in the differential amplifier
100
. Elements
205
and
206
are load resistors in the differential amplifier
200
L. Elements
107
and
207
are constant current sources of differential amplifiers
100
and
200
L.
The element
151
is a pair of differential input terminals of an amplifying unit
150
L including two stages of differential amplifiers
100
and
200
L. The element
152
is a pair of differential output terminals of the amplifying unit
150
L.
In the differential amplifier
100
, each of load resistors
105
and
106
is connected between a constant positive voltage source Vcc and each of collectors of transistors
101
and
102
.
In the differential amplifier
200
L, each of load resistors
205
and
206
is connected between a constant positive voltage source Vcc and each of collectors of transistors
201
and
202
.
The constant current source
107
is connected between a center junction of two feedback resistors
103
and
104
connected in series and a constant negative voltage source V
EE
. The constant current source
207
is connected between a center junction of two feedback resistors
203
and
204
connected in series and a constant negative voltage source V
EE
.
The collector of the transistor
101
in the differential amplifier
100
is connected to a base of the transistor
202
in the differential amplifier
200
L. The collector of the transistor
102
is connected to a base of the transistor
201
in the differential amplifier
200
L.
In the amplifying unit
150
L of
FIG. 1
, the relation between the input voltage Vicom and the output voltage Vocom can be shown as follows.
Vocom>Vicom
Therein, Vicom is a common-mode voltage applied to the input terminals
151
and Vocom is a common-mode voltage delivered from the output terminals
152
.
In each of the differential input of the input terminals
151
and the differential output of the output terminals
152
, a voltage change in common-mode is so called the common-mode voltage.
As already stated, the amplifying circuits in measuring instruments such as oscilloscopes amplify signals from DC to high frequency. In such a large bandwidth, it is desirable that the common-mode output voltage is 0V. It is, therefore, required that the common-mode output voltage Vocom is kept 0V or so.
In
FIG. 2
, there is shown a circuit diagram of another prior art amplifying unit. A differential amplifier
200
M of the second stage in the amplifying unit
150
M includes two pnp transistors
208
and
209
. It is able to keep the same output voltage Vocom of the terminals
152
as the input voltage Vicom of the input terminals
151
in common-mode.
However, pnp transistors are, generally, inferior to npn transistors in high frequency characteristics. The amplifying unit
150
M including pnp transistors
208
and
209
can not, therefore, obtain the same large bandwidth as that of npn transistors.
In the amplifying circuit used for measuring instruments like an oscilloscopes, a level-shift circuit is employed. The circuit shifts the DC level of the common-mode output voltage Vocom to 0V approximately. The circuit is connected to the output stage of an amplifying unit.
In
FIG. 3
, there is shown a circuit diagram of a prior art amplifying circuit with a level-shift circuit. The level-shift circuit
300
is appended to the amplifying unit
150
L of FIG.
1
. In
FIG. 3
, elements similar to those previously described with reference to
FIG. 1
are denoted by the same reference numerals.
A pair of differential output terminals
162
is output terminals of the amplifying circuit
160
L with the level-shift circuit. The level-shift circuit
300
is constituted of a couple of two level-shifters having the same composition. A level-shifter includes an npn transistor
301
, a diode group
303
of n diodes connected in series and a constant current source
305
. Another level-shifter includes an npn transistor
302
, a diode group
304
of n diodes connected in series and a constant current source
306
. Each of diode groups
303
and
304
containing one diode or more connected in series functions as a constant voltage diode.
A base of the transistor
301
is connected to the collector of the transistor
202
in the differential amplifier
200
L. A base of the transistor
302
is connected to the collector of the transistor
201
in the differential amplifier
200
L. Each of bases of transistors
301
and
302
is connected to the output terminals
152
of the differential amplifier
150
L.
A collector of the transistor
301
is connected to a positive voltage source VCC. A collector of the transistor
302
is connected to a positive voltage source VCC.
An emitter of the transistor
301
is connected to an anode of the first diode in the diode group
303
. An emitter of the transistor
302
is connected to an anode of the first diode in the diode group
304
.
A cathode of the last diode in the diode group
303
is connected to one end of the constant current source
305
. A cathode of the last diode in the diode group
304
is connected to one end of the constant current source
306
. Another end of each of constant current sources
305
and
306
is connected to each of negative voltage sources V
EE
s.
The output of the amplifying circuit
160
L with a level-shift circuit
300
is delivered from each of cathodes of the last diodes in diode groups
303
and
304
. The cathodes of the last diodes are connected to a pair of differential output terminals
162
.
The base-emitter voltage of each of transistors
301
and
302
is Vbe. The anode-cathode voltage (forward-voltage) per one diode in diode groups
303
and
304
is Vf. A voltag
Iwatsu Electric Co. Ltd.
Nguyen Patricia
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