Amplifiers – With semiconductor amplifying device – Including push-pull amplifier
Reexamination Certificate
2000-03-10
2001-07-31
Mottola, Steven J. (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including push-pull amplifier
C330S273000
Reexamination Certificate
active
06268771
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an amplifying device, and more particularly to an improvement for achieving both larger dynamic range and easier control of idling current.
2. Description of the Background Art
In many cases, a power amplifying device has a configuration of a push-pull type class AB amplifying device in which two transistors are combined. A class AB amplifying device, comprising a transistor for drawing an output current out from a positive power supply line to a load and a transistor for drawing the output current from the load into a negative power supply line, alternately turns the two transistors on to achieve draw-out and draw-in of the output current and turns both the transistors on in some degree at switching between the draw-out and draw-in.
Therefore, the class AB amplifying device has an advantage of saving a consumption current since only a very small amount of idling current flows from one transistor to the other when the output current is zero. Further, since the two transistors do not turn off simultaneously, it is advantageously possible to suppress crossover distortion and improve switching characteristics.
A background-art class AB amplifying device using MOSFETs has a configuration of combining a source follower of an n-channel type MOSFET and that of a p-channel type MOSFET.
FIG. 26
is a circuit diagram showing an exemplary configuration of such a background-art class AB amplifying device. In the device
150
, an n-channel type MOSFET
161
and a p-channel type MOSFET
162
of which source electrodes are connected to each other are disposed between a positive power supply line
163
and a negative power supply line
164
. An output terminal
165
is connected to a node between the two source electrodes.
Further, between a positive power supply line
170
and a negative power supply line
171
disposed is a series circuit in which a resistance element
169
, an n-channel type MOSFET
166
, a p-channel type MOSFET
167
and an n-channel type MOSFET
168
are connected in this order. Drain electrodes of the MOSFETs
166
and
167
are connected to each other.
A gate electrode and a source electrode of the MOSFET
166
are connected to each other and similarly a gate electrode and a source electrode of the MOSFET
167
are connected to each other. A node between the resistance element
169
and the source electrode of the MOSFET
166
is connected to a gate electrode of the MOSFET
161
, and a node between a drain electrode of the MOSFET
168
and the source electrode of the MOSFET
167
is connected to a gate electrode of the MOSFET
162
. Further, an input terminal is connected to a gate electrode of the MOSFET
168
.
When a voltage to make the MOSFET
168
full-on is applied as an input voltage Vin, a large amount of current flows in the resistance element
169
through the MOSFETs
166
and
167
. That increases a voltage drop across the resistance element
169
, and hence gate voltages of the MOSFETs
161
and
162
drop. As a result, the MOSFET
161
turns off and the MOSFET
162
turns on. An output current is thereby drawn from a load into the negate power supply line
164
through the output terminal
165
.
On the other hand, when a voltage to make the MOSFET
168
full-off is applied as the input voltage Vin, only a small amount of current flows in the resistance element
169
. That decreases the voltage drop across the resistance element
169
, and hence the gate voltages of the MOSFETs
161
and
162
rise. As a result, the MOSFET
161
turns on and the MOSFET
162
turns off. The output current is thereby drawn out from the positive power supply line
163
to the load through the output terminal
165
.
Thus, in response to the input voltage Vin, draw-out of the output current (current source) and draw-in thereof (current sink) are performed. The MOSFETs
166
and
167
serve to create a potential difference between the gate electrodes of the MOSFETs
161
and
162
. At switching between the current sink and the current source, a current proportional to a current flowing in the MOSFETs
166
and
167
flows from the MOSFET
161
to the MOSFET
162
as an idling current. That achieves a class AB operation where the MOSFETs
161
and
162
do not turn off simultaneously.
The device
150
has a problem in being incorporated as an IC (Integrated Circuit) in a single semiconductor chip as follows. In the IC, usually, the negative power supply lines
164
and
171
are formed as a common ground power supply line. That causes a problem that an output voltage Vout can not be less than a value obtained by adding a source-drain voltage of the MOSFET
168
and the gate-source voltage of the MOSFET
162
when the MOSFET
168
is made full-on to a negative power supply potential −Vcc of the negative power supply line
164
.
Specifically, a dynamic range of the output voltage Vout is disadvantageously limited smaller than a potential difference (power supply voltage) between a positive power supply potential Vcc and the negative power supply potential −Vcc. In a largely-rated amplifying device, a gate-source voltage to make the MOSFET full-on is high and when the power supply voltage is low, a ratio of lost dynamic range to the power supply voltage is not negligible. The problem is pronounced when an IC is used for portable electronics using battery as power supply.
As a solution of this problem known is an amplifying device comprising two n-channel type MOSFETs connected in series between a positive power supply line and a negative power supply line, two preliminary amplifiers to separately control these two MOSFETs and another MOSFET to prevent these two MOSFETs from turning on simultaneously, to achieve a class AB operation. This device, however, has a problem that it is not easy to control the idling current of the two MOSFETs and switching distortion and a through current (excessive idling current) are likely to be caused.
SUMMARY OF THE INVENTION
The present invention is directed to an amplifying device. In the present invention, “conductivity type” of a transistor means pnp-type or npn-type in a case of a bipolar transistor and means n-channel type or p-channel type in a case of a MOSFET. According to a first aspect of the present invention, the amplifying device comprises: a first MOSFET having a drain electrode connected to a first power supply line; a second MOSFET of the same channel type as the first MOSFET, having a source electrode connected to a second power supply line and a drain electrode connected to a source electrode of the first MOSFET; a third MOSFET of the same channel type as the first MOSFET, having a source electrode connected to the second power supply line and a gate electrode connected to a gate electrode of the second MOSFET; a first resistance element having two ends, one end being connected to a gate electrode of the first MOSFET and the other end being connected to the source electrode of the first MOSFET; a control circuit having first to third electrodes, the first electrode being connected to a drain electrode of the third MOSFET, the second electrode being connected to the gate electrode of the first MOSFET, a potential difference between the first electrode and the second power supply line is determined by a voltage applied across the third electrode and the second power supply line, and a current flowing in the second electrode being in proportion to a current flowing in the first electrode; and a first constant current source having an output end connected to the second electrode.
According to a second aspect of the present invention, the amplifying device comprises: a first MOSFET having a drain electrode connected to a first power supply line; a second MOSFET of the same channel type as the first MOSFET, having a source electrode connected to a second power supply line and a drain electrode connected to a source electrode of the first MOSFET; a third MOSFET of the same channel type as the first MOSFET, having a source electrode connected
Burns Doane , Swecker, Mathis LLP
Mitsubishi Denki & Kabushiki Kaisha
Mottola Steven J.
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