Amplifiers – With semiconductor amplifying device – Including push-pull amplifier
Reexamination Certificate
2000-10-31
2001-09-25
Pascal, Robert (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including push-pull amplifier
C330S268000
Reexamination Certificate
active
06294958
ABSTRACT:
BACKGROUND OF THE INVENTION
An integrated amplifier circuit such as an opamp is usually constructed from a small chip of semiconductor material upon which an array of active/passive components have been constructed and connected together to form a functioning circuit. An integrated amplifier circuit is generally encapsulated in a plastic housing (chip) with signal, power supply, and control pins accessible for connection to external electronic circuitry. Typically, input signals transmitted to the integrated amplifier circuit via selected input pins are processed by active and passive components in different stages, e.g., input and turn-around, and the processed signals are then applied to selected output pins using an output stage.
The enormous growth of high-speed communication and high data rate image processing applications, requiring high-speed, low power and small size, has created a demand for miniaturized high-speed amplifiers that can operate at low voltages. To maximize the dynamic range at low supply voltages for this use, it is desirable that the output voltage range for this type of amplifier should be as large as possible. Preferably, the output voltage range of the amplifier would extend from one rail to the other rail of the power supply.
Class-AB circuitry is used in amplifiers that employ both bipolar and MOS components. A Class AB circuit can deliver to and pull from a load a current that is larger than the DC quiescent current flowing into the circuit. For example, the drive current in a Class AB circuit may be 100 milliamps and the quiescent current could be 1 milliamp. Class AB circuitry is preferred in output stage of a low-power high speed amplifier because it improves power efficiency by maximizing the output drive current with a relatively low quiescent current. Also, class-AB circuitry enables the output stage to exhibit good linearity over the entire output voltage range.
However, class-AB circuitry can be prone to cross-over distortion caused by the non-linear operation of transistors in a push-pull amplifier. For example, when changing (crossing over) the active operation of one transistor (turn off) to another transistor (turn on) in an output stage, distortion of the output signal can be caused by a less than ideal (non-linear) operation of a transistor. In
FIG. 1
, a graph illustrates the effects of crossover distortion in an output signal.
To minimize power consumption when a low-power high-speed amplifier is operated at higher supply voltages, the quiescent current of the amplifier's output stage should remain constant when the supply voltage increases from a minimum to a maximum predetermined voltage. However, stabilization of this quiescent current over a range of supply voltages for an amplifier circuit typically requires some type of compensation for the Early effect (second order effect) which can cause increased quiescent current at elevated supply voltages. Typically, as the physical size of the base width of a transistor is decreased so will the Early voltage, which causes the collector current to increase as collector emitter voltage increases.
FIG. 2
illustrates a graph of the effect of the Early voltage on the operation of a transistor. The x-axis represents values for a collector emitter voltage and the y-axis represents a collector current. Two different Early voltages V
A1
and V
A2
are indicated on the x-axis at 100 volts and 20 volts, respectively. When the Early voltage has a value of 100 volts (V
A1
), the collector current does not significantly increase as the collector emitter voltage increases. Alternatively, when the base width is significantly decreased and the Early voltage is decreased to 20 volts (V
A2
), the collector current increases steeply as the collector emitter voltage increases.
Compensation for the Early effect has often been provided by physically matching sizes of NPN and PNP bipolar transistors in an integrated circuit. However, matching the sizes of bipolar transistors of different polarities is a process dependent technique that can be relatively imprecise in a highly miniaturized integrated circuit.
SUMMARY OF THE INVENTION
In accordance with the invention, an apparatus for an output stage of bipolar transistors that includes a pair of buffers that are coupled opposite ends of a resistor and a pair of input terminals that are adapted to receive a differential signal. A pair of output transistors are separately coupled between different sides of a supply voltage and a single ended output terminal. Each output transistor is a different type and separately turns in response to a polarity of the differential signal. A pair of clamp transistors have different types and they are separately associated with one of the output transistors that have the same type. Each clamp transistor clamps the turn off voltage to a predetermined value at a base of each associated output transistor. Also, a collector and an emitter for each clamp transistor is coupled between the bases of each output transistor. Additionally, a pair of source transistors that are different types are coupled between a pair of current mirrors that are separately coupled to opposite sides of the supply voltage and each buffer. The arrangement of the pair of source transistors enables a floating current source for turning on each current mirror to mirror a current on top of the current flowing through the resistor in response to a polarity of the differential signal. The size of each source transistor is matched to the size of each clamp transistor of the same type so that Early effect of the clamp transistors over a range of supply voltages is compensated for.
In accordance with another aspect of the invention, a pair of voltage sources are separately coupled to a base of each source transistor and separately coupled to separate sides of the supply voltage. Also, another pair of voltage sources are separately coupled to a base of each clamp transistor and separately coupled to a side of the supply voltage that is coupled to an emitter of the associated output transistor. Each voltage source may include two stacked diodes that are biased at a fixed current to match the base-emitter voltages of the associated clamp transistor and output transistors.
In accordance with yet another aspect of the invention, a coupling of a collector and an emitter for each clamp transistor between the bases of each output transistor causes a reduction in the cross over distortion of an output signal at the single ended output terminal. The floating current source provides a quiescent current that is stable over a range of supply voltages. Additionally, when the polarity of the differential signal turns on an output transistor, the other output transistor is kept from turning off. Similarly, when another polarity of the differential signal turns on the other output transistor, the output transistor is kept from turning off.
In accordance with still another aspect of the invention, a base for each clamp transistor is coupled to a separate voltage source. This separate voltage source is coupled to a side of the supply voltage that is coupled to the emitter of the associated output transistor. Also, each current mirror is coupled to an opposite side of the supply voltage.
In accordance with another aspect of the invention, the output stage may be employed with an amplifier and a comparator. Also, the output stage provides Class AB operation. Additionally, the types of the transistors include NPN and PNP.
The invention may also be implemented as methods that perform substantially the same functionality as the embodiments of the invention discussed above and below.
These and other features as well as advantages, which characterize the invention, will be apparent from a reading of the following detailed description and a review of the associated drawings.
REFERENCES:
patent: 4560946 (1985-12-01), Yokoyama
patent: 5754078 (1998-05-01), Tamagawa
patent: 6163216 (2000-12-01), Murray et al.
“160 MHz Rail-to-Rail Amplifier With Disable”, DataSheet AD8041, Analog Devic
Branch John W.
Choe Henry
Merchant & Gould P.C.
National Semiconductor Corporation
Pascal Robert
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