Amplifiers – With semiconductor amplifying device – Including differential amplifier
Patent
1994-03-04
1995-06-27
Mullins, James B.
Amplifiers
With semiconductor amplifying device
Including differential amplifier
330255, 330258, 330261, H03F 345
Patent
active
054283167
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a power amplifier with a differential output and differential input, for example, suitable for use as a driver circuit for moving coil transducers, such as earpieces and speakers employed in telephone sets.
Modern telephone circuitry may be implemented in CMOS integrated circuits. Suitable drivers must be provided to interface with discrete moving coil transducer components, such as the earpieces and speakers employed in modern telephone sets. Generally, the drivers must be capable of supplying high power to the load at low D.C. bias voltages, and for this purpose fully differential amplifiers, which have a differential output and differential input, can be employed to advantage because of their ability to produce a high output power from the 5 V CMOS voltage supply (No special power supply is required). The drivers must, however, have linear transfer characteristics, a very high input impedance in order to interface with the CMOS circuitry, a stable frequency response and be insensitive to process variations.
Two major problems must be addressed, namely the control of the common mode voltage associated with fully differential designs and the D.C. quiescent current associated CMOS amplifiers with large output transistors. The high load currents and large dimensions of the output transistors in the amplifier imply a high sensitivity of quiescent current to process variations and biasing. Small deviations in the gate voltages of the output transistors can lead to quiescent currents that greatly exceed desired requirements. Typical such cicuits are described in EP 0325 299, NEUES AUS DER TECHNIK, no. 1, 20 Feb. 1987, page 1, "OPERATIONSVERSTARKER", and IEEE INTERNATIONAL SOLID STATE CIRCUITS CONFERENCE, Vol. 34, February 1991, New York, U.S., pages 278-279 G Nicollini et al. "A-78db THD 100 OHM DIFFERENTIAL DRIVER FOR ISDN APPLICATIONS".
The driver should also have high gain, inherent stability and simplicity of implementation. These features have proved difficult in the past to achieve.
According to the present invention there is provided a power amplifier comprising an input stage having a fully differential amplifier with differential inputs and differential outputs, and a high input impedance closed loop common mode feedback control circuit deriving common mode feedback control signals from said differential outputs; and an output stage comprising for each differential output of the input stage a class AB amplifier having source followers interfacing with said fully differential amplifier and a quiescent current control input for maintaining a desired quiescent current, means for maintaining the common mode output voltage of the class AB amplifiers at a desired level, and a quiescent current regulator for supplying a control signal to said quiescent current control inputs, said quiescent current regulator comprising means for mimicking the behaviour of one of said source followers to derive said control signal.
The quiescent current regulator preferably includes a pair of transistors that mimic the N-channel source follower of one of the class AB amplifiers. The gate of one of the mimic transistors is set to the quiescent state voltage of the input stage output. The drain voltage of the second mimic transistor corresponds to the drain voltage of one of a pair of transistors forming the N-channel follower connected to the gate of the N-channel driver transistor in the output stage and thus represents its gate voltage. The gate voltage of the second mimic transistor is used to bias the N-channel source follower of the output stage. A reference voltage generated at the input of the quiescent current regulator is effectively mirrored into the N-channel driver transistor of the output stage and can thus be used to determine the quiescent current of the output stage. This circuit results in an amplifier quiescent current that is insensitive to process variations.
The present invention is of two-stage design, in which the input stage is a preferably a folded cascode amplifier, which is
REFERENCES:
patent: 4533876 (1985-08-01), Haque et al.
patent: 4933644 (1990-06-01), Fattaruso et al.
patent: 5166635 (1992-11-01), Shih
Mitel Corporation
Mullins James B.
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