Amplifiers – With semiconductor amplifying device – Including differential amplifier
Reexamination Certificate
1999-05-27
2001-02-06
Shingleton, Michael B (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including differential amplifier
C330S268000, C330S265000, C330S267000, C330S263000
Reexamination Certificate
active
06184750
ABSTRACT:
DESCRIPTION
1. Technical Field
The present invention is related to electronic circuits for amplifying electrical signals. In particular, the present invention teaches a variety of power amplifying output stages, control circuits for such stages, and methods for controlling these devices.
2. Background Art
Amplification circuitry can be loosely divided into two types: high bandwidth and low power amplification circuitry. In the creation of high bandwidth and low power amplification circuitry, designers have different goals in mind and thus concern themselves with different issues when designing the different types of circuitry.
For example, in the design of high bandwidth circuitry, designers are concerned with achieving high frequency and low distortion. However, most high bandwidth circuitry applications are not concerned with power efficiency issues. Thus, maintaining a low quiescent current or operating with a low supply voltage generally are not design constraints in high bandwidth circuitry.
In the design of low power circuitry, on the other hand, designers are concerned about proper operation with a low supply voltage and low supply current. However, most low power circuitry applications are not beset with frequency or distortion issues. Thus, maintaining a high frequency or low distortion generally are not design constraints in low power circuitry.
Prior Art
FIG. 1
is an example of a conventional rail-to-rail output stage
10
that is a low bandwidth, low voltage circuit. The conventional rail-to-rail output stage
10
includes a common source output transistor
12
, a current mirror
14
, and a current source
20
. The current mirror
14
is formed having a pair of transistors
16
and
18
. The current source
20
drives the current mirror
14
with a bias current I
b
, which determines the quiescent current I
q
in output transistor
12
. Gate drive, which is provided by a preceding stage through an input
22
, is amplified at output
24
by the gain of output transistor
12
. However, load transistor
18
provides no symmetrical drive for sourcing output current. Consequently, output stage
10
creates significant distortion while providing a maximum output voltage swing. However, this is usually not a drawback since low supply voltage, low bandwidth designs are usually not required to have low distortion.
Prior Art
FIG. 2
is an example of a conventional class AB complementary source follower output stage
30
that is a high bandwidth circuit. The class AB output stage
30
includes first and second output transistors
32
and
34
connected in series as source followers with respect to nodes A and B, respectively. A bias circuit
31
for output transistors
32
and
34
includes diode-connected transistors
40
and
42
, and first and second current sources
36
and
38
. Circuit input
44
is formed by the current programming input of current source
38
. Although the class AB complementary output stage
30
is beneficial when a wide bandwidth, low distortion circuit having high output current is required, it does not allow the output voltage to swing close to either supply.
In both high bandwidth and low power amplification circuitry, designers use numerous techniques to accomplish various design goals. One such technique often used for increasing output current in a low quiescent current circuit has been the use of a Darlington transistor configuration.
Prior Art
FIG. 3
illustrates a Darlington emitter-follower circuit
50
. The Darlington circuit
50
introduces a second npn transistor
52
having a base
54
connected in series with the emitter
56
of the first npn transistor
58
. In contrast to the basic output stage
10
of
FIG. 1
, in
FIG. 3
the emitter
56
of the first npn transistor
58
drives the second npn transistor
52
which in turn supplies the output current I
out
, To do so, the second npn transistor
52
draws a base current I
E2
from the emitter
56
of the first npn transistor
58
.
This configuration allows additional amplification of a base current such as I
B2
. For example, although I
E2
is limited to &bgr;
1
*I
B2
, the emitter current I
E2
is amplified by the second emitter-follower transistor
52
to generate the output current I
out
. Thus, assuming the first and second npn transistors
58
and
52
have current gains of &bgr;
1
and &bgr;
2
, respectively, the maximum value of I
out
=&bgr;
1
*&bgr;
2
*I
B2
. Hence the potential available output current I
out
is greatly increased without increasing the quiescent current of I
s
.
However, the additional amplification stage embodied in the second npn transistor
52
introduces its own problems. For example, an additional V
BE
voltage drop is introduced across the base
54
and the emitter
60
of the second npn transistor
52
. As a result, the maximum output voltage reduces to V
OUT
=V
+
−V
SAT
−2*V
BE
. The added V
BE
reduction of V
OUT
imposes a significant limitation to Darlington circuits
50
, especially those running from lower-voltage power sources.
In summary, a variety of circuits exist for normal high bandwidth and low power circuitry. High bandwidth and low power circuitry applications normally have different requirements. However, there has been very little development in circuitry that has the requirements of both a high bandwidth circuit and a low power circuit. An application requiring portability would be one example where both types of requirements would need to be met to create a superior product. Since portable applications run on batteries, it would be very advantageous to use a circuit that requires a minimum number of battery cells and increases cell life by requiring a low supply current. A high speed amplifier that requires a low supply voltage would have the normal requirements of high speed circuitry, specifically high frequency and low distortion. It would also need, however, to operate on a minimum supply voltage and current.
What is needed and desirable is a circuit that allows high frequency with low distortion, but can operate with a low supply voltage and low quiescent current.
SUMMARY OF THE INVENTION
In order to achieve the foregoing and in accordance with the present invention, a variety of output stages and methods for amplifying electrical signals are disclosed.
According to a first embodiment of the present invention, a control circuit driven by a differential input voltage includes first and second input stages coupled in parallel. Each input stage has a differential input coupled to the differential input voltage and a single-ended output substantially proportional to the differential input voltage. The control circuit provides a substantially symmetrical signal pair to drive a complementary push-pull output stage, while decreasing the supply voltage requirement. The complementary push-pull output stage includes first and second push-pull amplifiers each driven by one of the input stages, with the outputs of the push-pull coupled together.
Another aspect of the present invention describes a method for controlling a complementary push-pull output stage whereby the output of the stage is formed by the coupling of the outputs of the first and second push-pull amplifiers. Two symmetrical signal paths are provided from differential inputs to one singe-ended output.
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patent: 5070308 (1991-12-01), Padi
patent: 5475343 (1995-12-01), Bee
patent: 5825228 (1998-10-01), Gross
patent: 5907262 (1999-05-01), Graeme et al.
patent: 6028481 (2000-02-01), Gerstenhaber et al.
Charles A. Holt “Electronic Circuits Digital and Analog” John Wiley & Sons Copyright 1978, pp. 431, 432, 526 and 527.
M. Jeroen Fonderie, Johan H. Huijsing;Design of Low-Voltage Bipolar Operational Amplifiers; Kluwer Academic Publishers.
J. L. Linsley Hood;Symmetry in audio amplifier circuitry; Electronics & Wireless World, Jan. 1985.
Gain Technology, Inc.
Hickman Coleman & Hughes LLP
Shingleton Michael B
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