Amplifiers – Wheatstone bridge with amplifier in at least one arm
Reexamination Certificate
1999-05-26
2001-06-05
Pascal, Robert (Department: 2817)
Amplifiers
Wheatstone bridge with amplifier in at least one arm
C330S127000
Reexamination Certificate
active
06242977
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to electronic signal amplifier circuits. In particular, the present invention is an audio frequency preamplifier that boosts the magnitude of signals obtained from an audio source, or instrumentation source in a similar frequency range, to form input signals for an audio frequency power amplifier that can be directly connected thereto without use of capacitors or coupling transformers. The present invention circuitry arrangement can also be used for a power amplifier that boosts the magnitude of audio frequency signals obtained from a preamplifier.
Preamplifier and power amplifiers for audio frequency signals are well known in the prior art. For example, U.S. Pat. No. 4,229,706 granted to Bongiorno in October 1980 and U.S. Pat. No. 4,719,431 granted to Karsten in January 1988 both disclose power amplifiers for this purpose. Some preamplifiers have been shown to be advantageous in being capable of transferring output signals therefrom over a balanced line interconnection arrangement directly connected thereto at its output. For example, the so called “Circlotron” circuit is such a high performance preamplifier.
An audio power amplifier
10
of the prior art, the “Circlotron” circuit, is illustrated in the FIG.
1
. Power amplifier
10
includes first and second triode electron tubes
12
and
14
and first and second power supplies
16
and
18
. Power amplifier
10
also includes first and second input terminals
20
and
22
as well as first and second output terminals
24
and
26
. An output load
28
in operation is placed across the first and second output terminals
24
and
26
. Finally, power amplifier
10
includes first and second stabilizing resistors
30
and
32
as well as first and second reference output resistors
34
and
36
.
First and second power supplies
16
and
18
are constant polarity floating power supplies, that is, they do not have a ground reference with respect to the outputs thereof connected in power amplifier
10
. Power supplies
16
and
18
include input terminals
17
and
19
suited for connection to ordinary 60 Hz single phase commercial alternating current (ac) sources, power transformers
21
and
23
, full-wave bridge and rectifiers
25
and
27
. Sources connected to terminals
17
and
19
provide a sinusoidal voltage waveform of substantially fixed amplitude to power amplifier
10
. Power transformers
21
and
23
step down the voltage to an appropriate level for amplifier
10
. Rectifying diode bridges
25
and
27
convert these ac waveforms to constant polarity waveforms between positive and negative output terminals having a selected nominal voltage value suited for operating the remainder of the circuit.
First triode
12
includes plate
40
, grid
42
, and cathode
44
. Grid
42
is connected to input terminal
20
via resistor
30
. Plate
40
is connected to the positive output terminal of first power supply
16
. Cathode
44
is connected to first output terminal
24
and the negative output terminal of power supply
18
. Second triode
14
includes plate
50
, grid
52
, and cathode
54
. Grid
52
is connected to second input terminal
22
via resistor
32
. Plate
50
is connected to the positive output terminal of second power supply
18
. Cathode
54
is connected to second output terminal
26
and the negative output terminal of power supply
16
. Resistor
34
is connected between first output terminal
24
and ground, and resistor
36
is connected between second output terminal
26
and ground. The cathode heater circuits for triodes
12
and
14
are not shown.
Power amplifier
10
typically receives from the input signal source in operation two balanced magnitude input voltage signals that are complements of each other, that is, one is the negative of the other ignoring the average values of each resulting from providing biasing for tubes
12
and
14
. A first of these balanced input signals is received by first input terminal
20
and the second input signal is received by second input terminal
22
. When the first input signal at terminal
20
is relatively high, the second input signal at terminal
22
is relatively low. Under these conditions, grid
42
of triode
12
has a relatively high voltage signal such that current increases through triode
12
to output terminal
24
, and grid
52
of triode
14
has a relatively low voltage signal such that current decreases through tri ode
14
to output terminal
26
leaving a net voltage across the load. In this way, power amplifier
10
operates to amplify the difference between the first and second input signals to thereby provide a substantial differential current gain, though little voltage gain with tubes
12
and
14
each connected as cathode followers.
Audio power amplifier
10
has desirable characteristics such as wide frequency bandwidth, fast transient response and low total distortion for reasons set out below. Similarly, this design has desirable characteristics in preamplifier applications including wide frequency bandwidth, low total distortion, the ability to transfer output signals over relatively long interconnection cables, good reliability and low cost.
This design achieves wide frequency bandwidth because it has a relatively simple design that uses a small number of components. This relatively small number of components allows a circuit design with low parasitics. This design has fast transient response because, unlike other preamplifier designs, the relatively low distortion of this design allows avoiding the use of negative output signal feedback to correct distortion effects. Negative feedback introduces significant damping that inhibits transient response. Also, this design has low total distortion because the follower arrangement keeps the voltage across the load substantially in the linear portion of its characteristics, and since the signals are substantially balanced they cancel distortion effects in each other. Finally, this circuit can operate its load through relatively long interconnection cables because of its current gain and low output impedance.
However, there are still several problems to overcome even using prior art power amplifier
10
. First, electrostatic noise from first and second power supplies
16
and
18
will result in noticeable amounts of noise at output terminals
24
and
26
. Electrostatic noise is caused by the electrostatic coupling of the alternating current line voltage across the power transformers. Electrostatic noise tends to be a problem since even very small amounts of electrostatic coupling to the alternating current line voltage across from the power transformers can result in very noticeable amounts of noise at the output. Even where multiple electrostatic shields are used with the transformer, this typically does not eliminate all the problems. Furthermore, great care must be used in applying electrostatic shields to achieve even marginal improvement in amplifier noise reduction. The present invention greatly reduces or eliminates this problem.
In addition to this problem with power amplifier
10
, average offset (DC) signal values of one polarity or the other occur at output terminals
24
and
26
due to circuit imbalances such as result from component parameter magnitude variations. Circuit imbalances may occur when there is a difference in gain between triodes that are not exactly matched or differences in voltage from the power supplies due to filter capacitor or resistor values being slightly off the proper value. The present invention also substantially overcomes these problems of the prior art.
BRIEF SUMMARY OF THE INVENTION
The present invention is a circuit for use as a preamplifier or amplifier. The circuit includes first and second input terminals configured to receive first and second input signals. The circuit also includes first and second active devices for controlling output signals. Each output control element has a source, a control, and an output. The control of the first active device is coupled to the first input
Atma-Sphere Music Systems, Inc.
Kinney & Lange , P.A.
Nguyen Khanh Van
Pascal Robert
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