Amplifiers – Modulator-demodulator-type amplifier
Reexamination Certificate
2003-05-13
2004-10-26
Choe, Henry (Department: 2817)
Amplifiers
Modulator-demodulator-type amplifier
C330S146000, C330S20700P
Reexamination Certificate
active
06809586
ABSTRACT:
TECHNICAL FIELD
This invention relates to power amplifiers and more particularly to bridge amplifiers.
BACKGROUND
As is known in the art, power amplifiers have a wide range of applications. One such power amplifier is a bridge amplifier. An early bridge amplifier is described in U.S. Pat. No. 2,235,677 Entitled “Amplifier for Signal Transmission” issued Mar. 18, 1941, inventor S. Gubin. The bridge amplifier described in the Gubin patent used thermionc emission devices. With the advent of transistors, the amplifier of Gubin was further improved by including a feedback signal to sense mismatches in the transistors and thereby apply a compensation voltage to the control electrodes of the transistors and thereby maintain apparent identity in operating characteristics of the transistors. Such improvement is described in U.S. Pat. No. 3,157,839, issued Nov. 17, 1064, entitled “Transistor Bridge Amplifier with a Bias Compensation Circuit Therefore”, inventor H. B. Brown et al. Further improvement in the bridge amplifier were made by using, inter alia, a pair of feedback signals, one proportional to current passing through the load connected to the output of the amplifier and one proportional to the voltage produced across the load. Such improved bridge amplifier is described in U.S. Pat. No. 4,092,610, issued May 30, 1978 entitled “Modulated Carrier Amplifying System”, inventors Benjermin J. White, George Moreau, Robert E. Dworkin, assigned to the same assignee as the present invention.
As described in U.S. Pat. No. 4,092,610, a signal to be amplified, after being combined with the feedback signal, is modulated with a carrier signal and the resulting signal is fed to the bridge amplifier. One such amplifier circuit
10
is shown in FIG.
1
. Thus, the input signal is fed to a summing amplifier
12
analog with a pair of feedback signals. One such feedback signal V
FB1
is a function of the voltage, V
L
, produced across output terminals
14
a
,
14
b
of a bridge amplifier
13
and the other feedback signal V
FB2
is a function of the current I
L
, to the output terminals
14
a
,
14
b
. The output of the summing amplifier
12
is combined with the modulating signal, here, for example a square wave of sawtooth wave voltage, produced by carrier signal generator
15
, in a pair of comparators
16
a
,
16
b
, as shown. The comparators
16
a
,
16
b
produce a pair of pulse width modulated (PWM) switching signals PWM
1
and PWM
2
, respectively. Inverters
18
a
,
18
b
are provided to generate a complementary pair of pulse width modulated signals PWM
1
′ and PWM
2
′, respectively. The four PWM signals are fed to the switching bridge amplifier
13
, as shown. The switching bridge amplifier
13
includes four amplifier elements, here transistors
22
a
,
22
b
,
22
c
and
22
d
arranged as shown in a bridge circuit having four nodes N
1
, N
2
, N
3
, and N
4
. Nodes N
1
and N
2
arc connected across a voltage source +V
s
and nodes N
3
and N
4
are connected to output terminals
14
a
,
14
b
. The output terminals
16
a
,
16
b
are connected to load here represented by a resistor R
L
, in a manner to be described in more detail below. Switching diodes
24
a
,
24
b
,
24
c
and
24
d
are arranged as shown. The operation and further details of the circuit shown in
FIG. 1
are provided in the above referenced U. S. Pat. No. 4,092,610, referred to above, the entire contents thereof being incorporated herein by reference.
As described in U.S. Pat. No. 4,092,610, the bridge amplifier
13
further comprises a transformer
17
having an input winding coupled via a capacitor to a pair of opposite nodes, here nodes N
2
and N
4
of the bridge circuit. A current sensor
19
is also coupled in series with the input winding for providing a voltage on line
21
having a magnitude proportional to the current in the input winding. The current sensor
19
comprises a transformer having a single turn of wire as its input winding and a multiple turn winding for its output winding with a potentiometer coupled across the terminals of the output winding. The current sensor
19
presents negligible impedance to the circuit of the input winding. The transformer
17
has a winding, which couples the signal of the winding via the lines
23
to a filter
25
. The winding provides a voltage between lines
23
, which is proportional to the voltage produced by the winding. The voltage on the lines
21
and
23
provide feedback signals which are summed together and with the input signal, in summing amplifier
12
, as described above.
As noted above, the output terminals
14
a
,
14
b
are coupled, via transformer
17
, to the filter
25
. The filter
25
comprises, by a way of example, a resonant tank circuit including an inductor and a capacitor, and a capacitive shunt, which includes a resistor and a capacitor. The tank resonates at the repetition frequency of the carrier frequency signal produced by generator
15
. The tank circuit thus blocks the passage of signals at the repetition frequency while the shunt shorts out harmonics of the input signal thereby providing a stop band which minimizes the appearance of carrier and harmonic frequency components at the load, here a transducer, or projector. A capacitor in series with the tank blocks direct current from the load. The load is shown as a resistive element, R
L
, which coats with the filter to provide the foregoing pass band.
In operation, when amplifying units (i.e., transistors
22
a
,
22
d
) conduct current simultaneously, the amplifying elements (i.e., transistors
22
b
,
22
c
) conduct no current. Conversely, when the amplifying elements (i.e., transistors
22
b
,
22
c
) are driven by switching signals to conduct current simultaneously, amplifying elements (i.e., transistors
22
a
,
22
d
) conduct no current. Thus, the direction of current in the input winding of the transformer
17
alternates in direction to provide both positive and negative pulses of current to the load, R
L
. In view of the filtering provided by the filter
25
and any reactance in both the load, R
L
and the elements of the bridge amplifier
10
, the signal appearing in the load, R
L
has an instantaneous amplitude which follows that of the input signal fed to the amplifier system
10
. The use of the four transistors in the bridge amplifier
13
permits application of a greater amount of power to the load than could be provided by simply one or two transistors. Further, the sequential operation of the branches of the bridge providing for a lower average value of power dissipation in each one of the four transistors.
One application of the amplifier system
10
shown in
FIG. 1
is in a sonar system. Such a system is shown in FIG.
2
. Here, a digital waveform generator
30
produces digital samples of acoustic wave energy. The digital samples are fed to a transmit bean forming network
32
. The beam forming network has a plurality of output ports, each producing the digital samples representing a burst of acoustic wave energy with a predetermined time delay in accordance with the desired transmit direction of the burst of energy. That is the beam forming network
32
is used to provide a collimated and directed beam of acoustic energy. The digital samples at each one of the output ports is fed to a digital to analog (D/A) converter
34
, as shown for conversion onto analog (i.e., time-continuous) bursts of acoustic energy. These bursts of energy are fed to a corresponding one of a plurality of the amplifier systems
10
shown in FIG.
1
. The amplified bursts of acoustic energy are fed to selected sets of transducers, or projectors
36
in an array of such projectors
36
through an aperture switching network
38
, as shown.
REFERENCES:
patent: 2235677 (1941-03-01), Gubin
patent: 3157839 (1964-11-01), Brown et al.
patent: 4092610 (1978-05-01), White et al.
patent: 6211735 (2001-04-01), Luu
patent: 6636113 (2003-10-01), Kirn
Cohen Martin L.
Hobboosh Namir W.
Choe Henry
Daly, Crowley & Mofford LLP
Raytheon Company
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