Audio amplifying circuit

Details Audio amplifying circuit Audio amplifying circuit

2002-10-04

2004-07-20

Mottola, Steven J. (Department: 2817)

Amplifiers

Combined with automatic amplifier disabling switch means

C330S069000

Reexamination Certificate

active

06765437

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an amplifying circuit used in audio systems.
2. Description of the Related Art
FIG. 1
shows a conventional bridge-connected audio amplifying circuit. The amplifying circuit includes first and second operational amplifiers
11
,
12
, connected as inverting amplifiers. The inverting input (−) of amplifier
11
is connected to an input terminal E of the system via a resistor
13
and a coupling capacitor
14
connected in series. Output O
1
of amplifier
11
is connected to the inverting input (−) via a resistor
15
. The inverting input (−) of amplifier
12
is connected to output O
1
of amplifier
11
via a resistor
16
and to its output O
2
via a resistor
17
. Outputs O
1
and O
2
of amplifiers
11
,
12
are connected across a load
18
, typically a loudspeaker able to give out sounds according to the current flowing therethrough. The non-inverting inputs (+) of amplifiers
11
,
12
are connected together to a node BP of a resistive divider including a resistor
19
connected between node BP and a supply terminal VCC, and a resistor
20
connected between node BP and ground GND. A capacitor
21
is connected in parallel with resistor
19
. Capacitor
21
has the function of filtering the noise generated by resistors
19
and
20
and for absorbing possible variations of the voltage at supply terminal VCC.
The gain of amplifier
11
is given by the ratio of resistances
15
and
13
. The gain of second amplifier
12
is generally chosen to be equal to −1 by setting an identical value for both resistances
16
and
17
.
The expression of voltage V
CH
across load
18
is given by the following equation:
V
CH
=V
O1
−V
O2
=−2(
R
15
/R
13
)*(
V
M
−V
BP
)
where R
13
and R
15
are the respective values of resistances
13
and
15
; and V
O1
, V
O2
, V
BP
, and V
M
are the voltages at outputs O
1
and O
2
of amplifiers
11
,
12
, at node BP, and at a node M between capacitor
14
and resistor
13
, respectively.
The divider formed of resistors
19
and
20
sets the voltage at node BP to a reference voltage. For example, the reference voltage may be chosen to be equal to VCC/2 and the values of resistances
19
,
20
are then set to a same value. In normal operation, in the absence of a signal at input terminal E, voltages V
M
and V
BP
are equal to the reference voltage and the voltage across the load is zero. When a voltage is applied to input terminal E, voltage V
M
is equal to the reference voltage plus the variable component of the input voltage, coupling capacitor
14
suppressing the D.C. component of the input voltage.
Accordingly, the voltage across load V
CH
is equal to the variable component of the input voltage multiplied by amplification gain −2R
15
/R
13
. By choosing an adapted ratio of the values of resistances
15
and
13
, the peak-to-peak load voltage can be significantly amplified.
FIG. 2
shows a circuit similar to that of
FIG. 1
further including a stand-by system
25
having the function of maintaining supply voltage VCC which may be used by stages upstream of the amplifying state, while reducing the specific consumption of the amplifying portion. Stand-by system
25
receives a control signal and is connected at a first output to inhibition terminals A
1
and A
2
of amplifiers
11
and
12
. An output of stand-by system
25
is further connected to a transistor
26
in series with resistor
19
. Upon reception of the specific control signal, stand-by system
25
also blocks transistor
26
to suppress the consumption of resistors
19
,
20
.
FIG. 3
shows the evolution of voltages along time at given points of the amplifying circuit of
FIG. 1
at the device power-on, that is, when the supply voltage passes from 0 to voltage V
CC
. Curve V
ALIM
shows the variation of the supply voltage along time. Curve V
M
shows the variation of the voltage at node M along time, curve V
BP
showing the variation of the voltage at node BP along time.
At the circuit power-on, supply voltage V
ALIM
almost instantaneously switches from 0 volt to VCC. The voltage at node BP settles at the reference voltage. The voltage at node V
M
also settles at the reference voltage, for example, VCC/2. It should be noted that curve V
M
reaches an equilibrium level in a time shorter than that of curve V
BP
.
The rise time of the voltage at node BP is mainly determined by the values of capacitance
21
and of resistances
19
,
20
. It generally is on the order of from 50 to 150 ms. It is generally not possible to guarantee an identical time constant “seen” by node M, which implies different rise times for the voltages at nodes M and BP.
On
FIG. 4
, curve V
M
−V
BP
shows the difference between the voltages at nodes M and BP and curve V
CH1
shows the voltage applied across load
18
for the circuit of FIG.
1
.
At the circuit power-on, the operational amplifiers supplied by the supply voltage being almost “instantaneously” on, the difference between the voltages at nodes M and BP is reflected on load
18
, multiplied by the amplifying gain. The voltage applied to the load, due to a high amplifying gain, is often sufficient to cause a characteristic audible and unpleasant noise.
In the case where the amplifier is equipped with a stand-by unit
25
, the problem is also posed upon switching from the stand-by state to a normal operation state, since in this switching, the voltage at node BP will settle to the reference voltage while the voltage at node M has already settled to the reference voltage.
Upon switching from a normal operating state to a stand-by state or from a normal operation state to an off state, the voltages at nodes M and BP switch from the reference level to a zero voltage in different durations, for the same reasons as mentioned previously. Generally, the supply of amplifiers
11
,
12
being “almost-instantaneously” interrupted, said amplifiers no longer amplify directly across the load the difference between the voltages at nodes M and BP. However, amplifiers
11
,
12
may stay on, so that a current generated by the difference between the voltages at nodes M and BP could cross the load. However, this current often is very small and only rarely translates as a characteristic audible and unpleasant noise of the loudspeaker. In the case where the switching of the supply voltage from VCC to a zero voltage exhibits a non-negligible time constant, amplifiers
11
,
12
and comparator
30
may remain supplied for some time. In this case, the voltage difference at nodes BP and M remains amplified across the load and may be the cause of a characteristic audible and unpleasant noise.
BRIEF SUMMARY OF THE INVENTION
The disclosed embodiment of the present invention provides an audio amplifying circuit that includes a system for reducing unwanted noise appearing at the circuit turning-on from an off state or a stand-by state.
Accordingly, the embodiment of the present invention provides an amplifying circuit having an amplifier circuit that receives an input voltage and a reference voltage circuit to generate a reference voltage equal to a fraction of the circuit supply voltage, a time constant circuit for generating a voltage, the amplifying circuit including means for, upon power-on, inhibiting the amplifier circuit for as long as the difference between the value of the provided reference voltage and the voltage at the output of the time constant circuit is greater than a determined threshold.
According to another feature of the present invention the amplifying circuit includes a first amplifier having a first amplifying gain, receiving at a first input the input voltage, and a second amplifier having a second amplifying gain, and receiving at a first input the output of the first amplifier, the outputs of the first and second amplifiers being connected to a load, second inputs of the first and second amplifiers being connected together to a node receiving the reference voltage.
According to another fea

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