Circuit for ensuring full saturation of amplifiers of a...

Details Circuit for ensuring full saturation of amplifiers of a... Circuit for ensuring full saturation of amplifiers of a...

1999-12-14

2001-10-23

Pascal, Robert (Department: 2817)

Amplifiers

Wheatstone bridge with amplifier in at least one arm

C330S084000

Reexamination Certificate

active

06307434

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates in general to amplifiers and, more particularly, to single-input bridge amplifiers.
BACKGROUND OF THE INVENTION
In many applications such as audio applications, it may be useful, in terms of maximizing the output power, to use one or several pairs of amplifiers to drive the load in a bridge mode.
FIG. 1
shows a typical single-input bridge configuration of a pair of single input operational amplifiers A and B. In the example shown, the noninverting input (+) of one of the two amplifiers is connected to ground, the B amplifier in the example, and the inverting inputs (−) of the two operational amplifiers A and B are connected with a resistor R
2
.
In order to maximize the output power, it is important to fully exploit the output voltage swing of both amplifiers of
FIG. 1
, when there are asymmetric characteristics of saturation of the two amplifiers A and B. By observing the scheme of
FIG. 1
, the gain of the amplifier A is:
 
Va=V
in*(1
+R
1
/
R
2
)
whereas the gain for the other amplifier B is:
Vb
=−(
R
1
/
R
2
).
The B amplifier is not directly driven by the input signal, but, by the voltage BV
1
present on the inverting input node (−) of the A amplifier. In these conditions, when the A amplifier reaches saturation, the voltage V
1
remains at the reached value and the B amplifier may not follow the other to a state of saturation. This Phenomenon is illustrated in FIG.
2
.
A commonly used approach is to make the gains of the two amplifiers substantially equal, eventually by using different values of resistance of the feedback resistors, R
1
, as shown for example in FIG.
3
. If the following equation is satisfied, a simultaneous saturation of the two outputs Va and Vb will be ensured:
Va
=1
+R
1
a/R
2
=
Vb
=1
R
1
b/R
21
.
However, owing to residual asymmetries of the resistors, and above all of the saturation characteristics of the two amplifiers A and B which may be different in terms of respectively high and low saturation characteristics, it is difficult to ensure an identical saturation of both.
SUMMARY OF THE INVENTION
In view of the prior art, an efficient circuit arrangement is provided which is capable of ensuring a complete saturation of both operational amplifiers of a single-input bridge amplifier, practically under any condition of operation. Essentially, this result is achieved by replacing the commonly used single resistor that is connected between the inverting inputs of the two amplifiers with a voltage divider, and by injecting a saturation current signal on the intermediate node of the voltage divider. Such a saturation current signal is obtained through dedicated sensing devices of the state of saturation reached by the transistors of the output stages of both amplifiers of the single-input bridge amplifier.
In practice, the currents that originate from these saturation sensing devices are added to the intermediate node of the voltage divider that connects the inverting inputs of the two amplifiers and act as an auxiliary driving signal that reconfirms the value of the input signal when the amplifier reaches the saturation state.


REFERENCES:
patent: 4254380 (1981-03-01), Guillien
patent: 4560946 (1985-12-01), Yokoyama
patent: 4758796 (1988-07-01), Verhoeven et al.
patent: 4791381 (1988-12-01), Lenz
patent: 5402084 (1995-03-01), Freiler et al.
patent: 5654668 (1997-08-01), Botti et al.
patent: 5903189 (1999-05-01), Huijser

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