Amplifiers – With semiconductor amplifying device – Including differential amplifier
Reexamination Certificate
2001-04-09
2002-09-10
Pascal, Robert (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including differential amplifier
C330S257000, C330S292000
Reexamination Certificate
active
06448853
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to circuitry for controlling distortion due to current variations in output stage driver transistors of an amplifier.
2. Background
Several factors can produce distortion in amplifiers. One of many is the output stage. Because the output stage must provide widely varying output load currents but only draw modest quiescent currents, the output and driver transistors undergo decades of current variation throughout the output drive range. The decades of current variation causes the output and driver transistor V
BE
S to change according to logarithmic law, causing distortion when superimposed on linear signals.
A typical amplifier block diagram is shown in FIG.
1
. In the amplifier of
FIG. 1
, an input differential voltage is applied across input stage amplifier
100
to drive a gain node
102
. The gain node
102
is buffered by an output stage buffer
104
forming an output buffer for the overall amplifier of FIG.
1
. The output buffer
104
has a nominal gain of +1. The frequency response is set by setting the capacitance Ccomp of capacitor
106
.
Distortion created by the output buffer
104
can be modeled as a voltage source V
DISTO
107
in series with the input of the output buffer
104
. To correct for V
DISTO
, some type of feedback is typically employed to adjust the differential input voltage.
FIG. 2
shows a block diagram of an amplifier with circuitry including correction amplifier
200
to correct for V
DISTO
by creating an error current which is injected into the gain node
102
. The gain of the correction amplifier
200
, Gm′, allows the error from the gain node to the final output to be observed. The gain of the amplifier
200
is typically set using an internal resistor. Previous designs employed the resistor with a value set so that the magnitude of Gm′ was less than the input differential Gm for stability. The Gm′ values of previous designs were also generally flat with frequency due to the resistor R controlling Gm′. Note that components carried over from
FIG. 1
to
FIG. 2
are similarly labeled, as will be components carried over in subsequent figures.
SUMMARY OF THE INVENTION
In accordance with the present invention an improved amplifier is provided with circuitry to correct for V
DISTO
by creating an error current from a correction amplifier which is injected into the gain node, similar to FIG.
2
. The circuit in accordance with the present invention includes components as in
FIG. 2
, with the correction amplifier including a correction capacitor to correct for V
DISTO
. Unlike typical circuits which included a resistor, with the correction capacitor the effective correction value will not be flat with frequency. The correction capacitor has a capacitance value C′ preferably set equal to the capacitance of the compensation capacitor Ccomp. With the value for C′ so set, Gm′≡sComp and the feedback current provided will be exactly the amount the differential input to amplifier
100
would have to be adjusted in feedback to supply the correct current to compensate for V
DISTO
. Thus, no input differential voltage is needed to cancel V
DISTO
, and the distortion is effectively cancelled. Another benefit of using the correction capacitor is that output impedance is reduced over frequency.
REFERENCES:
patent: 4462002 (1984-07-01), Schade, Jr.
patent: 4713628 (1987-12-01), Nelson
patent: 4881043 (1989-11-01), Jason
patent: 5990748 (1999-11-01), Tomasini et al.
Choe Henry
Elantec Semiconductor Inc.
Fliesler Dubb Meyer & Lovejoy LLP
Pascal Robert
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