Low power current feedback amplifier

Details Low power current feedback amplifier Low power current feedback amplifier

2002-11-27

2004-04-20

Choe, Henry (Department: 2817)

Amplifiers

With semiconductor amplifying device

Including current mirror amplifier

C330S257000, C323S315000

Reexamination Certificate

active

06724260

ABSTRACT:

FIELD OF INVENTION
The present invention relates to amplifier circuits. More particularly, the present invention relates to a low power current feedback amplifier with a lower output impedance input stage buffer.
BACKGROUND OF THE INVENTION
The increasing demand for higher performance amplifier circuits has resulted in the continued improvement of the precision and accuracy of the various devices and components within the amplifier circuits, as well the increased response and bandwidth, and lower power requirements, of the amplifier circuits. For example, current feedback amplifiers, such as those incorporated within various video and audio drivers, equalizing filters, ADC input drivers, and other communication devices, are being required to provide lower power with increased bandwidth.
It would be ideal if current feedback amplifiers had gain independent bandwidth; however, such is not the case in practical applications. One area that affects the bandwidth performance is the output impedance of the inverting input terminal of the current feedback amplifier. The inverting input terminal impedance of the current feedback amplifier typically ranges from moderate to high. This level of impedance increases as the quiescent current bias level within an input buffer of the current feedback amplifier is reduced.
For example, with reference to
FIG. 1
, a basic current feedback amplifier
100
having an input buffer
102
, a pair of current mirrors
104
and
106
, and an output stage
108
illustrated. Input buffer
102
provides input terminals INP and INN for current feedback amplifier
100
. A pair of output terminals of input buffer
102
drive current mirrors
104
and
106
, which can provide a mirrored current to output stage
108
.
Input buffer
102
comprises an open loop buffer, such as that illustrated in
FIG. 2
, including bias currents I
1
and I
2
, a pair of input diode-connected transistors Q
1
, and Q
2
, and a pair of output devices Q
3
and Q
4
. As discussed above, ideally, the bandwidth of current feedback amplifier
100
is independent of the gain of current feedback amplifier
100
; however, in reality, the bandwidth of current feedback amplifier
100
is dependent upon any compensation capacitance and the value of an external feedback resistor R
FBext
(not shown) that can be configured in a closed feedback loop with current feedback amplifier
100
. The output impedance of input buffer
102
gets multiplied by the noise gain of current feedback amplifier
100
and adds to the external discrete feedback resistor, to give an equivalent feedback resistor. For example:
R
FBext
+(Noise Gain * Input Buffer Output Impedance)=
R
FBequiv
Equivalent feedback resistor R
FBequiv
determines the bandwidth of current feedback amplifier
100
. Changing the noise gain of current feedback amplifier
100
in an effort to try and maintain bandwidth requires changes in the value of equivalent feedback resistor R
FBequiv
. As a result, the bandwidth is never constant due to other effects on the output impedance of input buffer
102
. In addition, for low power applications, the higher output impedance becomes even more problematic, i.e., lower quiescent current results in higher output impedance.
One attempt to solve the higher output impedance problems of input buffer
102
includes the increasing of biasing currents I
1
and I
2
to reduce the output impedance of input buffer
102
. However, increasing biasing currents I
1
and I
2
increases the quiescent power requirements, which is undesirable.
SUMMARY OF THE INVENTION
In accordance with various aspects of the present invention, an overall low power current feedback amplifier having a lower output impedance input stage is provided. To reduce the output impedance, the input stage comprises a closed-loop input buffer. The closed-loop input buffer realizes a low output impedance since the loop gain of the input buffer reduces the output impedance of the input buffer. With a lower output impedance, the bandwidth of the current feedback amplifier becomes more independent of the gain, even at low supply current implementations.
An exemplary input buffer can be configured in a current feedback amplifier or a voltage feedback amplifier arrangement. In accordance with an exemplary embodiment, the input buffer comprises another closed-loop current feedback amplifier configured within the overall current feedback amplifier, wherein the output of the input buffer corresponds to the inverting node of the overall current feedback amplifier. The closed-loop configuration of the input buffer is facilitated by the use of an internal feedback resistor coupled from an inverting input terminal of the input buffer to the output of the input buffer, which corresponds to the inverting input terminal of the overall current feedback amplifier. Thus, with a low impedance at the inverting input terminal, the overall current feedback amplifier realizes a gain independent bandwidth over a wide range of supply currents.


REFERENCES:
patent: 4536662 (1985-08-01), Fujii
patent: 4639685 (1987-01-01), Saller et al.
patent: 4818901 (1989-04-01), Young et al.
patent: 4897616 (1990-01-01), Wang et al.
Electronic Design, “Wideband Op Amp Unshackles Gain From Bandwidth,” Oct. 29, 2001, www.elecdesign.com.

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