Low voltage large swing/high linearity analog buffer with...

Details Low voltage large swing/high linearity analog buffer with... Low voltage large swing/high linearity analog buffer with...

2003-06-12

2004-10-26

Nguyen, Patricia (Department: 2817)

Amplifiers

With semiconductor amplifying device

Including differential amplifier

C330S085000, C330S110000

Reexamination Certificate

active

06809589

ABSTRACT:

BACKGROUND OF THE INVENTION
An analog buffer is coupled between the source of an input voltage and a load when the source of the input voltage is unable to directly drive an output load. The analog voltage buffer has high input impedance and low output impedance to provide a source and a sink current and supplies an output voltage which tracks the input voltage. A linear transconductor is typically used in an analog voltage buffer to perform linear voltage-to-current conversion. Linear transconductors are typically used in continuous-time filters, tuning circuitry, and phase locked loops.
FIG. 1
is a circuit diagram of a prior art differential-mode analog buffer
100
. A differential input voltage V
i
(V
ip
-V
in
) is applied to the respective non-inverting inputs of servo amplifiers
102
,
103
. The analog buffer
100
converts the differential input voltage V
i
(V
ip
-V
in
) to an output current i
o
that flows through the output terminals of the analog buffer through the load. The analog buffer
100
includes four constant current sources labeled I
1
, I
2
, I
3
, I
4
each constant current source is represented by a symbol having an arrow indicating the direction of the current flow on the input side of the current mirror.
Each of the servo amplifiers
102
,
104
has an NPN bipolar transistor in the feedback path between the inverting input and the output. The base of NPN bipolar transistor Q
1
in the feedback path of servo amplifier
102
is coupled to the output of the servo amplifier
102
and the emitter is coupled to the non-inverting input of the servo amplifier
102
. Constant current source I
1
is coupled to the collector of NPN transistor Q
1
and constant current source I
3
is coupled to the emitter of NPN transistor Q
1
.
The base of transistor Q
2
in the feedback path of servo amplifier
104
is coupled to the output of the servo amplifier
104
and the emitter is coupled to the non-inverting input of the servo amplifier
104
. Constant current source I
2
is coupled to the collector and constant current source I
4
is coupled to the emitter. A linear resistor R
E
is coupled between the emitter of transistor Q
1
and the emitter of transistor Q
2
. The load R
L
coupled to the differential output V
o
(V
op
-V
om
) of the analog buffer
100
is represented by load resistors R
L
/2 each coupled between the respective collector of the transistor Q
1
, Q
2
and a common mode output voltage V
cmout
.
A differential analog input voltage V
i
(V
ip
-V
im
) is applied to the analog buffer
100
through the respective non-inverting inputs of the servo amplifiers
102
,
104
. The corresponding differential output voltage V
o
(V
op
-V
om
) on the collectors of the respective NPN transistors Q
1
, Q
2
tracks the differential input voltage V
i
(V
ip
-V
im
)
If each of the constant current sources I
1
, I
2
, I
3
, I
4
supplies a constant current through the range of operation of the circuit, the differential input signal V
i
(V
ip
-V
im
) is essentially equal to the voltage V (V
p
-V
m
) applied across the linear resistor R
E
. Thus, the voltage V across R
E
more or less follows the differential input voltage V
i
(V
ip
-V
im
) dependent on the gain-bandwidth of the feedback loops constituted by the combination of the servo amplifiers
102
,
104
and the transistors Q
1
, Q
2
. Hence, the resulting output current i
o
=V
i
/R
E
flows through the output terminals. The same signal appears on V
op
, V
om
, but it is 180 degrees out of phase with the signal on the emitter; that is, V
p
, V
m
.
However, the output voltage V
op
, V
om
swing is limited. The lower output swing of V
op
, V
om
is limited because transistors Q
2
and Q
1
must always be in the active region to provide the feedback path for the respective servo amplifier so that the feedback loop is closed. In the active region, the base-emitter diodes in transistors Q
2
and Q
1
are forward biased and the collector-emitter diodes are reverse biased. Thus, with both transistors Q
1
, Q
2
in the active region, there is a common-mode direct current (DC) voltage at V
om
, V
op
that is at least V
be
above ground because of the need to keep at least a V
be
voltage at Vm, Vp to keep transistors Q
2
and Q
1
forward biased. With low power supply voltages, for example, 2V, a common-mode D.C. voltage of 0.8V is significant.
FIG. 2
illustrates the lower output swing limitation of the prior art analog buffer shown in FIG.
1
. The common mode voltage at V
B1
is at least V
be
above the common mode voltage at V
ip
to keep transistor Q
2
in the active region. Thus, the lower output swing of the output signal V
om
is limited because the output voltage V
om
must be greater than the voltage at V
B1
to keep transistor Q
2
in the active region.
Returning to
FIG. 1
, the upper output swing of V
op
, V
om
is limited because collector currents I
2
and I
1
must be kept constant. The requirement to keep the collector currents constant is dependent on the implementation of the constant current sources I
2
, I
1
, but it generally requires a voltage drop across the constant current sources. Thus, the power supply voltage V
dd
limits the upper output swing and the feedback path through transistors Q
1
, Q
2
limits the lower output swing. The limitations on the upper and lower output swing results in harmonic distortion of the output signal.
SUMMARY OF THE INVENTION
An analog buffer which offers low harmonic distortion for an output signal with a wide voltage swing and low power supply voltage is presented. The output voltage swing is increased by adding a voltage level shifter to the feedback path of a servo amplifier. The servo amplifier receives an input signal at a first input. A bipolar transistor is coupled to the output of the servo amplifier. The emitter of the bipolar transmitter is coupled through a feedback loop to a second input of the servo amplifier. A voltage level shifter is coupled in the feedback loop and a current source pushes current into the voltage level shifter to cause a voltage shift at the emitter of the bipolar transistor to increase output voltage swing. The voltage shift increases the lower voltage swing. The upper output voltage swing may be increased, by coupling a load between the collector of the bipolar transistor and the upper power supply voltage. The current source maintains a constant voltage at the second input of the servo amplifier through the diode to keep the bipolar transistor in an active region.
The voltage level shifter may be a diode-coupled NPN or PNP transistor or a V
be
multiplier. The bipolar transistor may be NPN or PNP. The diode may be a diode-coupled NPN bipolar transistor. The current source may be a cascode current mirror. A second current source may be coupled to the collector of the bipolar transistor to center the output voltage swing in the middle of the power supply voltage range.


REFERENCES:
patent: 4739281 (1988-04-01), Doyle
patent: 5631968 (1997-05-01), Frey et al.
patent: 6066985 (2000-05-01), Xu
patent: 6232805 (2001-05-01), Brandt
patent: 6459332 (2002-10-01), Uchida
patent: 6724169 (2004-04-01), Majumdar et al.
Analog Integrated Circuit Design, David Johns and Ken Martin eds., (John Wiley & Sons Inc.), Chapter 6, pp. 266-268 (1997).
Analysis and Design of Analog Integrated Circuits, Paul Gray and Robert G. Meyer eds.,(John Wiley & Sons Inc.), Chapter 4, pp. 336-337 (1993).

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