Amplifiers – With semiconductor amplifying device – Including push-pull amplifier
Reexamination Certificate
2002-05-21
2003-03-18
Mottola, Steven J. (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including push-pull amplifier
C330S268000
Reexamination Certificate
active
06535064
ABSTRACT:
FIELD OF THE INVENTION
The present disclosure relates to a current-feedback amplifier and more specifically to a current-feedback amplifier having reduced crossover distortion with phase delay eliminated.
RELATED ART
FIG. 1
depicts a generalized current-feedback amplifier (CFA). The circuit of
FIG. 1
includes an input stage
10
and an output stage
20
. The input stage
10
includes an input port (IN) providing a signal to the base of an NPN bipolar junction transistor (BJT)
104
and a PNP BJT
116
. The collector of the transistor
104
is connected directly to a first common voltage line V+, while its emitter is connected through a current sink
114
to a second common voltage line V−. The collector of the transistor
116
is connected directly to V−, while its emitter is connected through current sink
120
to V+.
The input stage
10
further includes emitter follower transistors
124
and
132
. A transistor
124
has a base connected to the emitter of transistor
116
, while transistor
132
has a base connected to the emitter of transistor
104
. The collector of the transistor
124
is connected to an input of a first current mirror
128
. The collector of the transistor
132
is connected to an input of a second current mirror
136
. The emitter transistor
124
and the emitter of transistor
132
are connected to form a common node n
10
. The current mirror
128
has an output connected to the output of the current mirror
136
at node n
20
. A capacitor
140
connects node n
20
to ground.
In the output stage
20
, the base of transistor
142
is connected to node n
20
, while its emitter is connected to V+ through a current source
146
and its collector is connected directly to V−. The base of a transistor
150
is also connected to node n
20
. The collector of the transistor
150
is connected directly to V+, while its emitter is connected to V− through a current sink
154
.
The output stage
20
further includes emitter follower transistors
158
and
160
. The base of transistor
158
is connected to the emitter of transistor
142
, and the base of transistor
160
is connected to the emitter of transistor
150
. The collector of transistor
158
is connected to V+, while the collector of transistor
160
is connected to V−. The emitter of transistor
158
and transistor
160
are connected together to form an output node n
30
. The output node n
30
is connected by a feedback resistor
172
, having a value R
F
, to node n
10
. A load resistor
176
, having a value R
L
, connects the output node n
30
to ground.
FIG. 2
depicts a simplified CFA that results if the fifth and sixth BJTs are replaced by diodes.
FIG. 2
depicts an input stage
10
A that is identical to the input stage
10
of FIG.
1
. Note that components carried over from
FIG. 1
to
FIG. 2
are similarly labeled, as will be components carried over from
FIG. 1
or other figures into subsequent figures. In the output stage
20
A, the capacitor
140
, transistors
142
and
150
and current sources
146
and
154
have been removed between the two current mirrors
128
and
136
, relative to FIG.
1
. The diodes
240
and
242
are connected in series between the current mirrors
128
and
136
. The base of transistor
158
now connects to the output of the current mirror
128
, while the base of transistor
160
connects to the output of current mirror
136
.
The diodes
240
and
242
of the simplified CFA of
FIG. 2
are replacements for emitter-followers used in the design of FIG.
1
. Eliminating emitter-followers eliminates phase delay over frequency due to limited FT in the transistors. Therefore, the potential bandwidth may be extended beyond that of the classic CFA which includes emitter-followers. However, the downside of the diode replacement is that there is less current gain around the feedback loop. Nevertheless, reduced current gain increases the amplifier's output impedance and provides less suppression of internal distortion.
Often such a simplified CFA, as shown in
FIG. 2
, is used as an output stage within an overall amplifier. The bandwidth increase in the simple CFA allows more bandwidth in the externally compensated overall amplifier. Furthermore, the simple CFA is also much more linear than a simple emitter-follower output stage.
FIGS. 3 and 4
depict half sine wave input signals
300
and
400
. The positive sine wave signal
300
is formed from a normal sine wave signal with all negative values of the sine wave attenuated. The negative sine wave signal
400
is formed from the same normal sign wave as is used to form the positive wave signal
300
, however in the negative sine wave signal, all positive values for the sine wave are attenuated. The positive sine wave signal
300
is passed to the positive current mirror
128
and the negative sine wave signal
400
is passed to the negative current mirror
136
. Each current mirror
128
and
136
replicates the signal on the output of the particular current mirror into which the signal is input.
Dynamic problems occur when trying to pass these half-sine waves
300
and
400
. While devices will traverse trajectories of on-state to off-state fairly faithfully, it is very difficult for a device to immediately traverse from fully-off to suddenly on. In particular, when a current mirror is turned off, voltages across the devices composing the current mirror lag to small values at rates limited by the capacitances within the devices of the current mirror. Thus, there will be enough time for the current mirror's devices to turn fully off in the off half-cycle of current. From the fully off state, the current mirror is called upon to turn on during its off half-cycle. To pass the current in an undistorted manner, the voltages across the devices within the current mirror must come to an on-state virtually immediately. However, capacitances of the device terminals prevent that immediate change of voltage. Thus, the output of the current mirror will not accurately respond to the signal. This signal distortion caused by the rapid change in states demanded of the current mirrors is a form of cross-over distortion. At every zero crossing of output current the switching of current from one set of devices, an output error will be introduced into the signal due to the lag in current mirrors' response in switching from a fully off state to a fully on state. Therefore, what is needed is an amplifier that produces an amplified signal exhibiting less distortion at zero cross-over points.
SUMMARY
The disclosure describes a current feedback amplifier (
FIG. 5
) output stage that contains an additional pair of emitter follower transistors
538
and
540
with a capacitor
548
connecting the emitters of transistors
538
and
540
to ground to reduce discontinuities in the output current. The introduction of the pair of transistors
538
and
540
and the capacitor
548
allows each current mirror
128
and
136
to be turned on prior to the time the particular current mirror is required to control the output of the amplifier. By having the non-dominant current mirror capacitively turned on prior to the time it is required to dominate the output signal allows the non-dominant current mirror to more accurately replicate the input signal at the time it is required to dominate the output. Thus, signal glitches due to switching on of the current mirror, as occur in classic AB amplifiers as shown in
FIG. 2
, are avoided and less signal distortion results in the output signal.
REFERENCES:
patent: 4446443 (1984-05-01), Johnson et al.
patent: 5128631 (1992-07-01), Feliz et al.
patent: 5973563 (1999-10-01), Seven
Elantec Semiconductor Inc.
Fliesler Dubb Meyer & Lovejoy LLP
Mottola Steven J.
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