Amplifiers – With semiconductor amplifying device – Including gain control means
Reexamination Certificate
2001-09-14
2002-07-30
Shingleton, Michael B (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including gain control means
C330S281000, C330S282000, C330S284000, C330S285000
Reexamination Certificate
active
06426677
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to bipolar junction transistor (BJT) amplifiers, and more particularly to linearization bias circuit for BJT amplifiers that extends the compression point by keeping transconductance constant in the presence of varying input voltage amplitudes of an input radio frequency (RF) signal.
DESCRIPTION OF RELATED ART
The 1 decibel (dB) compression point for a radio frequency (RF) bipolar junction transistor (BJT) amplifier has both an input and an output component. The output component results from the output power being limited, depending upon the load, either by voltage swing clipping or by the load current exceeding the DC collector current. The output limited compression point should be reasonably sharp. The input component of compression results from the exponential relationship between base-emitter voltage and collector current. As the peak of the AC component of the base-emitter voltage approaches the thermal voltage V
T
, the transconductance and input impedance of the BJT amplifier begin to drop off. These effects produce a gradual drop in gain which gives a softer compression point.
The key BJT relationships are now briefly discussed. The collector current i
c
of a BJT, when the base-emitter voltage V
BE
includes an AC term, is provided according to the following equations 1 and 2:
i
C
≈
I
s
exp
(
V
BE
V
T
)
=
I
s
exp
(
V
D
C
V
T
)
exp
[
V
P
V
T
cos
(
ω
t
)
]
V
BE
=
V
D
C
+
V
P
cos
(
ω
t
)
(
EQ
1
)
i
C
≈
i
D
C
[
I
0
(
x
)
+
2
I
1
(
x
)
cos
(
ω
t
)
+
2
I
2
(
x
)
cos
(
2
ω
t
)
+
2
I
3
(
x
)
cos
(
3
ω
t
)
+
…
]
(
EQ
2
)
where I
s
is a constant describing the transfer characteristic of the BJT, “exp” denotes the exponential function (natural logarithm), V
DC
is the average DC applied base-emitter voltage, V
P
is the peak voltage of the input RF signal, “Cos” denotes the cosine function, &ohgr; is the fundamental radian frequency of the input signal (&ohgr;=2&pgr;f, where “f” is the fundamental frequency), “t” denotes time, i
DC
is the DC collector current due to the average DC applied base-emitter voltage V
DC
, “x” is the signal level of an input signal V
IN
normalized to the thermal voltage V
T
, and I
n
(x) is the modified Bessel function of the first kind of order “n”.
The DC collector current i
CDC
and base current i
BDC
as well as the peak of the fundamental collector current i
C
(&ohgr;) are provided in the following equation 3:
i
C
D
C
=
i
D
C
I
0
(
x
)
i
B
D
C
=
i
D
C
I
0
(
x
)
β
i
C
(
ω
)
=
i
D
C
2
I
1
(
x
)
i
D
C
≡
I
s
exp
(
V
D
C
V
T
)
(
EQ
3
)
where beta (&bgr;) is the ratio of collector current to base current of the BJT. The zero order Bessel function I
0
(x) {modified of the first kind} is a scale factor for the DC that accounts for the increase due to rectification of the AC voltage applied to the base terminal of the BJT. Likewise the first order Bessel function I
1
(x) is a scale factor for the current at the fundamental frequency. The DC current i
DC
is defined as the collector current due to the average (DC) applied base-emitter voltage V
DC
.
The notation i
DCN
denotes the collector current due to the average base-emitter voltage of a particular transistor numbered “N”. Although i
DC
is not a function of the signal level x, the notation i
DC
(x) denotes the value of i
DC
forced by the bias circuit when a signal level x is applied to its base-emitter junction. Zero “0” will be substituted for “x” in the special case when no AC voltage is applied to the base-emitter junction.
A linear circuit has constant gain which means the transconductance g
m
should be constant and independent of signal level as indicated in the following equation 4:
g
m
=
i
C
(
ω
)
V
p
=
(
i
D
C
V
T
)
[
2
I
1
(
x
)
x
]
(
EQ
4
]
Many amplifiers are biased with constant base currents. For example, the base of the amplifier BJT is connected to a reference voltage through a larger value resistor or a current mirror. If the DC base current is constant, so is the DC collector current. For these currents to remain constant with increasing signal level, the DC base-emitter voltage must decrease as indicated by the following equations 5 and 6:
i
D
C
(
x
)
I
0
(
x
)
=
i
D
C
(
0
)
I
0
(
0
)
=
i
D
C
(
0
)
→
i
D
C
(
x
)
=
i
D
C
(
0
)
I
0
(
x
)
Δ
V
D
C
=
-
V
T
ln
(
I
0
(
x
)
)
(
EQ
5
)
g
m
(
x
)
=
[
i
D
C
(
0
)
V
T
]
[
2
I
1
(
x
)
xI
0
(
x
)
]
(
EQ
6
)
where “In” denotes that natural logarithm function. The transconductance decreases with increasing signal level starting for values of x of about one. A different bias condition is required for constant transconductance as indicated by the following equation 7:
g
m
=
[
i
D
C
(
x
)
V
T
]
[
2
I
1
(
x
)
x
]
=
i
D
C
(
0
)
V
T
→
i
D
C
(
x
)
=
i
D
C
(
0
)
[
x
2
I
1
(
x
)
]
(
EQ
7
)
Usually RF BJT amplifier stages have biases which apply either a constant (i.e. DC) base current through an RF isolating resistor or constant base-emitter voltage through an isolating inductor. The base current is often generated by forcing a reference current through a current mirror. A base-emitter voltage bias is often generated by forcing the reference current through a similar BJT, possibly with a different emitter area.
It is desired to provide a bias control circuit for a BJT amplifier that establishes linear operation in the presence of varying input voltage amplitude of an input RF signal.
SUMMARY OF THE PRESENT INVENTION
A linearization bias circuit for a bipolar junction transistor (BJT) amplifier according to an embodiment of the present invention includes a reference circuit, a current device and a transconductance amplifier. The BJT amplifier includes base, collector and emitter terminals in which the base terminal receives an input radio frequency (RF) signal. The reference circuit includes a reference terminal and a control terminal, where the control terminal is coupled to the base terminal of the BJT amplifier to control its operating point based on signal level of the RF signal. The current device provides a constant reference current to the reference terminal of the reference circuit, where the constant reference current has a level that is based on a desired collector current of the BJT amplifier. The transconductance amplifier has an input coupled to the reference terminal and an output coupled to the control terminal of the reference circuit, where the transconductance amplifier asserts its output to maintain the constant reference current into the reference terminal of the reference circuit. The reference circuit applies a predetermined relationship between DC and AC scale factors of collector current of the BJT amplifier. In this manner, the transconductance amplifier controls the base terminal to modify the operating point of the BJT amplifier to substantially maintain constant transconductance in the presence of varying input voltage amplitudes of the input RF signal.
The transconductance amplifier
Intersil America's Inc.
Shingleton Michael B
Stanford Gary R.
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