Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – Nonlinear amplifying circuit
Reexamination Certificate
2000-10-13
2002-09-24
Callahan, Timothy P. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
Nonlinear amplifying circuit
C327S359000
Reexamination Certificate
active
06456158
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to ways of controlling the transconductance of a differential stage with active load followed by a cascode current follower (transconductor) in discrete steps. More particularly, the present invention proposes a transconductor with a digitally programmable transconductance and substantially constant DC operating point. The present invention also proposes an accurate transconductance setting that depends on a master value and on ratios of similar components integrated on the same chip.
The basic setting of the transconductance of a differential stage is through a tail current. The DC operating point is also dependent on the value of the tail current. There are certain circuit configurations, like programmable amplifiers or filters, where changing the transconductance has to be done in discrete steps, and without affecting other parameters such as the distortion level.
FIG. 1
shows a conventional digitally-programmable transconductor circuit. The transconductor circuit presented in
FIG. 1
is derived from a source degenerated differential pair. It includes a current generator
30
, right and left precision transconductors
40
and
50
, and a degeneration resistance
60
. The current generator
30
includes a left current generator
32
and a right current generator
34
. The right and left precision transconductors
40
and
50
each include a right or left operational amplifier
44
,
54
and a right or left PMOS transistor
46
,
56
. The PMOS transistor
46
,
56
passes a right or left current I
L
or I
R
, and is controlled by the output of the corresponding operational amplifier
44
,
54
. Each of the right or left operational amplifier
44
,
54
accepts a corresponding left or right voltage V
L
or V
R
at a non-inverting input
42
,
52
and a feedback loop from the degeneration resistance
60
at a negative input
43
,
53
. The degeneration resistance
60
includes a plurality of degeneration resistors R
D1
, R
D2
, R
D3
, R
D4
, and R
D5
and a plurality of programming switches S
P1
, S
P2
, S
P3
, S
P4
, S
P5
, and S
P6
. The degeneration resistors can be classified as first and second left resistors R
D1
and R
D2
, a center resistor R
D3
, and first and second right resistors R
D4
and R
D5
.
The right and left precision transconductors
40
and
50
take their feedback from taps on the plurality of degeneration resistors R
D1
, R
D2
, R
D3
, R
D4
, and R
D5
through the plurality of programming switches S
P1
, S
P2
, S
P3
, S
P4
, S
P5
, and S
P6
. These switches are controlled by a plurality of switch control signals C
1
to C
3
.
Through the selection of a particular pair of taps the resulting degeneration resistance can be properly divided. The five degeneration resistors are divided by the switches into a central resistance R
C
, a right lateral resistance R
RL
, and a left lateral resistance R
LL
. The lateral resistances R
RL
and R
LL
are included in the respective feedback loops of the precision transconductors
40
and
50
, and the central resistance passes a side current I
S
. The feedback of the precision transconductors
40
and
50
forces the input voltage across the resultant center resistance R
C
.
Table 1 below shows an example of how the central resistance R
c
and the lateral resistances R
RL
and R
LL
are determined based on the status of the programming switches S
P1
, S
P2
, S
P3
, S
P4
, S
P5
, and S
P6
.
TABLE 1
S
P1
S
P2
S
P3
S
P4
S
P5
S
P6
R
RL
R
LL
R
C
OFF
ON
OFF
OFF
ON
OFF
R
D5
R
D1
R
D2
+ R
D3
+ R
D4
OFF
OFF
ON
ON
OFF
OFF
R
D4
+ R
D5
R
D1
+ R
D2
R
D3
The central resistance R
c
defines the AC current generated by the transconductor. By changing the position of the taps, the value of the resistor exposed to the input voltage changes. This yields an equivalent transconductance as follows:
g
m
=
I
R
-
I
L
V
R
-
V
L
=
1
R
C
(
1
)
Another drawback of this circuit becomes apparent at high frequency, where it is necessary to have high speed amplifiers drawing important currents for the feedback to be effective.
An implementation of a continuously adjustable transconductance circuit is presented in FIG.
2
. This continuously adjustable transconductance circuit includes first and second precision transconductors
210
and
220
, first through third tunable transistors T
TUN1
, T
TUN2
, and T
TUN3
, a plurality of resistors R connected between inputs of the transconductors
210
and
220
, a capacitor C connected between outputs of the transconductors
210
and
220
, and a variety of transistors T and current sources
260
.
The precision transconductors
210
and
220
each include an operational amplifier
212
,
222
and a transistor T
T1
, T
T2
, and the transconductors
210
and
220
are connected to have degeneration resistor.
The output currents i
out1
and i
out2
of the circuit are steered by the tunable transistors T
TUN1
, T
TUN2
, and T
TUN3
into the inputs of a folded-cascode. Complementary weighted currents are summed on the low impedance of the folded-cascode, providing opposite AC currents to the outputs.
Each of the tunable transistors T
TUN1
, T
TUN2
, and T
TUN3
provide a respective tunable resistance R
TUN1
, R
TUN2
, or R
TUN3
. The resistance presented by each of the tunable transistors T
TUN1
(R
TUN1
), T
TUN2
(R
TUN2
), and T
TUN3
(R
TUN3
) varies with first and second control voltages V
1
, and V
2
supplied to the inputs of the transistors T
TUN1
, T
TUN2
, and T
TUN3
. If, for example, the first and third tunable transistors T
TUN1
and T
TUN3
are identical, then the first and third tunable resistances will also be identical (R
TUN1
=R
TUN3
), since they both receive the first control voltage V
1
. For differential output currents from the transconductor i
1
=i
i
, i
2
=(−i
i
), we have:
i
A
=
(
R
TUN2
2
R
TUN1
+
R
TUN2
)
i
1
(
1
)
i
B
=
-
(
R
TUN2
2
R
TUN1
+
R
TUN2
)
i
1
(
2
)
The fraction
R
TUN2
2
R
TUN1
+
R
TUN2
of the current generated by the input transconductor that is distributed to the output changes with R
TUN1
=R
TUN3
, R
TUN2
, i.e., this fraction of the current is a function of R
TUN1
, R
TUN2
, and R
TUN3
. The global transconductance appears as a fraction of the input stage transconductance. This ratio is voltage controlled. The dependence of the output current on the individual “resistor” values is not linear unless by electronic means the sum (2R
TUN1
+R
TUN2
) is kept constant.
The current sources
260
are preferably bias current sources, and the resistors R form a main transconductance setting. In this case, the transconductance of the stage is a fraction (depending upon V
1
, and V
2
) of (1/R).
Another way of steering the current of the input transconductor is shown in FIG.
3
. The circuit of
FIG. 3
includes an input transconductor
305
, voltage control current steering circuit
310
, a common mode feedback circuit
330
, and a plurality of transistors T.
The input transconductor
305
includes first and second sections
350
and
360
, each functioning as a differential amplifier. The first section
350
includes first through fourth transistors T
1
, T
2
, T
3
, and T
4
. The second section
360
includes fifth through seventh transistors T
5
, T
6
, and T
7
.
The voltage controlled current steering circuit
310
includes eighth through eleventh transistors T
8
, T
9
, T
10
, and T
11
, formed into two differential pairs. The eighth and ninth transistors T
8
and T
9
form one differential pair, and the tenth and eleventh transistors T
10
and T
11
, form the other differential pair.
A fraction of the current generated by the input transconductor
305
is transmitted to the outputs i
out1
and i
out2
through a voltage controlled current steering circuit composed of the two differential pairs (formed from the differential transistors T
8
, T
9
, T
10
, and T
11
). The circuit has the disadvantages of requiring a high supply voltage to accommodate the various stacked stages, and e
Callahan Timothy P.
Nguyen Linh
Oki America Inc.
Volentine & Francos, PLLC
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