Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Amplitude control
Reexamination Certificate
2002-10-17
2004-04-06
Wells, Kenneth B. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Amplitude control
C327S323000, C327S332000
Reexamination Certificate
active
06717449
ABSTRACT:
This application claims benefit of Japanese Application No. 2001-324970 filed in Japan on Oct. 23, 2001, the contents of which are incorporated by this reference.
BACKGROUND OF THE INVENTION
The present invention relates to variable resistance circuit implemented on an integrated circuit and having an equivalent resistance between terminals capable of being controlled (regulated) by an electrical signal to be applied to a control terminal, and also relates to application circuits using the variable resistance circuit such as an integrating (filter) circuit, gain control amplifier, automatic gain control circuit.
Among known filter circuits capable of regulating passband (Q-factor) are:
{circle around (1)} Switched Capacitor type filter;
{circle around (2)} Operational Transconductance Amplifier (OTA)-C type filter; and
{circle around (3)} MOSFET-C type filter.
These filters can be implemented on an integrated circuit.
Here, the passband or Q-factor in each filter is regulated (controlled) by:
{circle around (1)} clock (frequency) in Switched Capacitor filter;
{circle around (2)} regulating voltage or current of OTA in OTA-C filter; or
{circle around (3)} the gate voltage of MOSFET in MOSFET-C filter.
Shown in
FIGS. 1A
,
1
B are the configurations of a single-end MOSFET-C type filter and totally balanced MOSFET-C type filter as disclosed in FIGS. 7 and 2 of U.S. Pat. No. 4,509,019. The passband fc thereof and resistance Rds between the source and drain of MOS transistor are given by formulas (1), (2). Included in
FIGS. 1A
,
1
B are: operational amplifier
501
; capacitance
502
; MOS transistor
503
; control voltage source
504
; totally balanced operational amplifier
511
; capacitances
512
n
,
512
p
; MOS transistors
513
n
,
513
p
; control voltage source
514
; and reference voltage source
515
.
fc=
1/(2&pgr;×
C×Rds
) (1)
Rds=
1/{&mgr;
n×Cox×W/L×
(
Vx−Vth
)} (2)
&mgr;n: mobility of electron
Cox: capacitance of gate oxide film of MOS transistor
W: gate width of MOS transistor
L: gate length of MOS transistor
Vth: threshold voltage of MOS transistor The above formulas (1), (2) indicate that MOSFET-C type filter is an integrating circuit having resistance Rds formed between the drain and source of MOS transistor and controlled by voltage Vx to be applied between the gate and source and time constant achieved by capacitance C. The above formulas (1), (2) hold only when the MOS transistor is caused operate in a triode region.
FIG. 2
superimposes the triode region over Vgs-Ids characteristic of MOS transistor.
FIGS. 3A and 3B
show the triode region in an enlarged manner and a type of the drain-to-source resistance.
The drain-to-source resistance Rds of MOS transistor is a function of threshold voltage Vth, and such value is changed by ambient temperatures and variance in the manufacture of MOS transistor. For this reason, the passband of MOSFET-C type filter to be determined by the combination of the drain-to-source resistance Rds and capacitance C is also changed by the ambient temperatures and the variance in the manufacture of MOS transistor. Such change exceeds ±50% of a set target value. Because of such characteristic, the filters of the conventional configuration shown in
FIGS. 1A
,
1
B cannot be used in those applications where a high accuracy in passband is required.
Shown in
FIG. 4
is the configuration of a totally balanced Tow/Thomas type Biquad filter using the drain-to-source resistance of MOS transistor based on a similar operation principle (disclosed in (1) J. Tow, “Active RC Filters—a State—space Realization,” Proc. IEEE, Vol. 56 pp1137-1139, 1968, (2) L. C. Thomas, “The Biquad: Part I—some Practical Design Consideration,” IEEE Trans. Circuits and Syst., vol. CAS-18, pp 350-357, 1971, (3) T. C. Thomas, “Biquad: Part II:—A Multipurpose Active Filtering System,” IEEE Trans. Circuits and Syst., Vol. CAS-18 pp. 358-361, 1971.).
In this filter, of the resistances in
FIG. 5
for showing the principle of the biquad filter, the resistances Ra, Rb, Rc, Rd, except Rr to be used in producing inverting signal are replaced by the drain-to-source resistance Rdsan, Rdsap, Rdsbn, Rdsbp, Rdscn, Rdscp, Rdsdn, Rdsdp of MOS transistors Man, Map, Mbn, Mbp, Mcn, Mcp, Mdn, Mdp. The passband fc and Q-factor are expressed by formulas (3) to (11). It is to be noted that
FIG. 4
includes the totally balanced operational amplifiers
521
,
522
; capacitance Can, Cap, Cbn, Cbp; control voltage source
523
; and reference voltage source
524
.
fc=
1/(2&pgr;×
Rdsb×Rdsc×Ca×Cb
) (3)
Q
={square root over ( )}(
Ca/Cb
)×{
Rdsb
2
/(
Rdsc×Rdsd
)} (4)
Rdsx=
1/{&mgr;
n×Cox×W/L×
(
Vx−Vth}
(5)
x: a, b, c, d (n, p)
Rdsap=Rdsan=Rdsa
(6)
Rdsbp=Rdsbn−Rdsb
(7)
Rdscp=Rdscn=Rdsc
(8)
Rdsdp=Rdsdn=Rdsd
(9)
Can=Cap=Ca
(10)
Cbp=Cbn=Cb
(11)
Like formulas (1), (2), the formulas (3) to (11) hold when all MOS transistors are caused to operate in the triode region. Formula (4) indicates that, unlike the passband given by formulas (1) and (2), Q of Biquad filter is determined by the ratio of capacitance and the ratio of drain-to-source resistance Rds between the plurality of MOS transistors. If this filter is used with setting a high Q-factor such as >4, the signal processing characteristic is affected by the capacitance obtained at an integrated circuit or by the variance to be determined by the ratio of the drain-to-source resistances of the plurality of MOS transistors.
As has been described above, MOSFET-C type filter shown in
FIGS. 1A
,
1
B or in
FIG. 4
has restrictions as follows:
{circle around (1)} MOS transistors within the circuit must be operated in the triode region.
{circle around (2)} The passband is shifted ±50% or more from the standard value due to ambient temperature and variance in the manufacture of MOS transistor.
{circle around (3)} Although Q-factor, when compared with the passband, is not likely to be affected by variance in the manufacture of transistor and temperatures, a small change in its value affects signal processing in a setting of Q>4.
A conventional automatic gain control circuits (AGC) will now be described. The automatic gain control circuits are used in various circuits including receiving circuits of communication equipment which require amplification of wide dynamic range signals, read circuit of magnetic or optical disk device, code reader devices, oscillation circuits, etc.
Shown in
FIG. 6
is the configuration of automatic gain control circuit shown as an example in Japanese patent laid-open application Hei-6-208644. This automatic gain control circuit includes: amplifier circuit
552
including gain control means
551
; feedback circuit
553
; subtraction circuit
554
; and instruction signal
555
. The gain A thereof is determined as in formulas (12) and (13) by the internal circuits {operational amplifier
551
a
, MOS transistor
551
b
, resistors
551
c
(R
1
) and
551
d
(R
2
)} of the gain control means
551
.
A
=R
2
/R
1
×{1/(1+R
2
/
Rds
)} (12)
Rds=
1/{&mgr;
n×Cox×W/L×
(
Vgs−Vth
) (13)
&mgr;n: mobility of electron
Cox: capacitance of gate oxide film of MOS transistor
W: gate width of MOS transistor
L: gate length of MOS transistor
Vgs: gate-to-source voltage of MOS transistor
Vth: threshold voltage of MOS transistor
The output voltage of the automatic gain control circuit (in stable state) becomes constant if the gain at the amplifier means
552
and at the feedback means
553
result in a sufficiently large negative-feedback loop. The gain of the system is determined by the characteristic of MOS transistor as indicated by formulas (12) and (13). For
Saito Masashi
Tamiya Kosei
Ueyama Tetsuji
Olympus Corporation
Pokotylo John C.
Straub & Pokotylo
Wells Kenneth B.
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