Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – With specific source of supply or bias voltage
Reexamination Certificate
2002-04-23
2003-12-02
Cunningham, Terry D. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
With specific source of supply or bias voltage
C323S315000
Reexamination Certificate
active
06657481
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to current source biasing circuitry for various electronic devices, including but not limited to electronic circuitry in mobile telecommunication terminals. More specifically, the present invention relates to a current mirror circuit having an input portion with a first transistor, an output portion with a second transistor, and a control portion between the input portion and the output portion for controlling the second transistor to generate an output current which is a function of a reference current established by the first transistor.
BACKGROUND ART
Generally, biasing serves to establish a selected operating range in the voltage-current characteristics of non-linear devices such as transistors and diodes, so that gross non-linearities of the non-linear devices will be avoided and so that, therefore, the non-linear devices will behave like or resemble linear devices.
Current source biasing serves to provide a constant DC current source for the circuit to be biased. Current mirrors provide current source biasing which is substantially independent of device temperatures and device parameters, such as the forward DC current gain &bgr;
f
of a bipolar junction transistor (BJT), or the conductance parameter (K) and the threshold voltage (V
TR
) for a field-effect transistor (FET). Current mirrors are described in detail in sections 7.8-7.10, pp 302-314 of “Microelectronic Circuits & Devices”, by Mark N Horenstein, Prentice-Hall, Inc., 1990, ISBN 0-13-584673-0, incorporated herewith by reference.
As is shown in
FIG. 2
, a prior art current mirror uses a pair of first and second three-terminal devices T
1
and T
2
, coupled “back-to-back” with their input ports connected in parallel. T
1
and T
2
may be of arbitrary type but are illustrated as bipolar junction transistors in FIG.
2
. T
1
and T
2
are connected to ground through their emitter terminals and first and second resistors R
1
, R
2
, respectively. The transistor T
1
and resistor R
1
form an input portion
152
, which receives an input current i
in
and establishes a reference current i
ref
. Correspondingly, the transistor T
2
and resistor R
2
form an output portion
154
, which serves to supply an output current i
out
to a circuit
160
to be biased. To this end, a control portion
156
is formed between the input portion
152
and the output portion
154
.
In the prior art solution according to
FIG. 2
, the control portion
156
simply consists of a connection between the collector terminal of transistor T
1
and the base terminal of transistor T
2
, having a drawback in that some current i
t
will be stolen from the input current i
in
to control the transistors T
1
and T
2
.
In another prior art solution, shown in
FIG. 3
, the above is remedied by introducing a third transistor T
3
into the circuit. Transistor T
3
is biased trough V
cc
and will enable the current mirror to operate without stealing current from i
in
. The transistor T
3
may be, for instance, a bipolar junction transistor or an NMOS field-effect transistor. If T
3
is a bipolar junction transistor, the stolen current i
t
will be close to 0, and if T
3
is an NMOS, i
t
will equal to 0 (wherein i
ref
will be equal to i
in
).
However, the present inventors have identified a remaining problem with the prior art solution according to FIG.
3
. At node
164
in the input portion
152
, when looking both towards transistor T
1
and towards input node
162
, the impedances seen will be very high, particularly when BICMOS circuits are used. Therefore, any noise introduced at node
164
will go entirely across the control portion
156
to transistor T
2
in the output portion
154
, thereby corrupting the output signal at node
166
. Corruptions in the output signal mean that the following circuit
160
will not be biased with a perfect DC current source, and if this circuit is designed to rely on a perfect DC current source, the operation thereof may be jeopardized.
SUMMARY OF THE INVENTION
In view of the above, an objective of the invention is to solve or at least reduce the problems discussed above and to provide an improved current mirror technique compared to the prior art.
Generally, the above objective is achieved by a current mirror circuit according to the attached independent claim.
Thus, a first embodiment of the invention concerns a current mirror circuit comprising: an input portion including a first transistor, wherein the first transistor is adapted to establish a reference current; an output portion including a second transistor; and a control portion between the input portion and the output portion, the control portion including a third transistor coupled for controlling the second transistor to generate an output current which is a function of the reference current while inhibiting current leakage from the input portion to the output portion.
The objective of the invention has been achieved, in the first embodiment, by introducing a lowpass filter in the control portion prior to the third transistor. The lowpass filter will filter the signal that controls the second transistor, thereby allowing any noise from the input portion to be filtered out before it is amplified by the second transistor to generate the output current. Additionally, including the lowpass filter within the current mirror circuit has an advantage compared to external filtration outside/after the current mirror, since additional power would be consumed in the latter case.
In a second, more sophisticated embodiment, the lowpass filter is an RC filter which, as its resistive part, comprises a fourth transistor which is biased by a feedback loop into a state of high impedance. The state of high impedance may be the resistive (also known as triode or ohmic) region of operation for the fourth transistor, which may be a PMOS field-effect transistor.
Using a PMOS field-effect transistor instead of a resistor in an RC filter is advantageous for the following reasons. Due to the very high impedances in the input portion, a resistor would need to be very large (in the order of 1 G&OHgr;) to establish a proper low filtering frequency. However, resistors with such high impedance are very hard to build. Alternatively, the capacitor in the RC filter could be made very large, but that would be very costly because of the physical size of a large capacitor—silicon chip space is very expensive.
Moreover, by selecting a PMOS transistor, the gate-source voltage for the third transistor can be chosen so that it is sufficient to bias the fourth transistor, wherein no other components are necessary. By biasing the fourth transistor in a feedback loop, the ohmic characteristics thereof will be stabilized. Consequently, this embodiment of the invention overcomes a known problem in the technical field—because of the inherently irregular (non-linear) ohmic characteristics of a PMOS transistor it is difficult to include a PMOS transistor in a lowpass filter of RC type.
In either of the embodiments the third transistor may be an NMOS field-effect transistor or another type of field-effect transistor. The first and second transistors may be bipolar junction transistors, but essentially any other three-terminal devices would also do.
In a third embodiment, the lowpass filter comprises a plurality of cascaded RC filters, each of which has a PMOS field-effect transistor as its resistive part. This arrangement allows efficient noise filtering at a low component cost.
Generally, the current mirror circuit according to the invention may be included in any application using current mirrors. In one embodiment the current mirror circuit according to the invention is included in a station for a mobile telecommunications network. The station may be a mobile terminal.
Other objectives, features and advantages of the present invention will appear from the following detailed disclosure, from the attached dependent claims as well as from the drawings.
Generally, all terms used in the claims are to be interpreted according to their ordinary meaning
Nielsen Ivan Riis
Rasmussen Carsten
Antonelli Terry Stout & Kraus LLP
Cunningham Terry D.
Nokia Corporation
Tra Quan
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