Miscellaneous active electrical nonlinear devices – circuits – and – Specific input to output function – Combining of plural signals
Reexamination Certificate
2003-07-10
2004-08-10
Callahan, Timothy P. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific input to output function
Combining of plural signals
C327S411000, C327S356000
Reexamination Certificate
active
06774699
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
This invention relates to analog mixers or multipliers, in particular, to electronic mixers or multipliers using bipolar transistors. Such mixers are used for multiplying two input signals or for converting two difference frequency signals into a beat frequency output.
(2) Brief Description of Related Art
In a conventional radio receiver, the incoming radio frequency is mixed with a local oscillator (LO) signal to produce a beat frequency, which is the intermediate frequency (IF). The IF is then amplified and filtered to attenuate other unwanted signals,
A popular mixer circuit is the differential pair or Gilbert multiplier. A bipolar transistor version of the Gilbert multiplier is shown in FIG.
1
. Basically, a differential amplifier with a differential pair Q
2
and Q
3
is fed from a current source Q
1
. The differential gain of the differential amplifier is proportional to the transconductance gm of Q
2
and Q
3
. This transconductance varies as the dc collector current of Q
2
and Q
3
, which is controlled by the collector current of Q
1
. The dc collector current I
C1
of Q
1
is controlled by the dc base current V
B1
of Q
1
. When a local oscillator signal V
LO
of frequency f
LO
is applied differentially to Q
2
an Q
3
(i.e. V
LO+
=Vin−/2 and V
LO−
=Vin+/2 respectively), and a radio frequency signal V
rf
of frequency f
rf
is applied to the base of Q
1
, the output current of the differential amplifier is equal to V
LO
*gm
2
, and the gm
2
is proportional to Vrf*gm
1
. When the V
rf
is multiplied by V
LO
, a beat frequency f
if
=f
rf
±f
LO
intermediate frequency signal V
if
is produced.
While the Gilbert multiplier is widely used, it has a number of drawbacks for low voltage and low power applications. In modern integrated circuit technology, the tendency is to use a low supply voltage V
CC
: for instance 25 V for 0.25 &mgr;m technology and 1.8 V for 0.18 &mgr;m technology. In the Gilbert mixer, the current source is operating in the active region of the V
CE1
vs I
C1
V-I characteristic Q
1
in
FIG. 2
to obtain a higher transconductance and is proportional to V
rf
. Therefore the dc collector voltage V
C1
for the current source Q
1
must not forward bias the collector junction, i.e. about the same as the dc base voltage (≈0.7V for silicon BJT) to operate in the active region.
Similarly, the differential pair Q
2
and Q
3
also must have its collector voltage high enough to operate in the active region. Since the dc base voltage is about 0.7V higher than the emitter voltage of Q
2
or V
C1
(≈0.7V), the dc base voltage of Q
2
, hence the dc collector voltage, must be at least about 2×0.7V or 1.4V. This voltage is higher than certain kind of battery voltage of 1.3 V.
If a resistor is used as a load, another voltage drop V
L
will be added to be supplied by the power supply V
CC
. These three stacks of voltages, V
CE1
, V
CE2
and V
L
, dictate that the supply voltage cannot be made very low. For a typical base-to-emitter voltage of 0.7 V, there is hardly any “head room” for signal voltage swing. Besides, a higher supply voltage consume more power.
In U.S. Pat. No. 6,388,501, Chien disclosed an MOSFET mixer operating at the knee of the drain V-I characteristics of the MOSFET. Since a MOSFET is gate voltage control device while a BJT is a base current control device and since the drain and source characteristics of a MOSFET are symmetrical but the collector and emitter characteristics of a BJT are not symmetrical, Chien's circuit is not applicable to a BJT mixer.
SUMMARY OF THE INVENTION
An object of this invention is to design a BJT mixer which requires a lower supply voltage than the Gilbert mixer or similar structure. Another object of this invention is to reduce the power consumption of the BJT mixer. Still another object of this invention is to provide a high conversion gain of the mixer. A further object of this invention is to set the operating point of the mixer at its optimum conversion gain automatically
These objects are achieved by mixing the RF signal and the local oscillator signal at the knee of the output V
CE
-I
C
characteristic of a BJT as shown in
FIG. 2
with constant base current I
B1
. At the knee, the characteristic has the sharpest curvature. The nonlinearity produces a maximum beat frequency signal. For implementation, a mixer BJT is biased at the knee of the V
CE
-I
C
characteristic. The LO (or RF) signal voltage V
LO
(or V
rf
) is applied at the base of the mixer BJT, and the RF (or LO) signal voltage V
rf
(or V
LO
) is injected at the collector of the mixer BJT. Then a beat frequency drain current is produced. Specifically, the gate of a single-ended mixer BJT is fed with a local oscillator signal and the collector of the mixer is dc coupled to a single-ended emitter follower with the base fed from a radio frequency signal or vise versa.
REFERENCES:
patent: 5784692 (1998-07-01), Kleinberg
patent: 6388501 (2002-05-01), Chien
Callahan Timothy P.
Cox Cassandra
Lin H. C.
Maryland Semiconductor Inc.
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