Low-voltage, current-folded signal modulators and methods

Details Low-voltage, current-folded signal modulators and methods Low-voltage, current-folded signal modulators and methods

2000-04-06

2001-10-09

Mis, David (Department: 2817)

Modulators

Amplitude modulator

Including discrete semiconductor device

C323S315000, C330S288000, C455S333000

Reexamination Certificate

active

06300845

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to apparatus and methods for signal modulation. More specifically, this invention relates to apparatus and methods for low-voltage, current-folded signal modulator circuits.
A signal modulator circuit multiplies a first signal by a second signal, and may be used to shift the frequency components of the first signal from one frequency band to another frequency band. For example, a signal modulator circuit may be used to modulate a low frequency baseband signal (the first signal) by a high frequency carrier signal (the second signal), such as modulating a telephone signal that has frequency components from approximately 300-3400 Hz onto a higher frequency carrier signal (e.g. a 2 MHZ carrier signal). A modulator also may be used to shift a low frequency voice signal to a high frequency radio carrier signal for transmission over long distances. Modulating a signal onto a higher frequency carrier signal is called up-conversion. A signal modulator also may be used to down-convert a signal to lower frequencies.
One previously known signal modulator circuit
10
, commonly called a Gilbert-cell mixer, is shown in schematic diagram form in FIG.
1
. Gilbert-cell mixer
10
includes transconductance amplifier
12
and cross-coupled differential pair
14
. Gilbert-cell mixer
10
receives differential first signal (V
IN
+
−V
IN
−
) and second signal (V
LO
+
−V
LO
−
), and provides differential output signal (V
OUT
+
−V
OUT
−
). First signal (V
IN
+
−V
IN
−
) may be a baseband signal, and second signal (V
LO
+
−V
LO
−
) may be a high frequency modulation signal generated by a local oscillator. Output signal (V
OUT
+
−V
OUT
−
) is the modulated output.
Transconductance amplifier
12
includes current source
11
, emitter resistors
13
A and
13
B, and transistors
15
and
16
. Cross-coupled differential pair
14
includes differential pair transistors
17
and
18
, differential pair transistors
19
and
20
, and resistors
21
and
22
. Differential pair transistors
17
and
18
are cross coupled with differential pair transistors
19
and
20
.
Transistor
15
has a collector coupled to emitters of transistors
17
and
18
, a base coupled to input V
IN
+
, and an emitter coupled to a first terminal of resistor
13
A. Transistor
16
has a collector coupled to emitters of transistors
19
and
20
, a base coupled to input V
IN
−
, and an emitter coupled to a first terminal resistor
13
B. The second terminals of resistors
13
A and
13
B are coupled to GROUND through current source
11
. Transistor
17
has a base coupled to input V
LO
+
and a collector coupled to supply voltage V
CC
through resistor
21
. Transistor
18
has a base coupled to input V
LO
−
, and a collector coupled to supply voltage V
CC
through resistor
22
. Transistor
19
has a base coupled to input V
LO
−
, and a collector coupled to supply voltage V
CC
through resistor
21
. Transistor
20
has a base coupled to input V
LO
+
, and a collector coupled to supply voltage V
CC
through resistor
22
. Modulated output signals V
OUT
+
and V
OUT
−
are provided at the collectors of transistors
17
and
20
, respectively. Transconductance amplifier
12
converts differential first signal (V
IN
+
−V
IN
−
) into differential current signal I
X
=(I
X
+
−I
X
−
). Cross-coupled differential pair circuit
14
modulates differential current signal (I
X
+
−I
X
−
) by second signal (V
LO
+
−V
LO
−
) to produce differential output signal (V
OUT
+
−V
OUT
−
)
For many applications (e.g. battery powered cellular telephones), it is desirable to implement a signal modulator that consumes as little power as possible, thereby minimizing its energy needs. The power consumption of a signal modulator circuit is proportional to the supply voltage used to power the circuit. Thus, using a lower supply voltage advantageously reduces the power consumption of the circuit. There are, however, inherent constraints in the Gilbert-cell mixer that set a lower limit on the supply voltage for the circuit.
For example, in circuit
10
, supply voltage V
CC
may be expressed as:
V
CC
=V
R-21
+V
CE-17
+V
CE-15
+V
R-13A
+V
I-11
  (1)
where V
R-21
is the voltage drop across resistor
21
, V
CE-17
is collector-emitter voltage of transistor
17
, V
CE-15
is the collector-emitter voltage of transistor
15
, V
R-13A
is the voltage drop across resistor
13
A, and V
I-11
is the voltage drop across current source
11
. Transistors
17
and
15
enter saturation when their collector-emitter voltage drops below V
CE-SAT
(e.g., 0.4 volts). If V
IN
+
has a DC voltage of 1.4 volts and a voltage swing of ±0.25 (i.e., V
IN
+
has a maximum value of 1.65 volts and a minimum value of 1.15 volts), V
CE-15
should be greater than V
CE-SAT
plus the voltage swing of V
IN
+
. Therefore, V
CE-15
is at least 0.65 volts to prevent transistor
15
from entering saturation and causing distortion in V
OUT
. If V
LO
+
has a voltage swing of 200 mV, then V
CE-17
should be at least 0.60 volts (i.e., V
CE-SAT
+200 mV) to prevent transistor
17
from entering saturation. V
R-21
may be, for example, 0.5 volts; V
I-11
is typically 0.4-0.6 volts; and V
R-13A
equals the voltage swing in V
IN
+
(e.g., 0.25 volts). For these exemplary signal values, V
CC
must be at least 2.4-2.6 volts.
If a lower supply voltage is used, output signals V
OUT
+
and V
OUT
−
may not have sufficient room to reach their peak amplitude. Also, a low supply voltage may cause transistors in circuit
10
to saturate, resulting in a non-linear output response that causes distortion in output signal (V
OUT
+
−V
OUT
−
. With a low supply voltage, transistors in circuit
10
saturate for large values of V
IN
+
and V
IN
−
. Thus, lowering the supply voltage results in a trade-off: the more that the supply voltage is lowered to save power, the more distortion may be present in the output signal. Therefore, the peak amplitude of V
IN
+
and V
IN
−
and maximum distortion requirements in output signals V
OUT
+
and V
OUT
−
are constraints that set a lower limit on the supply voltage of circuit
10
.
It would, however, be desirable to provide signal modulator circuits that consume less power than previously known Gilbert-cell mixer circuits, such as circuit
10
. In particular, it would be desirable to provide signal modulator circuits that consume low power by operating at a low supply voltage.
It also would be desirable to provide signal modulator circuits that produce an output signal with reduced distortion at low supply voltages.
It further would be desirable to provide signal modulator circuits that allow for greater input voltage swings at low supply voltages.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide signal modulator circuits that consume low power by operating at a low supply voltage.
It also is an object of the present invention to provide signal modulator circuits that produce an output signal with reduced distortion at low supply voltages.
It further is an object of the present invention to provide signal modulator circuits that allow for a greater input voltage swings at low supply voltages.
These and other objects of the present invention are provided by signal modulator circuits that comprise a transconductance amplifier, a current amplifier, and a differential pair circuit. The transconductance amplifier converts a first voltage signal to a current signal. The current amplifier has a low input impedance and provides a current output signal. The differential pair circuit modulates the current output signal of the current amplifier by a second signal to produce a frequency-modulated output signal with minimal distortion. Modulator cir

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