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CMOS Differential Signal Modulator and Variable Gain Amplifier Disclosure Number: IPCOM000015648D
Original Publication Date: 2002-Aug-18
Included in the Prior Art Database: 2003-Jun-20
Document File: 3 page(s) / 35K

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CMOS Differential Signal Modulator and Variable Gain Amplifier


    Analog circuits utilizing merged division and multiplication
functions offer flexibility in design of advanced integrated
systems for performing various signal processing functions such
as signal modulation, noise rejection and gain control. The
realization of these microelectronic circuits in CMOS technology
commonly employs a current pooling technique, such that
differential signal division and multiplication are obtained
while signal flows are balanced and reached in an equilibrium
state. These circuit structures can be referenced to Ref. 1 and
Ref. 2. In fact, the current pooling circuit requires a high
gain amplification for high-bandwidth operations that is
difficult to achieve with low voltage CMOS implementations. The
high gain operation can result an increase of power consumption
with poor voltage switching characteristics and often causes a
stability problem.

    Fig. 1 presents a compact current-mode low-voltage CMOS
modulator circuit for an improved design to provide
high-bandwidth differential signal processing operations. The
advantages of the circuit is low power consumption, good
stability and excellent dynamic characteristics.

    The modulator (Fig. 1) contains two folded current squarer
circuits with PFET devices of P1, P2 and P3, also P4, P5 and P6.
The physical sizes of these PFET's are identical. The squaring
function of the PFET circuits can be referenced to Ref. 3 and
Ref. 4 for the unfolded NFET structures. The modulation signal
is shown as an input current Iw in the diagram. The differential
high frequency signal, Vin- and Vin+, is merged through the NFET
pair of N1 and N2. The bias current Im is for establishing a
current bias for the differential circuit structure mirrored
(1:1) through transistors N7, N9 and N10. The current mirror
produces the paired bias currents of Id9 and Id10 at the drains
of N9 and N10. The drain currents of N1 and N2 are provided as
Id1 = Id9 - Iin and Id2 = Id10 + Iin, where Iin is the
differential signal current produced by the differential signal
inputs of Vin- and Vin+, and Id9 = Id10 = Im. The output current
of the first squarer, which is sum of the drain currents of P2

2and P3 can be derived as Io1 = Iw + (Id1)/(4*Iw); and the second
squarer output current is the sum of drain currents of P5 and P6,


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2is Io2 = Iw + (Id2)/(4*Iw). The drain currents of P7, P2 and P5
are mirrored from the modulation input of Iw.

    The current mode signal division and multiplication is
completed with the differential circuit of N3 and N4. It can be
shown that the differential output current is Io = Io2 - Io1 =

2(1/(4*Iw))*( (Id2) - (Id1)2 ) = ( 1/Iw) * (Im * Iin) = (Im/Iw) *
(Vin/Rin). This provides a standard modulator or mixer function
of S(t) = (m(t)/A(t))*f(wt), as defined commonly for producing a
modulated output signal Vo with the modulation functions of m(t)
and A(t).

    The modulator circuit can be exte...