Merged variable gain mixers

Details Merged variable gain mixers Merged variable gain mixers

1998-06-05

2001-04-03

Tsang, Fan (Department: 2746)

Telecommunications

Receiver or analog modulated signal frequency converter

Frequency modifying or conversion

C455S323000, C455S326000, C455S209000, C455S253200, C455S315000, C455S293000, C327S560000, C327S356000, C327S359000, C327S346000

Reexamination Certificate

active

06212369

ABSTRACT:

DESCRIPTION
1. Technical Field
The present invention is related to signal mixing and amplifying or attenuating circuitry. More specifically, the present invention teaches a variety of merged variable gain mixers capable of mixing and variably amplifying or attenuating an input signal.
2. Background Art
Many electronics applications require processing an input frequency signal with both a mixing function and a variable gain function. For example, typical wireless communication system transmitters and receivers utilize separate mixer and variable gain amplifier devices in order to operate upon the input signal as desired. In many applications, such circuitry must operate with low supply voltages (e.g., 2.7 Volts) and at extremely low temperatures (e.g., −40 degrees C.).
FIG. 1
illustrates a prior art mixing and gain control circuitry
100
suitable for mixing and variably amplifying an input signal VIN to generate a desired output signal Vout. The circuitry
100
includes a so called Gilbert cell mixer
102
, a variable gain amplifier
104
, blocking capacitors C
1
and C
2
, and a bias source formed by the resistor pair R
1
and R
2
. The Gilbert cell mixer
102
includes three differentially connected transistor pairs Q
1
-Q
2
, Q
3
-Q
4
, and Q
5
-Q
6
, a current source I
1
, and a pair of resistors R
3
and R
4
. Those skilled in the art will be familiar with the operation of the Gilbert cell mixer
102
, the functionality of which is defined primarily by the linear transconductor
103
(formed by the transistor pair Q
5
-Q
6
) and the current switch or switching quad
105
(formed by the transistor quad Q
1
-Q
4
) which causes the mixing of the input current.
The input signal Vin is coupled to the bases of transistors Q
5
and Q
6
so that the linear transconductor
103
may convert it into a current signal. The local oscillator signal LO is connected to the bases of differential transistor pairs Q
1
-Q
2
and Q
3
-Q
4
. The blocking capacitors C
1
and C
2
may serve to disassociate any DC bias present on the LO signal.
The mixer
102
operates as follows. The current signal out of the transconductor
103
is multiplied by +
1
and −
1
at the frequency of the local oscillator signal LO. This happens as the local oscillator signal LO switches the switching quad
105
ON and OFF at the frequency of the local oscillator signal LO. The resulting output current into the load (the load here being represented for convenience by R
3
and R
4
) is a current which contains signals with frequencies that are the sum and difference, respectively, of the frequencies of the local oscillator signal LO and the input signal Vin. Typically only one of the sum and difference signals is later used, the other being eliminated by simple filtering (not shown in FIG.
1
). Hence, as will be appreciated, the mixer
102
is basically a multiplier that takes a local oscillator signal LO and the input signal Vin and frequency translates the input signal Vin to a new differential signal at the mixer output terminals M
1
and M
2
.
Depending upon the application, a variable gain function can either precede or follow the mixer function in a radio transceiver. In a transmit application the mixer would serve as an upconvertor. That is, the input would be either at baseband or intermediate frequency (IF), typically 0-20 MHz and 10-400 MHz, respectively. The output signal from the mixer would be at much higher RF frequencies. Typically, any variable gain function preceding the mixer is called IF variable gain and any variable gain function after the mixer is called RF variable gain.
In the prior art Gilbert cell mixers there are implementations of IF variable gain wherein the gain of the input transconductor can be varied by controlling the gain of the current source I
1
. However, in
FIG. 1
we illustrate an RF variable gain control function (i.e., the gain function is implemented after the mixer). Accordingly, the differential signal generated by the mixer at terminals M
1
and M
2
is coupled to the input of the variable gain amplifier
104
. As will be appreciated, the amplifier
104
shown in
FIG. 1
is simply a conceptual representation of a voltage controlled variable gain amplifier. That is, as the control voltage V
gc
varies, the gain of the amplifier
104
varies.
Prior art
FIG. 2
is a schematic showing in more detail one typical variable gain amplifier
104
. The amplifier
104
of
FIG. 2
includes three differentially connected transistor pairs Q
7
-Q
8
, Q
9
-Q
10
, and Q
11
-Q
12
, a pair of resistors R
5
and R
6
, and a second current source
12
. The mixer outputs M
1
and M
2
are coupled to the bases of transistors Q
7
and Q
8
. The emitters of Q
7
and Q
8
are coupled at a first terminal of a second current source
12
. Hence the voltage signal on M
1
, and M
2
is once again converted into a current signal by another transconductance stage formed by the transistor pair Q
7
-Q
8
. Those of skill in the art will appreciate that a linear transconductance stage such as this may take on innumerable forms.
The control voltage V
gc
is coupled to the bases of transistor pairs Q
9
-Q
10
and Q
11
-Q
12
. The emitters of Q
9
and Q
10
are coupled to the collector of transistor Q
7
. The emitters of Q
11
and Q
12
are coupled to the collector of transistor Q
8
. The collectors of transistors Q
9
and Q
12
, and first terminals of the resistors R
5
and R
6
are coupled to the supply voltage V
cc
. A second terminal of the resistor R
5
is coupled to the collector of transistor Q
10
and a second terminal of the resistor R
6
is coupled to the collector of transistor Q
11
. The output signal O
f
is therefore generated at the collectors of Q
10
and Q
11
.
Typical of the Prior Art, the mixing and gain circuitry
100
shown in
FIGS. 1 and 2
has several shortcomings. One major shortcoming is the non-linearity introduced by the two transconductance stages (i.e., Q
5
-Q
6
and Q
7
-Q
8
). Another serious shortcoming is the power loss due to the current sources I
1
and I
2
. This power loss is particularly problematic when designing for supplies at 2.7V (and lower) and cold temperature operating conditions.
FIG. 3
illustrates a second mixing and gain control circuitry
200
of the Prior Art. The circuitry
200
is motivated by a recognition that the second transconductance stage formed by the transistor pair Q
7
and Q
8
is unnecessary if one connects the Gilbert cell mixer
102
directly in series with the transistor pairs Q
9
-Q
10
and Q
11
-Q
12
. Doing so, as shown in
FIG. 3
, allows the gain control voltage signal V
gc
to directly control the current flow through the Gilbert cell mixer
102
. This eliminates any non-linearities introduced by the second transconductance stage as well as improving power efficiency by eliminating the second current source I
2
.
The improvements in linearity and power efficiency of circuitry
200
are not free. Connecting the Gilbert cell mixer
102
in series with the variable gain amplifier of
FIG. 3
forms a circuit that suffers from a four transistor voltage drop, specifically, the three transistor drop across the Gilbert cell mixer
102
and the single transistor drop across transistor pairs Q
9
-Q
10
and Q
11
-Q
12
. This makes use of the circuitry
200
with low voltage supplies problematic.
What is needed is a mixer and variable gain amplifier circuit that lacks the non-linearity and power inefficiencies of multiple transconductance stages, yet is capable of operating properly when provided low supply voltages.
DISCLOSURE OF THE INVENTION
The present invention teaches parallel coupling what are herein termed a “switching stage” and a “steering stage,” thereby arranging the mixer and variable gain amplifier circuitry as a single merged circuit. The merged variable gain mixers of the present invention provide mixing and gain functionality utilizing only that power needed for a basic mixer function and only the transconductance of the basic mixer function (thereby eliminating non-linearities introduced by additional transcon

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