Amplifiers – With semiconductor amplifying device – Including differential amplifier
Reexamination Certificate
1999-10-01
2001-05-08
Pascal, Robert (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including differential amplifier
C330S254000, C327S359000
Reexamination Certificate
active
06229395
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the use of a transistor differential pair and its use as the input to many integrated circuit amplifiers. It has specific application to Radio Frequency Integrated Circuit (RFIC) amplifiers and double balanced (Gilbert cell) mixers, as well as many other possible uses. Such mixers are used in numerous applications, including, but not limited to, upconverters and downconverters for cellular or other wireless communications.
A conventional differential amplifier is shown in
FIG. 1
, and a typical double balanced mixer is shown in FIG.
2
. Though these examples show bipolar transistors, the analysis is identical for Field Effect Transistors (FETs i.e., MOSFETS, MESFETS, JFETS . . . ), and this invention is applicable to both bipolar and field effect transistor circuits. These examples also show resistive loads RL
1
, RL
2
, RL
3
and RL
4
, but there are a wide variety of loads that could be used, for example, current combiners, transformers, inductors, tuned networks and similar circuits.
As seen in
FIG. 1
, transistors Q
1
and Q
2
have their emitters tied and supplied through current source CS
1
. An input voltage, VIN
1
, is applied across their bases, and a collector voltage VCC
1
applied through load resistors RL
1
and RL
2
determines the output signal VOUT
1
as the amplified differential of VIN
1
.
In
FIG. 2
, the Gilbert cell has transistors Q
5
, Q
6
, Q
7
, and Q
8
arranged with the emitters of Q
5
and Q
6
tied together and the emitters of Q
7
and Q
8
tied together. The bases of Q
6
and Q
7
receive one side of local oscillator signal LO_IN
1
, and the bases of Q
5
and Q
8
receive the other pole of LO_IN
1
. The collectors of Q
6
and Q
7
are tied as one pole of the output signal VOUT
2
, and the collectors of Q
6
and Q
8
are tied as the other output polarity of VOUT
2
. The two tied collector pairs are connected respectively through load resistors RL
3
and RL
4
to the collector voltage supply VCC
2
.
The tied emitters of Q
5
and Q
6
are connected to the collector of transistor Q
3
and the tied emitters of Q
7
and Q
8
are connected to the collector of transistor Q
4
. Input signal VIN
2
is connected across the bases of Q
3
and Q
4
. The emitters of transistors Q
3
and Q
4
are tied to a constant current source CS
2
.
As can be seen, the transistors Q
3
and Q
4
act as a differential amplifier component of the Gilbert cell circuit, which on the whole acts to multiply the voltage VIN
2
by the local oscillator signal LO_IN
1
to provide the output signal VOUT
2
.
The small signal gain of circuit in
FIG. 1
is: gm*RL
1
. (Small signal is defined to be about 10 mV or less.) For input signals larger than about 50 mV, the differential input pair suffers from linearity problems. The typical solution to this problem is to “degenerate” the input pair. This is done by adding impedance Z
1
as shown in
FIG. 3
or
FIG. 4
, which show the amplifier sections only, for simplicity.
In
FIG. 3
, two identical impedances Z
1
have been interposed between the transistor emitters and the current source CS
1
. In
FIG. 4
, two current sources CS
3
and CS
4
are provided, one tied to each transistor emitter, with the two emitters coupled through an impedance equal to twice Z
1
.
The equivalent gm of the input pair changes to: Gm=1/(1(/gm)+Z
1
). If Z
1
is large compared to 1/gm (which is typically the case), than the gain is dominated by this degeneration impedance. The gain is also less than it was without the degeneration, but the input dynamic range will be greater.
The circuit in
FIG. 3
tends to have less noise than that of
FIG. 4
, because the noise contribution from the bias current source CS
1
is common-moded out. But,
FIG. 3
has the disadvantage in that, if Z
1
is resistive, DC current flows through this impedance, and a substantial voltage drop will result, reducing the dynamic range. Also, since DC current must flow, Z
1
can not be capacitive. One common solution to this problem is to make Z
1
purely inductive, or use the circuit in FIG.
4
. The problem with using the
FIG. 3
circuit with Z
1
purely inductive is that at lower frequencies, the impedance becomes less, and therefore gain becomes greater. This causes lower frequency noise, especially at the image frequency, to be gained up more than the signal frequency of interest.
The circuit in
FIG. 4
solved the problem of DC current flowing through the degeneration by adding a second current source. The big problem with this approach is that the noise from the current sources is no longer common-moded out, and can add substantial noise to the entire amplifier and/or mixer. Thus, prior degeneration efforts have been plagued with one detrimental side effect or another, leaving an unresolved need.
SUMMARY OF THE INVENTION
The present invention fulfills this need in the art by providing a differential transconductance amplifier circuit including a transistor differential pair having an output and an alternating current input port with a degeneration feedback circuit that includes a series of inductive and capacitive impedances. The transistors may be field effect transistors or bipolar transistors.
The amplifier preferably includes a resistance in series with the series of inductive and capacitive impedances to de-Q resonance of the series of inductive and capacitive impedances. Typically, the differential transistor pair is arranged with the alternating current input port as the bases of the transistors.
An oscillator may be included to generate a frequency signal at the alternating current input and the series of inductive and capacitive impedances preferably have values so as to resonate at about the frequency signal. In one embodiment the frequency signal is in the radio frequency band. Other frequencies can be used.
The series of inductive and capacitive impedances preferably includes two inductive impedances with a capacitive impedance between them. Conductors may be connected between the capacitive impedance and each of the inductive impedances so that the currents of the differential amplifier transistors are determined by currents in the conductors. Typically, the conductors are connected to bias current sources so that the currents are determined by the bias current sources.
In a preferred embodiment, the alternating current input port includes first and second lines, the pair of transistors is arranged with the first alternating current line as the base of one of the transistors, the second alternating current line as the base of the other of the transistors, and the emitters of the transistors are connected to bias current sources through the degeneration feedback circuit.
The invention also provides a differential transconductance amplifier circuit including a Gilbert cell having an intermediate frequency port, a local oscillator port and a frequency port, with the Gilbert cell including a differential amplifier with a degeneration circuit that includes a series of inductive and capacitive impedances. An oscillator may be included to generate a frequency signal supplied to the frequency port and the series of inductive and capacitive impedances typically have values so as to resonate at about the frequency signal.
In a preferred embodiment the Gilbert cell includes two pairs of transistors, the intermediate frequency port includes first and second lines, with the second intermediate frequency line tied to a collector of one transistor of each pair and the first intermediate frequency line tied to a collector of the other transistor of each pair. The local oscillator port includes first and second lines, with the second oscillator line tied to a base of one transistor of each pair and the first oscillator line tied to a base of the other transistor of each pair.
The frequency port may include first and second lines, with the differential amplifier including a pair of transistors arranged with the first frequency line tied to the base of one of the differential amplifier transistors and the second radio
Choe Henry
Pascal Robert
RF Micro Devices, Inc.
Rhodes & Mason P.L.L.C.
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