Amplifiers – With semiconductor amplifying device – Including gain control means
Reexamination Certificate
1999-05-19
2003-09-16
Choe, Henry (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including gain control means
C330S282000, C375S222000
Reexamination Certificate
active
06621346
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention is in the field of amplifier circuits, and is more specifically directed to programmable gain amplifiers.
Programmable gain amplifiers, particularly those utilizing operational amplifiers, are well known in the art for providing amplification of analog electrical signals. Those of ordinary skill in the art will recognize that programmable gain amplifiers are particularly useful in the amplification of input signals that may be received over a wide dynamic range; the programmability of the gain of the amplifier permits adjustment of the amplifier operating characteristics according to the amplitude of the input signals being received thereby over time. Additionally, many communications systems are operable according to multiple standards or protocols, such that the specified range and characteristics of the input signals may vary widely among the standards; in such systems, it is useful to have a programmable gain amplifier for receiving and amplifying the input signals, such that the gain of the amplifier may be programmably adjusted according to the desired standard. Also, in many applications such as in the field of analog data communications, programmable gain amplifiers are often used in applying a relatively fine adjustment to incoming signals, prior to such processes as analog-to-digital conversion.
Referring now to
FIG. 1
, conventional programmable gain amplifier
2
will now be described. In this conventional arrangement, programmable gain amplifier
2
includes operational amplifier
20
, which has a non-inverting input connected to ground, and an inverting input that receives an input signal from terminal IN via capacitor
18
and input series resistor R
IN
. In this conventional arrangement, the output of operational amplifier
20
is presented on terminal OUT, and is also fed back, as negative feedback, to the inverting input via series resistors RA, RB, RC.
According to this conventional arrangement, the programmability of amplifier
2
is effected by metal-oxide-semiconductor (MOS) transistors
22
,
24
. MOS transistor
22
has its source-drain path connected across resistor RA, while MOS transistor
24
has its source-drain path connected across both of resistors RA, RB; the gates of transistors
22
,
24
are controlled by signals at terminals G
1
, G
2
, respectively. In this example, a high logic level at terminal G
1
(and a low logic level at terminal G
2
) will cause resistor RA to be shorted out by transistor
22
; similarly, a high logic level at terminal G
2
will cause both of resistors RA, RB to be shorted out by transistor
24
. As is fundamental in the art, the inverting gain of an operational amplifier is proportional to the ratio between the feedback resistance and the input resistance. Accordingly, the feedback resistance of programmable amplifier
2
, and thus its gain, is determined by the state of signals G
1
, G
2
; in this example, amplifier
2
may have any one of the resistances of RA+RB+RC, RB+RC, or RC as its feedback resistance, depending upon the state of control terminals G
1
, G
2
.
It has been observed, according to the present invention, that significant distortion can be produced by amplifier
2
according to this conventional arrangement of FIG.
1
. It is contemplated that this distortion is because switching transistors
22
,
24
, when on, conduct the signal current itself. As illustrated in
FIG. 1
, when either one of transistors
22
or
24
is turned on, current is conducted therethrough between the input and output terminals IN, OUT, depending upon the signal levels at each (considering that the inverting input of operational amplifier
20
typically has an extremely high input impedance). Because the source-drain resistance of an MOS transistor depends upon the current conducted therethrough, the feedback resistance presented by the ones of series resistors RB, RC not shorted out plus the source-drain resistance of the shorting transistor
22
,
24
will vary with signal current. Particularly in high precision applications such as high frequency modems, this distortion in programmable gain amplifiers such as amplifier
2
may not be tolerable.
FIG. 2
illustrates another conventional programmable gain amplifier
25
, in which the distortion due to signal current being conducted by the shorting transistors of the example of
FIG. 1
is avoided. In this example, input line IN is capacitively coupled to an integrated circuit containing programmable gain amplifier
25
via external high-pass coupling capacitor
18
connected to bond pad BP of the integrated circuit (boundary B of
FIG. 2
illustrating the chip boundary of the integrated circuit). Programmable gain amplifier
25
has its gain programmably set through operation of switches S
12
, S
23
, S
3
X, which are connected between the inverting input of operational amplifier
30
and nodes between resistors R
1
, R
2
, R
3
, RX, which are connected in series between the output of amplifier
30
and external coupling capacitor
18
. The values of resistors R
1
, R
2
, R
3
, RX will typically vary among themselves, depending upon the range and resolution of programmable gain levels desired for amplifier
25
. The non-inverting input of amplifier
30
is biased to ground, and the output of amplifier
30
is presented at terminal OUT.
Similarly as in the case described above relative to
FIG. 1
, switches S
12
, S
23
, S
3
X are generally implemented by way of MOS transistors, with a control signal connected to the gate of each that sets the state of each switch S
12
, S
23
, S
3
X. The state of switches S
12
, S
23
, S
3
X determine the gain of programmable gain amplifier
25
, by setting the ratio between feedback and input resistance as seen by amplifier
30
. As noted above, the gain of amplifier
25
is proportional to the ratio between its feedback resistance and its input resistance. For example, if switch S
23
is closed and all other switches S
12
, S
3
X are open, the gain of programmable gain amplifier
25
will be proportional to
RX
+
R3
R1
+
R2
.
Other combinations of switches S
12
, S
23
, S
3
X will select different ratios of feedback to input resistance and thus implement a different gain.
Programmable gain amplifier
25
of FIG
2
avoids one type of distortion, namely that caused by the switching transistors conducting signal current as in the case described above relative to FIG.
1
. This is because one may safely consider the inverting input of operational amplifier
30
as having extremely high impedance. The high input impedance of operational amplifier
30
limits the current that must be conducted by any one of switches S
12
, S
23
, S
3
X, implemented as MOS transistors, as switches S
12
, S
23
, S
3
X are connected between the inverting input of operational amplifier
30
and a node along the resistor chain of the input and feedback resistors. As such, signal current is never conducted by switches S
12
, S
23
, S
3
X, and thus no current-dependent changes are presented thereby. As such, the low frequency behavior of programmable gain amplifier
25
is of quite high fidelity.
However, changes in the gain of programmable gain amplifier
25
will also change its high frequency response. Specifically, the high-pass filter established by external capacitor
18
of capacitance C
1
8
will have a pole determined by
1
R
in
C
18
,
where R
in
is the input resistance. These changes in high frequency response will thus modulate the frequency response of the overall circuit from the ideal, causing distortion in the amplified signal at terminal OUT. As discussed above, especially in high precision communications applications such as high data rate modems, distortion due to programmable gain amplifiers is quite undesirable. Indeed, this conventional programmable gain amplifier
25
of
FIG. 2
requires adjustment in the input signal level presented thereto according to the selected gain in order to avoid this high frequency distortion.
BRIEF SUMMARY OF THE INVENTION
It is therefore an object o
Islam Kazi I.
Mukherjee Subahashish
Nabicht Joseph T.
Richardson Donald C.
Brady III Wade James
Choe Henry
Mosby April M.
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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