Datasheet

Texas Instruments Incorporated
Amplifiers: Op Amps
24
Analog Applications Journal
Analog and Mixed-Signal Products www.ti.com/sc/analogapps 3Q 2003
introduce errors into the system due to
the bias current and the dynamic signal
current flowing through this impedance;
but these effects are reasonably small as
long as the impedance is minimized.
Adding impedance Z can affect input
offset voltage due to the dc input bias
current, which is typically 1 to 10 µA,
multiplied by the impedance Z. This
resulting voltage gets multiplied by the
noise gain of the circuit. Additionally, when
a signal appears at the output, the CFB
amplifier (as the name implies) relies on
an error current flowing through the
inverting node through the impedance Z,
producing a signal error. However, since
the transimpedance of most CFB ampli-
fiers is well over 100 kΩ and sometimes as
high as several megohms, this error is also
minimized if the impedance is kept low. The
drift of this circuit now also relies on the
temperature characteristics of impedance
Z and should not be used as a precision
amplifier; but most CFB amplifiers are not
used as precision amplifiers anyway due to
their inherent topology limitations. Overall, these issues
are minimal and, for most systems, can be effectively
ignored in favor of the CFB amplifier’s advantages as
previously stated.
Testing with different Z values
The easiest way to see if the circuit is stable is to use a
network analyzer frequency sweep. Instability can typical-
ly be seen as sharp rises in the frequency response at the
amplifier’s bandwidth limitations. If the peaking is smooth,
or there is no peak, then the amplifier should be stable.
Figure 3 shows the frequency response of the system with
different values of resistors for the variable Z.
The response of the THS4012 is also shown for reference
to easily compare the performance of the two systems. It
is interesting that no matter what resistance is used for Z,
the responses below 20 MHz look identical to each other.
This is the ultimate goal of this configuration—no differ-
ences in signal performance. For the stability part of the
circuit, the area above 20 MHz must be examined.
Examining the circuits in Figures 1 and 2 shows us that
the feedback impedance is dictated by the capacitor CF.
Above 20 MHz, this impedance is very small—essentially
creating a short from the output to the summing node. This
configuration is commonly referred to as a unity buffer with
the signal gain set to 1. The data sheet for the THS3112
4
recommends that, in a gain of +1 under the circuit condi-
tions utilized, the feedback resistance be 1 kΩ. Thus, it is
no surprise to see that when Z = 1 kΩ, the response looks
very smooth and well behaved, indicating a very stable
system. However, when Z = 681 Ω, the response also looks
very reasonable and helps minimize the potential issues
stated previously. This shows that there is a reasonably wide
range of acceptable values for Z and does not imply that the
selection for Z is highly critical. Figure 3 also illustrates a
common trait for current-feedback amplifiers—as the feed-
back impedance is decreased, the peaking will increase. If
the impedance is too low, there is a good chance that the
circuit will become unstable and oscillate, as illustrated by
the response when Z = 200 Ω.
Output noise
One element that may be very important in a system is the
output noise. Adding a resistance in the manner discussed
only makes the output noise worse. The inverting current
noise of the amplifier goes through the resistance at Z and
creates a voltage noise. This noise then becomes multiplied
by the circuit’s gain, which is frequency-dependent.
For a CFB amplifier, the inverting current noise is typi-
cally the highest noise component of the amplifier. Although
the CFB amplifier voltage noise is inherently very low,
typically less than 3 nV/
√
Hz
——
, the inverting current noise of
most CFB amplifiers is generally around 15 to 20 pA/
√
Hz
——
.
The noninverting current noise is only noticeable if the
source impedance is high. Using a 50-Ω environment
minimizes the noninverting current noise.
The THS3112 was designed to have very low noise. The
voltage noise is 2.2 nV/
√
Hz
——
, the noninverting current noise
is 2.9 pA/
√
Hz
——
, and the critical inverting current noise is a
low 10.8 pA/
√
Hz
——
. However, multiplying the inverting current
noise by 1 kΩ and then multiplying by the gain can alone
produce a very substantial output noise of about 54 nV/
√
Hz
——
in the pass band. To quantify the output noise of the system,
the circuits shown in Figures 1 and 2 were tested for output
10 k 100 k 1 M 10 M 100 M 1 G
Frequency (Hz)
V (dB)
OUT
25
20
15
10
5
0
–5
–10
–15
Z = 200 Ω
Z = 475 Ω
Z=1kΩ
THS4012
Z = 681 Ω
Figure 3. Frequency responses with resistors (gain = +5)