Datasheet

Texas Instruments Incorporated
Amplifiers: Op Amps
25
Analog Applications Journal
3Q 2003 www.ti.com/sc/analogapps Analog and Mixed-Signal Products
noise (see Figure 4). For comparison, the THS4012, with a
respectable voltage noise of 7.5 nV/
√
Hz
——
and both current
noises of 1 pA/
√
Hz
——
, is also shown in Figure 4.
Note that the output noise of the THS4012 is the same
as when using the THS3112 with Z = 475 Ω. Again, these
responses are just like those of a VFB amplifier in the tradi-
tional configuration, showing that the basic functionality is
sound—there are no differences between a VFB amplifier
and this configuration. Figure 4 shows that although using
Z = 1 kΩ produces a very stable amplifier, the output
noise is 20 nV/
√
Hz
——
higher than that of the THS4012.
Keep in mind that the THS3112 has very low overall
noise but that many other CFB amplifiers will probably
produce much higher noise. The only way to get around
this is if the unity-gain stability of the amplifier requires a
very small resistor of, say, only 500 Ω or less. But what if
there was another way to make the CFB amplifier stable
and have low noise at the same time?
Fundamentally speaking, the circuit needs high impedance
within the feedback path only at the amplifier’s bandwidth
limit. At frequencies below this point, it really does not
matter what the impedance is, and the amplifier will work
fine. The issues stated previously are also
minimized, resulting in an even better
system than one using pure resistors.
The first solution that comes to mind is
to use an inductor. Inductors have low
impedance at low frequencies and high
impedance at high frequencies—exactly
what is desired; but their relatively large
size and high cost are generally considered
prohibitive. An alternative component
that minimizes these disadvantages and
still functions the same is the ferrite chip.
Testing with ferrite chips
used for Z
Ferrite chips have been available for several
years, are relatively low-cost, and are
available in very small sizes—0402 and
larger. Although several manufacturers
produce ferrite chips, testing was done
with what was available in the test lab—
ferrite chips from Murata’s BLM series.
Examining the impedance characteristics
of these ferrites revealed several possible
components that could be utilized.
The first factor in determining the proper
component was the ferrite’s impedance at
the amplifier’s bandwidth limit. For the
THS3112, this implied an impedance of
at least 600 Ω at about 150 MHz to meet
stability. This can vary, as the first test
results showed (see Figure 3).
Additionally, the Q of the ferrite chips
varies from grade to grade. Some have a
low Q with a fairly smooth rise to the
resonance point that then subsides due to
inherent properties and parasitics, while
other chips have a relatively high Q with a
sharp rise and fall in impedance associated
with them. Although either style may
meet the impedance requirements, testing
was required to see if this Q had an effect
on the circuit. Again, the best way to show
the results was to graph the frequency
response of the system, as shown in
Figure 5. The responses below 10 MHz
were all identical to the original configu-
ration. This figure concentrates on the
stability portion of the responses above
10 MHz. For comparison purposes, the
681-Ω, pure-resistance response is shown.
10 M 100 M 1 G
Frequency (Hz)
V (dB)
OUT
35
30
25
20
15
10
5
0
–5
–10
–15
–20
Z = 681 Ω
Z = BLM18HD601SN1
Z = BLM18HG601SN1
Z = BLM18AG601SN1
Figure 5. Frequency responses above 10 MHz with
ferrite chips (gain = +5)
10 k 100 k 1 M 10 M
Frequency (Hz)
Output Noise (nV/ Hz)√
—–
70
60
50
40
30
20
10
0
Z = 681 Ω
Z = 200 Ω
Z =1kΩ
THS4012; alsoZ=475Ω
Figure 4. Output noise (gain = +5)