Datasheet

Texas Instruments Incorporated
Amplifiers: Op Amps
27
Analog Applications Journal
3Q 2003 www.ti.com/sc/analogapps Analog and Mixed-Signal Products
exactly the same as before. Figures 8 and 9 show the test
circuits for this configuration. The signal gain was kept at
a gain of 5.
The same concepts apply for this CFB configuration as
for the noninverting configuration. The advantage of this
circuit is that the attenuation is not limited to unity gain,
or 0 dB, like the noninverting gain circuit. Figure 10 shows
the frequency responses of this configuration with varying
pure resistor values for Z. The THS4012 response is shown
for comparison purposes.
As expected, the responses all look comparable to each
other below 10 MHz. Additionally, the resistance values
affect the stability and again show that the higher the
resistance is, the better the stability. Using a resistance as
low as 475 Ω actually shows respectable performance in this
configuration. Remember that for oscillations to occur, the
gain must be above unity gain, or 0 dB. As long as the peak
is below 0 dB, oscillations should not occur. As in the non-
inverting case, using 200 Ω shows a large narrow peak that
will most likely result in stability issues and/or oscillations.
However, notice that above 10 MHz the same general
shape occurs for both the CFB and VFB amplifiers. This is
caused by the amplifiers’ input and output impedances
becoming very high above their bandwidth limit. When
this occurs, there is a path for the input signal to flow
through R
G
, through C
F
, and then to feed forward to the
load. Of course, the amplifiers’ own input and output
capacitances also affect the amount of feed-through in the
circuit; but it is important to remember that this occurs
above the amplifiers’ usable bandwidths.
Just as for the noninverting configuration, using ferrite
chips has several advantages for the inverting configuration.
10 k
100 k
1 M 10 M 100 M 1 G
Frequency (Hz)
V (dB)
OUT
15
10
5
0
–5
–10
–15
–20
–
25
Z = 475 Ω
Z = 200 Ω
Z=1kΩ
THS4012
Figure 10. Frequency responses with resistors (gain = –5)
C = 220 pF
F
THS4012
R = 750
F
Ω
R
100
L
Ω
R
75
Term
Ω
–15 V
+15 V
V
IN
V
OUT
R
150
G
Ω
Figure 8. Inverting gain of 5 VFB configuration
C = 220 pF
F
THS3112
R = 750
F
Ω
R
100
L
Ω
R
75
Term
Ω
–15 V
+15 V
V
IN
V
OUT
R
150
G
Ω
Z
Figure 9. Inverting gain of 5 CFB configuration