THS4211 Datasheet, PDF (20/50 Pages) Texas Instruments – LOW-DISTORTION HIGH-SPEED VOLTAGE FEEDBACK AMPLIFIER

THS4211
THS4215
SLOS400E – SEPTEMBER 2002 – REVISED SEPTEMBER 2009 ................................................................................................................................... www.ti.com
WIDEBAND, INVERTING GAIN OPERATION
Since the THS4211 and THS4215 are
general-purpose, wideband voltage-feedback
amplifiers, several familiar operational-amplifier
applications circuits are available to the designer.
Figure 76 shows a typical inverting configuration
where the input and output impedances and noise
gain from Figure 75 are retained in an inverting circuit
configuration. Inverting operation is a common
requirement and offers several performance benefits.
The inverting configuration shows improved slew
rates and distortion due to the pseudo-static voltage
maintained on the inverting input.
5 V _+VS
100 pF
0.1 µF
+
6.8 µF
C_T
0.1 µF
R_T
200 Ω
50 Ω Source
R_g
V_I
392 Ω
R_M
57.6 Ω
+
THS4211
_
R_f
392 Ω
100 pF
V_O
499 Ω
0.1 µF 6.8 µF
+
-5 V ^-VS
Figure 76. Wideband, Inverting Gain
Configuration
In the inverting configuration, some key design
considerations must be noted. One is that the gain
resistor (R_g) becomes part of the signal-channel input
impedance. If input impedance matching is desired
(beneficial when the signal is coupled through a
cable, twisted pair, long PCB trace, or other
transmission line conductor), R_g may be set equal to
the required termination value and R_f adjusted to give
the desired gain. However, care must be taken when
dealing with low inverting gains, as the resultant
feedback resistor value can present a significant load
to the amplifier output. For an inverting gain of 2,
setting R_g to 49.9 Ω for input matching eliminates the
need for R_M but requires a 100-Ω feedback resistor.
This has the advantage that the noise gain becomes
equal to 2 for a 50-Ω source impedance—the same
as the noninverting circuit in Figure 75. However, the
amplifier output now sees the 100-Ω feedback
resistor in parallel with the external load. To eliminate
this excessive loading, it is preferable to increase
both R_g and R_f, values, as shown in Figure 76, and
then achieve the input matching impedance with a
third resistor (R_M) to ground. The total input
impedance becomes the parallel combination of R_g
and R_M.
The next major consideration is that the signal source
impedance becomes part of the noise gain equation
and hence influences the bandwidth. For example,
the R_M value combines in parallel with the external
50-Ω source impedance (at high frequencies),
yielding an effective source impedance of 50 Ω || 57.6
Ω = 26.8 Ω. This impedance is then added in series
with R_g for calculating the noise gain. The result is
1.9 for Figure 76, as opposed to the 1.8 if R_M is
eliminated. The bandwidth is lower for the inverting
gain-of-2 circuit in Figure 76 (NG=+1.9), than for the
noninverting gain of 2 circuit in Figure 75.
The last major consideration in inverting amplifier
design is setting the bias-current cancellation resistor
on the noninverting input. If the resistance is set
equal to the total dc resistance looking out of the
inverting terminal, the output dc error, due to the input
bias currents, is reduced to (input offset current) × R_f
in Figure 76, the dc source impedance looking out of
the inverting terminal is 392 Ω || (392 Ω + 26.8 Ω) =
200 Ω. To reduce the additional high-frequency noise
introduced by the resistor at the noninverting input,
and power-supply feedback, R_T is bypassed with a
capacitor to ground.
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