THS4032IDR Datasheet, PDF (24/47 Pages) Texas Instruments – 100-MHz LOW-NOISE HIGH-SPEED AMPLIFIERS

THS4031
THS4032
SLOS224G – JULY 1999 – REVISED MARCH 2010
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OPTIMIZING FREQUENCY RESPONSE
Internal frequency compensation of the THS403x was selected to provide very wide bandwidth performance and
still maintain a very low noise floor. In order to meet these performance requirements, the THS403x must have a
minimum gain of 2 (–1). Because everything is referred to the noninverting terminal of an operational amplifier,
the noise gain in a G = –1 configuration is the same as a G = 2 configuration.
One of the keys to maintaining a smooth frequency response, and hence, a stable pulse response, is to pay
particular attention to the inverting terminal. Any stray capacitance at this node causes peaking in the frequency
response (see Figure 53 and Figure 54). Two things can be done to help minimize this effect. The first is to
simply remove any ground planes under the inverting terminal of the amplifier, including the trace that connects
to this terminal. Additionally, the length of this trace should be minimized. The capacitance at this node causes a
lag in the voltage being fed back due to the charging and discharging of the stray capacitance. If this lag
becomes too long, the amplifier will not be able to correctly keep the noninverting terminal voltage at the same
potential as the inverting terminal's voltage. Peaking and possible oscillations will then occur if this happens.
OUTPUT AMPLITUDE
vs
FREQUENCY
OUTPUT AMPLITUDE
vs
FREQUENCY
10
V_CC = ± 15 V
9 Gain = 2
R_F = 300 Ω
8 R_L = 150 Ω
V_O(PP) = 0.4 V
7
C_i− = 10 pF
4
V_CC = ± 15 V
3 ^{Gain = −1}
R_F = 360 Ω
2 RL = 150 Ω
V_O(PP) = 0.4 V
1
C_i−= 10 pF
6
No C_i−
5
(Stray C Only)
0
No C_i−
−1
(Stray C Only)
4
3
2
1
0
100 k
C_i−
300 Ω
V_I
300 Ω
_
+
50 Ω
V_O
150 Ω
1M
10 M
100 M 500 M
f − Frequency − Hz
−2
−3 _VI
360 Ω
−4 ^{56 Ω Ci−}
−5
360 Ω
_
+
V_O
150 Ω
−6
100 k
1M
10 M
f − Frequency − Hz
100 M
500 M
Figure 53.
Figure 54.
The second precaution to help maintain a smooth frequency response is to keep the feedback resistor (R_f) and
the gain resistor (R_g) values fairly low. These two resistors are effectively in parallel when looking at the ac
small-signal response. But, as can be seen in Figure 26 through Figure 37, a value too low starts to reduce the
bandwidth of the amplifier. Table 1 shows some recommended feedback resistors to be used with the THS403x.
Table 1. Recommended Feedback Resistors
GAIN
1
2
–1
5
R_f for V_CC = ±15 V and ±5 V
50 Ω
300 Ω
360 Ω
3.3 kΩ (low stray-c PCB only)
24
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