THS4031 Datasheet, PDF (23/38 Pages) Texas Instruments – 100-MHz LOW-NOISE HIGH-SPEED AMPLIFIERS

THS4031, THS4032
100-MHz LOW-NOISE HIGH-SPEED AMPLIFIERS
APPLICATION INFORMATION
SLOS224C – JULY 1999 – REVISED APRIL 2000
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 46 and Figure 47). 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
10
VCC = ± 15 V
9 Gain = 2
RF = 300 Ω
8 RL = 150 Ω
VO(PP) = 0.4 V
7
Ci– = 10 pF
6
No Ci–
5
(Stray C Only)
4
3
2
1
0
100 k
Ci–
300 Ω
VI
300 Ω
_
+
50 Ω
VO
150 Ω
1M
10 M
f – Frequency – Hz
Figure 46
100 M
500 M
OUTPUT AMPLITUDE
vs
FREQUENCY
4
VCC = ± 15 V
3 ^{Gain = –1}
RF = 360 Ω
2 ^{RL = 150 Ω}
VO(PP) = 0.4 V
1
Ci–= 10 pF
0
No Ci–
–1
(Stray C Only)
–2
–3 _VI
360 Ω
–4 ^{56 Ω Ci–}
–5
360 Ω
_
+
VO
150 Ω
–6
100 k
1M
10 M
f – Frequency – Hz
Figure 47
100 M
500 M
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. This is why in Figure 30, a feedback resistor of 3.9 kΩ with a gain resistor of 1 kΩ only
shows a small peaking in the frequency response. The parallel resistance is only 800 Ω. This value, in
conjunction with a very small stray capacitance test PCB, forms a zero on the edge of the amplifier’s natural
frequency response. To eliminate this peaking, all that needs to be done is to reduce the feedback and gain
resistances. One other way to compensate for this stray capacitance is to add a small capacitor in parallel with
the feedback resistor. This helps to neutralize the effects of the stray capacitance. To keep this peaking out of
the operating range, the stray capacitance and resistor value’s time constant must be kept low. But, as can be
seen in Figures 26 – 29, 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.
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