THS4031 Datasheet, PDF (24/38 Pages) Texas Instruments – 100-MHz LOW-NOISE HIGH-SPEED AMPLIFIERS

THS4031, THS4032
100-MHz LOW-NOISE HIGH-SPEED AMPLIFIERS
SLOS224C – JULY 1999 – REVISED APRIL 2000
APPLICATION INFORMATION
optimizing frequency response (continued)
Table 1. Recommended Feedback Resistors
GAIN
2
–1
5
Rf for VCC = ±15 V and ± 5 V
300 Ω
360 Ω
3.3 kΩ (low stray-c PCB only)
unity-gain concerns
THS403x was designed for extremely low noise with a minimum gain of 2 (–1). If the amplifier were to be
configured for unity gain, the output would tend to oscillate because the open-loop intersection on a Bode
diagram is at a –40 dB/decade slope instead of the –20 dB/decade slope required for stable operation. But, it
is sometimes desirable to have a low-noise unity gain buffer. There is a way to accomplish this feat with the
THS403x with some added complexity (see Figure 48). The lag compensation circuit shown in Figure 48
increases the noise gain of the amplifier without increasing the signal gain. Another way to look at this is that
the open-loop gain is effectively reduced by the 1 + R_F/R_C gain. This reduction causes the –40 dB/decade pole
to be shifted down into an open-loop gain of less than 1. The drawbacks of this circuit are the decreased
frequency response, the increased noise, and the increased output offset voltage (V_OO). One way to eliminate
the V_OO increase is to add a capacitor (C_C) in series with R_C, with the added requirement that the time constant
of C_C and R_C be set low enough for the Bode plot intersection to be at a –20 dB/decade slope. Typically, a 1
+ R_F /R_C gain of 2 to 3 yields a smooth frequency response for the THS403x. If C_C is used, it is desirable to have
the 1/(2 π R_C C_C) frequency 5 to 10 times lower than the amplifier’s natural bandwidth. One additional advantage
this circuit provides is that it makes driving capacitive loads much easier. A capacitive load causes the
–40 dB/decade intersection (because of the phase lag), to be above unity gain, the same as described
previously. In general, this R_C and C_C modification can be used in both an inverting and noninverting
configuration with the same results.
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