THS4031 datasheet(24/38 Pages) TI

24 / 38 page

THS4031, THS4032

100-MHz LOW-NOISE HIGH-SPEED AMPLIFIERS

SLOS224C – JULY 1999 – REVISED APRIL 2000

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

APPLICATION INFORMATION

optimizing frequency response (continued)

Table 1. Recommended Feedback Resistors

GAIN

Rf for VCC = ±15 V and ± 5 V

300

Ω

–1

360

Ω

3.3 k

Ω (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 + RF/RC 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 (VOO). One way to eliminate

the VOO increase is to add a capacitor (CC) in series with RC, with the added requirement that the time constant

of CC and RC be set low enough for the Bode plot intersection to be at a –20 dB/decade slope. Typically, a 1

+ RF /RC gain of 2 to 3 yields a smooth frequency response for the THS403x. If CC is used, it is desirable to have

the 1/(2

π RCCC) 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 RC and CC modification can be used in both an inverting and noninverting

configuration with the same results.

THS4031 Datasheet(PDF) 24 Page - Texas Instruments