1

Application Notes

LM6172 Performance Discussion

The LM6172 is a dual high-speed, low power, voltage feed-

back amplifier. It is unity-gain stable and offers outstanding

performance with only 2.3 mA of supply current per channel.

The

combination

of

100

MHz

unity-gain

bandwidth,

3000V/µs slew rate, 50 mA per channel output current and

other attractive features makes it easy to implement the

LM6172 in various applications. Quiescent power of the

LM6172 Circuit Operation

The class AB input stage in LM6172 is fully symmetrical and

has a similar slewing characteristic to the current feedback

amplifiers. In the LM6172 Simplified Schematic, Q1 through

Q4 form the equivalent of the current feedback input buffer,

R

ers the inverting input. The triple-buffered output stage iso-

lates the gain stage from the load to provide low output im-

pedance.

LM6172 Slew Rate Characteristic

The slew rate of LM6172 is determined by the current avail-

able to charge and discharge an internal high impedance

node capacitor. This current is the differential input voltage

divided by the total degeneration resistor R

slew rate is proportional to the input voltage level, and the

higher slew rates are achievable in the lower gain configura-

tions.

When a very fast large signal pulse is applied to the input of

an amplifier, some overshoot or undershoot occurs. By plac-

ing an external series resistor such as 1 k

Ω to the input of

LM6172, the slew rate is reduced to help lower the over-

shoot, which reduces settling time.

Reducing Settling Time

The LM6172 has a very fast slew rate that causes overshoot

and undershoot. To reduce settling time on LM6172,a1k

Ω

resistor can be placed in series with the input signal to de-

crease slew rate. A feedback capacitor can also be used to

reduce overshoot and undershoot. This feedback capacitor

serves as a zero to increase the stability of the amplifier cir-

cuit. A 2 pF feedback capacitor is recommended for initial

evaluation. When the LM6172 is configured as a buffer, a

feedback resistor of 1 k

Ω must be added in parallel to the

feedback capacitor.

Another possible source of overshoot and undershoot

comes from capacitive load at the output. Please see the

section “Driving Capacitive Loads” for more detail.

Driving Capacitive Loads

Amplifiers driving capacitive loads can oscillate or have ring-

ing at the output. To eliminate oscillation or reduce ringing,

an isolation resistor can be placed as shown in

Figure 1. The

combination of the isolation resistor and the load capacitor

forms a pole to increase stability by adding more phase mar-

gin to the overall system. The desired performance depends

on the value of the isolation resistor; the bigger the isolation

resistor, the more damped (slow) the pulse response be-

comes. For LM6172, a 50

Ω isolation resistor is recom-

mended for initial evaluation.

DS012581-55

www.national.com

11