Datasheet

LM6172
SNOS792D –MAY 1999–REVISED MARCH 2013 www.ti.com
LM6172 Simplified Schematic (Each Amplifier)
Figure 42.
APPLICATION NOTES
LM6172 PERFORMANCE DISCUSSION
The LM6172 is a dual high-speed, low power, voltage feedback amplifier. It is unity-gain stable and offers
outstanding performance with only 2.3mA of supply current per channel. The combination of 100MHz unity-gain
bandwidth, 3000V/μs slew rate, 50mA per channel output current and other attractive features makes it easy to
implement the LM6172 in various applications. Quiescent power of the LM6172 is 138mW operating at ±15V
supply and 46mW at ±5V supply.
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 Figure 42, Q1 through Q4 form the equivalent of the current feedback input buffer, R
E
the
equivalent of the feedback resistor, and stage A buffers the inverting input. The triple-buffered output stage
isolates the gain stage from the load to provide low output impedance.
LM6172 SLEW RATE CHARACTERISTIC
The slew rate of LM6172 is determined by the current available to charge and discharge an internal high
impedance node capacitor. This current is the differential input voltage divided by the total degeneration resistor
R
E
. Therefore, the slew rate is proportional to the input voltage level, and the higher slew rates are achievable in
the lower gain configurations.
When a very fast large signal pulse is applied to the input of an amplifier, some overshoot or undershoot occurs.
By placing an external series resistor such as 1kΩ to the input of LM6172, the slew rate is reduced to help lower
the overshoot, 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, a 1kΩ resistor can be placed in series with the input signal to decrease 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 circuit. A 2pF feedback capacitor is recommended for initial evaluation. When the
LM6172 is configured as a buffer, a feedback resistor of 1kΩ must be added in parallel to the feedback capacitor.
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