lm6172mdr National Semiconductor Corporation, lm6172mdr Datasheet - Page 13

• Manufacturer Part Number: lm6172mdr
• Description: Lm6172qml Dual High Speed, Low Power, Low Distortion, Voltage Feedback Amplifiers
• Manufacturer: National Semiconductor Corporation

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 2.3mA of supply current per channel.
The combination of 100MHz unity-gain bandwidth, 3000V/μs
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 (Page 2), Q1
through Q4 form the equivalent of the current feedback input
buffer, R
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 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.
E
the equivalent of the feedback resistor, and stage
Harmonic Distortion vs. Frequency
Crosstalk Rejection vs. Frequency
(V
S
= ±15V)
E
. Therefore, the
20159430
20159431
13
Maximum Power Dissipation vs. Ambient Temperature
slew rate, 50mA per channel output current and other attrac-
tive 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.
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 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 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 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.
Harmonic Distortion vs. Frequency
(V
S
= ±5V)
20159432
20159434
www.national.com