Datasheet

LM7171
www.ti.com SNOS760B –MAY 1999–REVISED MARCH 2013
APPLICATION NOTES
PERFORMANCE DISCUSSION
The LM7171 is a very high speed, voltage feedback amplifier. It consumes only 6.5 mA supply current while
providing a unity-gain bandwidth of 200 MHz and a slew rate of 4100V/μs. It also has other great features such
as low differential gain and phase and high output current.
The LM7171 is a true voltage feedback amplifier. Unlike current feedback amplifiers (CFAs) with a low inverting
input impedance and a high non-inverting input impedance, both inputs of voltage feedback amplifiers (VFAs)
have high impedance nodes. The low impedance inverting input in CFAs and a feedback capacitor create an
additional pole that will lead to instability. As a result, CFAs cannot be used in traditional op amp circuits such as
photodiode amplifiers, I-to-V converters and integrators where a feedback capacitor is required.
CIRCUIT OPERATION
The class AB input stage in LM7171 is fully symmetrical and has a similar slewing characteristic to the current
feedback amplifiers. In the LM7171 Simplified Schematic, 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.
SLEW RATE CHARACTERISTIC
The slew rate of LM7171 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. A curve of slew rate versus input voltage level is provided in the “Typical
Performance Characteristics”.
When a very fast large signal pulse is applied to the input of an amplifier, some overshoot or undershoot occurs.
By placing an external resistor such as 1 kΩ in series with the input of LM7171, the bandwidth is reduced to help
lower the overshoot.
SLEW RATE LIMITATION
If the amplifier's input signal has too large of an amplitude at too high of a frequency, the amplifier is said to be
slew rate limited; this can cause ringing in time domain and peaking in frequency domain at the output of the
amplifier.
In the “Typical Performance Characteristics” section, there are several curves of A
V
= +2 and A
V
= +4 versus
input signal levels. For the A
V
= +4 curves, no peaking is present and the LM7171 responds identically to the
different input signal levels of 30 mV, 100 mV and 300 mV.
For the A
V
= +2 curves, with slight peaking occurs. This peaking at high frequency (>100 MHz) is caused by a
large input signal at high enough frequency that exceeds the amplifier's slew rate. The peaking in frequency
response does not limit the pulse response in time domain, and the LM7171 is stable with noise gain of ≥+2.
LAYOUT CONSIDERATION
Printed Circuit Board and High Speed Op Amps
There are many things to consider when designing PC boards for high speed op amps. Without proper caution, it
is very easy to have excessive ringing, oscillation and other degraded AC performance in high speed circuits. As
a rule, the signal traces should be short and wide to provide low inductance and low impedance paths. Any
unused board space needs to be grounded to reduce stray signal pickup. Critical components should also be
grounded at a common point to eliminate voltage drop. Sockets add capacitance to the board and can affect high
frequency performance. It is better to solder the amplifier directly into the PC board without using any socket.
Using Probes
Active (FET) probes are ideal for taking high frequency measurements because they have wide bandwidth, high
input impedance and low input capacitance. However, the probe ground leads provide a long ground loop that
will produce errors in measurement. Instead, the probes can be grounded directly by removing the ground leads
and probe jackets and using scope probe jacks.
Copyright © 1999–2013, Texas Instruments Incorporated Submit Documentation Feedback 17
Product Folder Links: LM7171