LM6171

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SNOS745C – MAY 1998 – REVISED MARCH 2013

APPLICATION INFORMATION

LM6171 PERFORMANCE DISCUSSION

The LM6171 is a high speed, unity-gain stable voltage feedback amplifier. It consumes only 2.5 mA supply

current while providing a gain-bandwidth product of 100 MHz and a slew rate of 3600V/

μs. It also has other great

features such as low differential gain and phase and high output current. The LM6171 is a good choice in high

speed circuits.

The LM6171 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 will couple with feedback capacitor and

cause oscillation. As a result, CFAs cannot be used in traditional op amp circuits such as photodiode amplifiers,

I-to-V converters and integrators.

LM6171 CIRCUIT OPERATION

The class AB input stage in LM6171 is fully symmetrical and has a similar slewing characteristic to the current

feedback amplifiers. In LM6171 Figure 56, Q1 through Q4 form the equivalent of the current feedback input

output stage isolates the gain stage from the load to provide low output impedance.

LM6171 SLEW RATE CHARACTERISTIC

The slew rate of LM6171 is determined by the current available to charge and discharge an internal high

impedance node capacitor. The current is the differential input voltage divided by the total degeneration resistor

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 1 k

Ω to the input of LM6171, the bandwidth is reduced to help

lower the overshoot.

LAYOUT CONSIDERATION

Printed Circuit Boards 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 and frustrating 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 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.

Components Selection And Feedback Resistor

It is important in high speed applications to keep all component leads short because wires are inductive at high

frequency. For discrete components, choose carbon composition-type resistors and mica-type capacitors.

Surface mount components are preferred over discrete components for minimum inductive effect.

Large values of feedback resistors can couple with parasitic capacitance and cause undesirable effects such as

ringing or oscillation in high speed amplifiers. For LM6171, a feedback resistor of 510

Ω gives optimal

performance.

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