LTC6246 datasheet(17/24 Pages) LINER

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LT6236/LT6237

623637f

Functional Description

Figure 1 is a simplified schematic of the LT6236/LT6237,

which has a pair of low noise input transistors Q1 and Q2.

A simple current mirror Q3/Q4 converts the differential

signal to a single-ended output, and these transistors are

degenerated to reduce their contribution to the overall

noise. Capacitor C1 reduces the unity cross frequency

and improves the frequency stability without degrading

the gain bandwidth of the amplifier. Capacitor CM sets

the overall amplifier gain bandwidth. The differential drive

generator supplies current to transistors Q5 and Q6 that

provide rail-to-rail output swing.

Input Protection

Back-to-back diodes, D1 and D2, limit the differential input

voltage to ±0.7V. The inputs of the LT6236/LT6237 do not

have internal resistors in series with the input transistors.

This technique is often used to protect the input devices

from over voltage that causes excessive current to flow.

The addition of these resistors would significantly degrade

the voltage noise of these amplifiers.

For instance, a 100Ω resistor in series with each input

would generate 1.8nV/√Hz of noise, and the total amplifier

noise voltage would rise from 1.1nV/√Hz to 2.1nV/√Hz.

Once the input differential voltage exceeds ±0.7V, steady

state current conducted through the protection diodes

should be limited to ±40mA. This implies 25Ω of protec-

tion resistance is necessary per volt of overdrive beyond

APPLICATIONS INFORMATION

±0.7V. These input diodes are rugged enough to handle

transient currents due to amplifier slew rate overdrive and

clipping without protection resistors. Figure 2 shows the

output response to an input overdrive with the amplifier

connected as a voltage follower. With the input signal

low, current source I1 saturates and the differential drive

generator drives Q6 into saturation so the output voltage

swings all the way to V–. The input can swing positive

until transistor Q2 saturates into current mirror Q3/Q4.

When saturation occurs, the output tries to phase invert,

but diode D2 conducts current from the signal source to

the output through the feedback connection. The output

is clamped a diode drop below the input. In Figure 2, the

input signal generator is limiting at about 20mA.

With the amplifier connected in a gain of AV ≥ 2, the output

can invert with very heavy overdrive. To avoid this inver-

sion, limit the input overdrive to 0.5V beyond the power

supply rails.

Figure 1. Simplified Schematic

ENABLE

DESD6

DESD5

–V

+VIN

–VIN

62367 F01

BIAS

DIFFERENTIAL

DRIVE GENERATOR

VOUT

–V

DESD3

–V

DESD4

DESD1

–V

DESD2

Figure 2. VS = ±2.5V, AV = 1 with Large Overdrive

2.5V

–2.5V

500μs/DIV

62367 F02

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