LT6201CS8 datasheet(20/26 Pages) LINER

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LT6200/LT6200-5

LT6200-10/LT6201

62001fd

APPLICATIONS INFORMATION

Ampliﬁer Characteristics

Figure 1 shows a simpliﬁed schematic of the LT6200

family, which has two input differential ampliﬁers in paral-

lel that are biased on simultaneously when the common

mode voltage is at least 1.5V from either rail. This topology

allows the input stage to swing from the positive supply

voltage to the negative supply voltage. As the common

mode voltage swings beyond VCC – 1.5V, current source I1

saturates and current in Q1/Q4 is zero. Feedback is main-

tained through the Q2/Q3 differential ampliﬁer, but with

an input gm reduction of one-half. A similar effect occurs

with I2 when the common mode voltage swings within

1.5V of the negative rail. The effect of the gm reduction is

a shift in the VOS as I1 or I2 saturate.

Input bias current normally ﬂows out of the “+” and “–”

inputs. The magnitude of this current increases when the

input common mode voltage is within 1.5V of the negative

rail, and only Q1/Q4 are active. The polarity of this current

reverses when the input common mode voltage is within

1.5V of the positive rail and only Q2/Q3 are active.

The second stage is a folded cascode and current mir-

ror that converts the input stage differential signals to a

single ended output. Capacitor C1 reduces the unity cross

frequency and improves the frequency stability with-

out degrading the gain bandwidth of the ampliﬁer. The

differential drive generator supplies current to the output

transistors that swing from rail-to-rail.

The LT6200-5/LT6200-10 are decompensated op amps

for higher gain applications. These amplifiers maintain

identical DC specifications with the LT6200, but have a

reduced Miller compensation capacitor CM. This results

in a significantly higher slew rate and gain bandwidth

product.

Input Protection

There are back-to-back diodes, D1 and D2, across the

+ and – inputs of these amplifiers to limit the differential

input voltage to ± 0.7V. The inputs of the LT6200 family

do not have internal resistors in series with the input

transistors. This technique is often used to protect the

input devices from overvoltage that causes excessive

currents to flow. The addition of these resistors would

significantly degrade the low noise voltage 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 0.95nV/√Hz

to 2.03nV/√Hz. Once the input differential voltage ex-

ceeds ±0.7V, steady-state current conducted though

the protection diodes should be limited to ±40mA.

This implies 25Ω of protection resistance per volt of

continuous overdrive beyond ±0.7V. The input diodes

are rugged enough to handle transient currents due to

amplifier slew rate overdrive or momentary clipping

without these resistors.

DIFFERENTIAL

DRIVE

GENERATOR

Q10

Q11

DESD2

DESD4

DESD3

DESD1

DESD5

DESD8

DESD7

DESD6

–

–V

V+

V–

6203/04 F01

BIAS

VSHDN

Figure 1. Simpliﬁed Schematic

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