Linear Technology LT6200CS6-5#TRMPBF Linear IC LT6200CS6-5#TRMPBF データシート

LT6200/LT6200-5

LT6200-10/LT6201

20

62001ff

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 V

CC 

– 1.5V, current source I

1

 

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

tained through the Q2/Q3 differential amplifier, but with 

an input g

m

 reduction of one-half. A similar effect occurs 

with I

2

 when the common mode voltage swings within 

1.5V of the negative rail. The effect of the g

m

 reduction is 

a shift in the V

OS

 as I

1

 or I

2

 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 C

M

. 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

R1

R2

R3

R4

R5

Q2

Q3

Q5

Q6

Q9

Q8

Q7

Q10

Q11

Q1

Q4

I

1

I

2

D3

D2

D1

DESD2

DESD4

DESD3

DESD1

DESD5

DESD8

DESD7

DESD6

+

–

C

M

C1

+V

–V

+V

+V

+V

–V

–V

–V

V

+

V–

6203/04 F01

BIAS

V

SHDN