LT6200-10 Linear Technology Corporation, LT6200-10 Datasheet - Page 20

• Manufacturer Part Number: LT6200-10
• Description: 165mhz, Rail-to-rail Input And Output, 0.95nv/rthz Low Noise, Op Amp Family
• Manufacturer: Linear Technology Corporation

LT6200/LT6200-5
LT6200-10/LT6201
APPLICATIO S I FOR ATIO
Amplifier Characteristics
Figure 1 shows a simplified schematic of the LT6200
family, which has two input differential amplifiers in par-
allel 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
I
maintained through the Q2/Q3 differential amplifier, but
with an input g
with I
1.5V of the negative rail. The effect of the g
a shift in the V
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 mirror
that converts the input stage differential signals to a single
ended output. Capacitor C1 reduces the unity cross
frequency and improves the frequency stability without
degrading the gain bandwidth of the amplifier. The
differential drive generator supplies current to the output
transistors that swing from rail-to-rail.
20
1
saturates and current in Q1/Q4 is zero. Feedback is
2
when the common mode voltage swings within
DESD1
DESD3
OS
m
+
–
reduction of 1/2. A similar effect occurs
as I
–V
–V
U
1
+V
+V
or I
D1
DESD2
DESD4
U
2
saturate.
CC
D2
– 1.5V, current source
W
Q1
R3
R1
Q2
m
I
I
1
2
reduction is
Figure 1. Simplified Schematic
U
Q3
R2
Q4
R4
The LT6200-5/LT6200-10 are decompensated op amps for
higher gain applications. These amplifiers maintain iden-
tical DC specifications with the LT6200, but have a reduced
Miller compensation capacitor C
cantly 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 transis-
tors. 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 100 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 exceeds 0.7V, steady-state
current conducted though the protection diodes should be
limited to 40mA. This implies 25 of protection resis-
tance per volt of continuous overdrive beyond 0.7V. The
input diodes are rugged enough to handle transient cur-
rents due to amplifier slew rate overdrive or momentary
clipping without these resistors.
Figure 2 shows the input and output waveforms of the
LT6200 driven into clipping while connected in a gain of
Q8
Q5
C1
R5
D3
+V
Q9
Q7
Q6
BIAS
DIFFERENTIAL
C
GENERATOR
M
DRIVE
6203/04 F01
Q11
Q10
M
V –
–V
. This results in a signifi-
DESD7
–V
+V
DESD6
V
DESD8
DESD5
+
V
SHDN
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