Datasheet

10
LT1207
U
S
A
O
PP
L
IC
AT
I
WU
U
I FOR ATIO
and for higher gains in the noninverting mode, the signal
amplitude on the input pins is small and the overall slew
rate is that of the output stage. The input stage slew rate
is related to the quiescent current and will be reduced as
the supply current is reduced. The output slew rate is set
by the value of the feedback resistors and the internal
capacitance. Larger feedback resistors will reduce the
slew rate as will lower supply voltages, similar to the way
the bandwidth is reduced. The photos (Figures 5a, 5b and
5c) show the large-signal response of the LT1207 or
various gain configurations. The slew rate varies from
860V/µs for a gain of 1, to 1400V/µs for a gain of –1.
When the LT1207 is used to drive capacitive loads, the
available output current can limit the overall slew rate. In
the fastest configuration, the LT1207 is capable of a slew
rate of over 1V/ns. The current required to slew a capacitor
Figure 5b. Large-Signal Response, A
V
= –1
LT1207 • F05b
R
F
= RG = 750Ω
R
L
= 50Ω
V
S
= ±15V
The amplifier’s supply current will be approximately 40
times the current in the Shutdown pin. The voltage across
the resistor in this condition is V
+
– 3V
BE
. For example, a
60k resistor will set the amplifier’s supply current to 10mA
with V
S
= ±15V.
The photos (Figures 4a and 4b) show the effect of reducing
the quiescent supply current on the large-signal response.
The quiescent current can be reduced to 5mA in the
inverting configuration without much change in response.
In noninverting mode, however, the slew rate is reduced
as the quiescent current is reduced.
Figure 4a. Large-Signal Response vs I
Q
, A
V
= –1
Figure 4b. Large-Signal Response vs I
Q
, A
V
= 2
LT1207 • F04a
R
F
= 750Ω
R
L
= 50Ω
I
Q
= 5mA, 10mA, 20mA
V
S
= ±15V
LT1207 • F04b
R
F
= 750Ω
R
L
= 50Ω
I
Q
= 5mA, 10mA, 20mA
V
S
= ±15V
Slew Rate
Unlike a traditional op amp, the slew rate of a current
feedback amplifier is not independent of the amplifier gain
configuration. There are slew rate limitations in both the
input stage and the output stage. In the inverting mode,
LT1207 • F05a
R
F
= 825Ω
R
L
= 50Ω
V
S
= ±15V
Figure 5a. Large-Signal Response, A
V
= 1