LT1207
APPLICATI
12
10
8
S I FOR ATIO
V
S
=
±15V
R
F
= 1.2k
COMPENSATION
VOLTAGE GAIN (dB)
6
4
2
0
–2
–4
–6
–8
1
R
F
= 2k
NO COMPENSATION
R
F
= 2k
COMPENSATION
10
FREQUENCY (MHz)
100
LT1207 • F01
Figure 1.
The network has the greatest effect for C
L
in the range of
0pF to 1000pF. The graph of Maximum Capacitive Load vs
Feedback Resistor can be used to select the appropriate
value of the feedback resistor. The values shown are for
0.5dB and 5dB peaking at a gain of 2 with no resistive load.
This is a worst-case condition, as the amplifier is more stable
at higher gains and with some resistive load in parallel with
the capacitance. Also shown is the –3dB bandwidth with the
suggested feedback resistor vs the load capacitance.
Although the optional compensation works well with
capacitive loads, it simply reduces the bandwidth when it
is connected with resistive loads. For instance, with a 30Ω
load, the bandwidth drops from 55MHz to 35MHz when
the compensation is connected. Hence, the compensation
was made optional. To disconnect the optional compensa-
tion, leave the COMP pin open.
Shutdown/Current Set
If the shutdown feature is not used, the Shutdown pins
must be connected to ground or V
–
.
Each amplifier has a separate Shutdown pin which can be
used to either turn off the amplifier, which reduces the
amplifier supply current to less than 200µA, or to control
the supply current in normal operation.
The supply current in each amplifier is controlled by the
current flowing out of the Shutdown pin. When the Shut-
down pin is open or driven to the positive supply, the
amplifier is shut down. In the shutdown mode, the output
looks like a 40pF capacitor and the supply current is
ENABLE
V
OUT
U
typically 100µA. Each Shutdown pin is referenced to the
positive supply through an internal bias circuit (see the
Simplified Schematic). An easy way to force shutdown is
to use open drain (collector) logic. The circuit shown in
Figure 2 uses a 74C904 buffer to interface between 5V
logic and the LT1207. The switching time between the
active and shutdown states is less than 1µs. A 24k pull-up
resistor speeds up the turn-off time and insures that the
amplifier is completely turned off. Because the pin is
referenced to the positive supply, the logic used should
have a breakdown voltage of greater than the positive
supply voltage. No other circuitry is necessary as the
internal circuit limits the Shutdown pin current to about
500µA. Figure 3 shows the resulting waveforms.
15V
V
IN
W
U
UO
+
–
1/2 LT1207
SHDN
–15V
V
OUT
R
F
15V
5V
ENABLE
74C906
24k
R
G
LT1207 • F02
Figure 2. Shutdown Interface
A
V
= 1
R
F
= 825Ω
R
L
= 50Ω
R
PU
= 24k
V
IN
= 1V
P-P
LT1207 • F3
Figure 3. Shutdown Operation
For applications where the full bandwidth of the amplifier
is not required, the quiescent current may be reduced by
connecting a resistor from the Shutdown pin to ground.
9
LINER [ LINEAR TECHNOLOGY ]
