LT1217 데이터 시트보기 (PDF) - Linear Technology
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LT1217 Datasheet PDF : 8 Pages
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LT1217
TYPICAL PERFOR A CE CHARACTERISTICS
Settling Time to 10mV vs
Output Step
10
8 VS = ±15V
6 RF = RG = 3k
4
INVERTING
2
NON-INVERTING
0
–2
NON-INVERTING
–4
–6
–8
–10
0
INVERTING
50 100 150 200 250 300
SETTLING TIME (ns)
LT1217 • TPC19
Settling Time to 1mV vs
Output Step
10
8 VS = ±15V
RF = RG = 3k
6
4
NON-INVERTING
INVERTING
2
0
–2
NON-INVERTING
–4
–6
–8
–10
0
INVERTING
100 200 300 400 500
SETTLING TIME (ns)
LT1217 • TPC20
Supply Current vs Supply Voltage
1.4
T = 125°C
1.2
1.0
T = 25°C
0.8
T = –55°C
0.6
T = –55°C
0.4
T = 25°C, 125°C
0.2
SHUTDOWN
PIN 8 AT GND
0.0
0 2 4 6 8 10 12 14 16 18
SUPPLY VOLTAGE (±V)
LT1217 • TPC21
APPLICATI S I FOR ATIO
Current Feedback Basics
The small signal bandwidth of the LT1217, like all current
feedback amplifiers, isn’t a straight inverse function of the
closed loop gain. This is because the feedback resistors
determine the amount of current driving the amplifier’s
internal compensation capacitor. In fact, the amplifier’s
feedback resistor (RF) from output to inverting input
works with internal junction capacitances of the LT1217 to
set the closed loop bandwidth.
Even though the gain set resistor (RG) from inverting input
to ground works with RF to set the voltage gain just like it
does in a voltage feedback op amp, the closed loop
bandwidth does not change. This is because the equivalent
gain bandwidth product of the current feedback amplifier
is set by the Thevenin equivalent resistance at the inverting
input and the internal compensation capacitor. By keeping
RF constant and changing the gain with RG, the Thevenin
resistance changes by the same amount as the change in
gain. As a result, the net closed loop bandwidth of the
LT1217 remains the same for various closed loop gains.
The curve on the first page shows the LT1217 voltage gain
versus frequency while driving 100Ω, for five gain settings
from 1 to 100. The feedback resistor is a constant 3k and
the gain resistor is varied from infinity to 30Ω. Second
order effects reduce the bandwidth somewhat at the
higher gain settings.
Feedback Resistor Selection
The small signal bandwidth of the LT1217 is set by the
external feedback resistors and the internal junction ca-
pacitors. As a result, the bandwidth is a function of the
supply voltage, the value of the feedback resistor, the
closed loop gain and load resistor. The characteristic
curves of bandwidth versus supply voltage are done with
a heavy load (100Ω) and a light load (1kΩ) to show the
effect of loading. These graphs also show the family of
curves that result from various values of the feedback
resistor. These curves use a solid line when the response
has less than 0.5dB of peaking and a dashed line when the
response has 0.5dB to 5dB of peaking. The curves stop
where the response has more than 5dB of peaking.
At a gain of two, on ±15V supplies with a 3kΩ feedback
resistor, the bandwidth into a light load is 13.5MHz with a
little peaking, but into a heavy load the bandwidth is
10MHz with no peaking. At very high closed loop gains, the
bandwidth is limited by the gain bandwidth product of
about 100MHz. The curves show that the bandwidth at a
closed loop gain of 100 is about 1MHz.
Capacitance on the Inverting Input
Current feedback amplifiers want resistive feedback from
the output to the inverting input for stable operation. Take
5
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