LT1228 Linear Technology, LT1228 Datasheet - Page 12

• Manufacturer Part Number: LT1228
• Description: 100MHz Current Feedback Amplifier with DC Gain Control
• Manufacturer: Linear Technology

LT1228
A
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
for the response with 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 750 feedback
resistor, the bandwidth into a light load is over 160MHz
without peaking, but into a heavy load the bandwidth
reduces to 100MHz. The loading has so much effect
because there is a mild resonance in the output stage that
enhances the bandwidth at light loads but has its Q
reduced by the heavy load. This enhancement is only
useful at low gain settings, at a gain of ten it does not boost
the bandwidth. At unity gain, the enhancement is so
effective the value of the feedback resistor has very little
effect on the bandwidth. At very high closed-loop gains,
the bandwidth is limited by the gain-bandwidth product of
about 1GHz. The curves show that the bandwidth at a
closed-loop gain of 100 is 10MHz, only one tenth what it
is at a gain of two.
12
PPLICATI
Current Feedback Amp Small-Signal Response
V
S
= 15V, R
O
U
S
F
= R
I FOR ATIO
U
G
= 750 , R
W
L
= 100
U
Capacitance on the Inverting Input
Current feedback amplifiers want resistive feedback from
the output to the inverting input for stable operation. Take
care to minimize the stray capacitance between the output
and the inverting input. Capacitance on the inverting input
to ground will cause peaking in the frequency response
(and overshoot in the transient response), but it does not
degrade the stability of the amplifier. The amount of
capacitance that is necessary to cause peaking is a func-
tion of the closed-loop gain taken. The higher the gain, the
more capacitance is required to cause peaking. For ex-
ample, in a gain of 100 application, the bandwidth can be
increased from 10MHz to 17MHz by adding a 2200pF
capacitor, as shown below. C
resistance, such as silver mica.
Boosting Bandwidth of High Gain Amplifier
V
IN
49
46
43
40
37
34
31
28
25
22
19
with Capacitance On Inverting Input
C
G
1
R
5.1
G
8
1
–
+
FREQUENCY (MHz)
CFA
510
R
C
G
10
F
G
C
must have very low series
= 0
G
= 4700pF
6
C
G
= 2200pF
LT1228 • TA09
LT1228 • TA08
V
100
OUT