LT1228 Linear Technology, LT1228 Datasheet - Page 17

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

Filters
Using the variable transconductance of the LT1228 to
make variable filters is easy and predictable. The most
straight forward way is to make an integrator by putting a
capacitor at the output of the transconductance amp and
buffering it with the current feedback amplifier. Because
the input bias current of the current feedback amplifier
must be supplied by the transconductance amplifier, the
set current should not be operated below 10 A. This limits
the filters to about a 100:1 tuning range.
The Single Pole circuit realizes a single pole filter with a
corner frequency (f
TYPICAL
A
PPLICATI
C
) proportional to the set current. The
O
LOWPASS
HIGHPASS
INPUT
U
INPUT
V
IN
S
V
IN
R3A
120
1k
–135
–180
–45
–90
R3
Single Pole Low/High/Allpass Filter
0
10k
Allpass Filter Phase Response
100 A SET CURRENT
3
2
R2
120
–
+
g
m
100k
I
f
f
SET
C
C
FREQUENCY (Hz)
5
=
= 10
10
2
1mA SET CURRENT
330pF
9
R
1k
I
SET
G
values shown give a 100kHz corner frequency for 100 A
set current. The circuit has two inputs, a lowpass filter
input and a highpass filter input. To make a lowpass filter,
ground the highpass input and drive the lowpass input.
Conversely for a highpass filter, ground the lowpass input
and drive the highpass input. If both inputs are driven, the
result is an allpass filter or phase shifter. The allpass has
flat amplitude response and 0 phase shift at low frequen-
cies, going to –180 at high frequencies. The allpass filter
has –90 phase shift at the corner frequency.
C
I
SET
FOR THE VALUES SHOWN
C
R2A
1k
1M
R
8
1
F
R
+ 1
G
LT1228 • TA17
+
–
CFA
R2 + R2A
R
1k
10M
F
R2
6
LT1228 • TA16
V
OUT
LT1228
17