LT1228 Linear Technology, LT1228 Datasheet - Page 9

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

The LT1228 contains two amplifiers, a transconductance
amplifier (voltage-to-current) and a current feedback am-
plifier (voltage-to-voltage). The gain of the transconduc-
tance amplifier is proportional to the current that is exter-
nally programmed into pin 5. Both amplifiers are designed
to operate on almost any available supply voltage from 4V
( 2V) to 30V ( 15V). The output of the transconductance
amplifier is connected to the noninverting input of the
current feedback amplifier so that both fit into an eight pin
package.
TRANSCONDUCTANCE AMPLIFIER
The LT1228 transconductance amplifier has a high imped-
ance differential input (pins 2 and 3) and a current source
output (pin 1) with wide output voltage compliance. The
voltage to current gain or transconductance (g
the current that flows into pin 5, I
is two forward biased diode drops above the negative
supply, pin 4. Therefore the voltage at pin 5 (with
respect to V
the set current (120mV/decade), see the characteristic
curves. The temperature coefficient of this voltage is
about –4mV/ C (–3300ppm/ C) and the temperature co-
efficient of the logging characteristic is 3300ppm/ C. It is
important that the current into pin 5 be limited to less than
15mA. THE LT1228 WILL BE DESTROYED IF PIN 5 IS
SHORTED TO GROUND OR TO THE POSITIVE SUPPLY. A
limiting resistor (2k or so) should be used to prevent more
than 15mA from flowing into pin 5.
The small-signal transconductance (g
times the value of I
holds over many decades of set current (see the character-
istic curves). The transconductance is inversely propor-
tional to absolute temperature (–3300ppm/ C). The input
stage of the transconductance amplifier has been de-
signed to operate with much larger signals than is possible
with an ordinary diff-amp. The transconductance of the
input stage varies much less than 1% for differential input
signals over a 30 mV range (see the characteristic curve
Small-Signal Transconductance vs DC Input Voltage).
A
PPLICATI
–
) is about 1.2V and changes with the log of
O
SET
U
S
(in mA/mV) and this relationship
I FOR ATIO
U
SET
. The voltage at pin 5
W
m
) is equal to ten
m
) is set by
U
Resistance Controlled Gain
If the set current is to be set or varied with a resistor or
potentiometer it is possible to use the negative tempera-
ture coefficient at pin 5 (with respect to pin 4) to compen-
sate for the negative temperature coefficient of the transcon-
ductance. The easiest way is to use an LT1004-2.5, a 2.5V
reference diode, as shown below:
The current flowing into pin 5 has a positive temperature
coefficient that cancels the negative coefficient of the
transconductance. The following derivation shows why a
2.5V reference results in zero gain change with tempera-
ture:
E
the loop for the set current gives:
g
is about 1.25V so the 2.5V reference is 2E
V
Temperature Compensation of g
Since g
and V
–
I
c
SET
0 001
be
.
LT1004-2.5
m
2
E
R
,
g
E
n
kT
g
–
q
–
2
3
akT
5
,
R
E
Ic
q
3 87
I
g
SET
g
.
m
I
SET
–
where a In
100
4
akT
q
10
A
or I
m
I
SET
with a 2.5V Reference
SET
cT
Ic
n
2.5V
2E
2
g
Rq
I
akT
SET
R
19 4
LT1228
g
.
. Solving
LT1228 • TA04
at
27
V
V
be
be
9
C