LT1228 Linear Technology, LT1228 Datasheet - Page 14

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

LT1228
A
Power Supplies
The LT1228 amplifiers will operate from single or split
supplies from 2V (4V total) to 18V (36V total). It is not
necessary to use equal value split supplies, however the
offset voltage and inverting input bias current of the
current feedback amplifier will degrade. The offset voltage
changes about 350 V/V of supply mismatch, the inverting
bias current changes about 2.5 A/V of supply mismatch.
Power Dissipation
The worst case amplifier power dissipation is the total of
the quiescent current times the total power supply voltage
plus the power in the IC due to the load. The quiescent
supply current of the LT1228 transconductance amplifier
is equal to 3.5 times the set current at all temperatures. The
quiescent supply current of the LT1228 current feedback
amplifier has a strong negative temperature coefficient
and at 150 C is less than 7mA, typically only 4.5mA. The
power in the IC due to the load is a function of the output
voltage, the supply voltage and load resistance. The worst
case occurs when the output voltage is at half supply, if it
can go that far, or its maximum value if it cannot reach half
supply.
Basic Gain Control
The basic gain controlled amplifier is shown on the front
page of the data sheet. The gain is directly proportional to
the set current. The signal passes through three stages
from the input to the output.
First the input signal is attenuated to match the dynamic
range of the transconductance amplifier. The attenuator
should reduce the signal down to less than 100mV peak.
The characteristic curves can be used to estimate how
much distortion there will be at maximum input signal. For
single ended inputs eliminate R2A or R3A.
The signal is then amplified by the transconductance
amplifier (g
the transconductance amplifier is:
14
TYPICAL
PPLICATI
m
) and referred to ground. The voltage gain of
A
O
PPLICATI
U
S
I FOR ATIO
U
O
U
W
S
U
For example, let’s calculate the worst case power dissipa-
tion in a variable gain video cable driver operating on 12V
supplies that delivers a maximum of 2V into 150 . The
maximum set current is 1mA.
The total power dissipation times the thermal resistance of
the package gives the temperature rise of the die above
ambient. The above example in SO-8 surface mount
package (thermal resistance is 150 C/W) gives:
Therefore the maximum junction temperature is 70 C
+57.75 C or 127.75 C, well under the absolute maximum
junction temperature for plastic packages of 150 C.
Lastly the signal is buffered and amplified by the current
feedback amplifier (CFA). The voltage gain of the current
feedback amplifier is:
The overall gain of the gain controlled amplifier is the
product of all three stages:
More than one output can be summed into R1 because the
output of the transconductance amplifier is a current. This
is the simplest way to make a video mixer.
P
P
1
Temperature Rise = P
g
A
D
D
m
V
R
R
0 252
2
2 12
G
F
.
R
V I
S SMAX
1 10
R
3
V
R
0 133
3
R A
.
3
7
I
mA
SET
3 5
.
= 57.75 C
I
0 385
SET
.
10
3 5 1
D JA
R
.
1
W
I
= 0.385W 150 C/W
V
SET
S
mA
–
V
R
OMAX
1
12
V
1
V
OMAX
–
R
2
L
R
R
V
F
G
150
2
V