LT1228I(Rev_C) 查看數據表（PDF） - Linear Technology

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LT1228I
(Rev.:Rev_C)

100MHz Current Feedback Amplifier with DC Gain Control

Linear Technology 

LT1228

APPLICATIO S I FOR ATIO

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 350µV/V of supply mismatch, the inverting

bias current changes about 2.5µA/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.

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 150Ω. The

maximum set current is 1mA.

( ) ( ) PD = 2VS ISMAX + 3.5ISET

+

VS – V OMAX

VOMAX

RL

[ ( )] ( ) PD = 2 • 12V • 7mA + 3.5 • 1mA

+ 12V – 2V 2V

150Ω

= 0.252 + 0.133 = 0.385W

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 pack-

age (thermal resistance is 150°C/W) gives:

Temperature Rise = PDθJA = 0.385W • 150°C/W

= 57.75°C

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.

TYPICAL APPLICATIO S

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 (gm) and referred to ground. The voltage gain of

the transconductance amplifier is:

gm • R1= 10 • ISET • R1

Lastly the signal is buffered and amplified by the current

feedback amplifier (CFA). The voltage gain of the current

feedback amplifier is:

1+ RF

RG

The overall gain of the gain controlled amplifier is the

product of all three stages:

AV

=

⎛

⎝⎜ R3

R3 ⎞

+ R3A⎠⎟

• 10

• ISET

⎛

• R1• ⎝⎜1+

RF ⎞

RG ⎠⎟

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.

1228fc

14

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