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LT1228
(Rev.:Rev_A)

100MHz Current Feedback Amplifier with DC Gain Control

Linear Technology 

LT1228

APPLICATI S I FOR ATIO

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.

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:

Temperature Compensation of gm with a 2.5V Reference

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 (gm) is set by

the current that flows into pin 5, ISET. The voltage at pin 5

is two forward biased diode drops above the negative

supply, pin 4. Therefore the voltage at pin 5 (with

respect to V–) is about 1.2V and changes with the log of

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 (gm) is equal to ten

times the value of ISET (in mA/mV) and this relationship

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).

gm

4

5

R

ISET

LT1004-2.5

V–

R

ISET

Vbe

2.5V

2Eg

Vbe

LT1228 • TA04

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:

Since gm

=

q

kT

×

ISET

3.87

=

10

× ISET

and

V be

=

Eg

–

akT

q

where

a

=



In

cTn 

Ic 

≈

19.4

at

27°C

( ) c = 0.001, n = 3, Ic = 100µA

Eg is about 1.25V so the 2.5V reference is 2Eg. Solving

the loop for the set current gives:

ISET

=

2E g

–

2





E

g

R

–

akT 

q 

or ISET

=

2akT

Rq

9

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