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LT1228

100MHz Current Feedback Amplifier with DC Gain Control

Linear Technology 

LT1228

APPLICATIONS INFORMATION

The LT1228 contains two amplifiers, a transconductance

amplifier (voltage-to-current) and a current feedback ampli-

fier (voltage-to-voltage). The gain of the transconductance

amplifier is proportional to the current that is externally

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 (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 nega-

tive 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 coefficient

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 given as

gm = 10 • ISET, with gm in (A/V) and ISET in (A).This rela-

tionship holds over many decades of set current (see the

characteristic curves). The transconductance is inversely

proportional to absolute temperature (–3300ppm/°C). The

input stage of the transconductance amplifier has been

designed to operate with much larger signals than is pos-

sible 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).

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 temperature

coefficient at Pin 5 (with respect to Pin 4) to compensate

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

gm

4

5

R

ISET

R

ISET

Vbe

2.5V

2Eg

Vbe

LT1004-2.5

V–

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 temperature:

Since

gm

=

q

kT

×

ISET

3.87

=

10

• ISET

akT

 cTn 

and Vbe = Eg –

q

where a = In



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

=

2Eg

–

2

Eg

–

akT

q





R

or ISET

= 2akT

Rq

1228fd

9

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