LT1228CS8 datasheet(10/20 Pages) LINER

10 / 20 page

LT1228

APPLICATI

I FOR ATIO

Substituting into the equation for transconductance gives:

m ==

194

The temperature variation in the term “a” can be ignored

since it is much less than that of the term “T” in the

equation for Vbe. Using a 2.5V source this way will main-

tain the gain constant within 1% over the full temperature

range of –55

°C to 125°C. If the 2.5V source is off by 10%,

the gain will vary only about

±6% over the same tempera-

ture range.

We can also temperature compensate the transconduc-

tance without using a 2.5V reference if the negative power

supply is regulated. A Thevenin equivalent of 2.5V is

generated from two resistors to replace the reference. The

two resistors also determine the maximum set current,

approximately 1.1V/RTH. By rearranging the Thevenin

equations to solve for R4 and R6 we get the following

equations in terms of RTH and the negative supply, VEE.

and R

TH EE









–

Temperature Compensation of gm with a Thevenin Voltage

LT1228 • TA05

1.24k

Ω

–15V

ISET

1.03k

Vbe

VTH = 2.5V

6.19k

Ω

Voltage Controlled Gain

To use a voltage to control the gain of the transconduc-

tance amplifier requires converting the voltage into a

current that flows into pin 5. Because the voltage at pin 5

is two diode drops above the negative supply, a single

resistor from the control voltage source to pin 5 will suffice

in many applications. The control voltage is referenced to

the negative supply and has an offset of about 900mV. The

conversion will be monotonic, but the linearity is deter-

mined by the change in the voltage at pin 5 (120mV per

decade of current). The characteristic is very repeatable

since the voltage at pin 5 will vary less than

±5% from part

to part. The voltage at pin 5 also has a negative tempera-

ture coefficient as described in the previous section. When

the gain of several LT1228s are to be varied together, the

current can be split equally by using equal value resistors

to each pin 5.

For more accurate (and linear) control, a voltage-to-

current converter circuit using one op amp can be used.

The following circuit has several advantages. The input no

longer has to be referenced to the negative supply and the

input can be either polarity (or differential). This circuit

works on both single and split supplies since the input

voltage and the pin 5 voltage are independent of each

other. The temperature coefficient of the output current is

set by R5.

LT1228 • TA19

IOUT

TO PIN 5

OF LT1228

50pF

R1 = R2

R3 = R4

IOUT =

= 1mA/V

–

(V1 – V2)

LT1006

Digital control of the transconductance amplifier gain is

done by converting the output of a DAC to a current flowing

into pin 5. Unfortunately most current output DACs

sink rather than source current and do not have output

LT1228CS8 bảng dữ liệu(PDF) 10 Page - Linear Technology

LT1228CS8 bảng dữ liệu(HTML) 10 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

bảng dữ liệu tải về

Link URL