9

LT1223

S

APPLICATI

I FOR ATIO

have about 2.5mV thermal tail on a 10V step. Unfortu-

nately, reducing the input signal and increasing the gain

always results in a thermal tail of about the same amount

for a given output step. For this reason we show separate

graphs of 10mV and 1mV non-inverting amplifier settling

times. Just as the bandwidth of the LT1223 is fairly

constant for various closed-loop gains, the settling time

remains constant as well.

Adjustable Gain Amplifier

To make a variable gain amplifier with the LT1223, vary the

controlled resistor, a FET, or any other low capacitance

be reduced at maximum gain and the circuit will oscillate

LT1223 • TA03

+

–

IN

V

OUT

V

LT1223

Adjustable Bandwidth Amplifier

Because the resistance at the inverting input determines

the bandwidth of the LT1223, an adjustable bandwidth

circuit can be made easily. The gain is set as before with

resistor is at a minimum.

Accurate Bandwidth Limiting The LT1223

It is very common to limit the bandwidth of an op amp by

A SMALL CAPACITOR FROM THE INVERTING INPUT OF

A CURRENT FEEDBACK AMPLIFIER TO ANYWHERE ELSE,

ESPECIALLY NOT TO THE OUTPUT. The capacitor on the

inverting input will cause peaking or oscillations. If you

need to limit the bandwidth of a current feedback amplifier,

use a resistor and capacitor at the noninverting input (R1

& C1). This technique will also cancel (to a degree) the

peaking caused by stray capacitance at the inverting input.

Unfortunately, this will not limit the output noise the way

it does for the op amp.

LT1223 • TA04

+

–

IN

V

OUT

V

LT1223

5k

LT1223 • TA05

+

–

IN

V

OUT

V

LT1223

R1

R1 = 300

C1

Ω

C1 = 100pF

BW = 5MHz

LT1223 • TA06

+

–

IN

V

OUT

V

LT1223

1k

OUT

V

IN

V

1

sC R

=

I

I

I

R

Current Feedback Amplifier Integrator

Since we remember that the inverting input wants to see

a resistor, we can add one to the standard integrator

circuit. This generates a new summing node where we can

apply capacitive feedback. The LT1223 integrator has

excellent large signal capability and accurate phase shift at

high frequencies.