7

LT1221

offset current have identical specifications. For this reason,

matching the impedance on the inputs to reduce bias

current errors is not necessary.

Capacitive Loading

The LT1221 is stable with capacitive loads. This is accom-

plished by sensing the load induced output pole and adding

compensation at the amplifier gain node. As the capacitive

load increases, both the bandwidth and phase margin

decrease. There will be peaking in the frequency domain as

shown in the curve of Frequency Response vs Capacitive

Load. The small-signal transient response will have more

overshoot as shown in the photo of the small-signal

response with 1000pF load. The large-signal response with

a 10,000pF load shows the output slew rate being limited

to 4V/

µs by the short-circuit current. The LT1221 can drive

coaxial cable directly, but for best pulse fidelity a resistor of

value equal to the characteristic impedance of the cable

(i.e., 75

Ω) should be placed in series with the output. The

other end of the cable should be terminated with the same

value resistor to ground.

Compensation

The LT1221 has a typical gain-bandwidth product of

150MHz which allows it to have wide bandwidth in high

gain configurations (i.e., in a gain of 10, it will have a

bandwidth of about 15MHz). The amplifier is stable in a

noise gain of 4 so the ratio of the signal at the inverting input

to the output must be 1/4 or less. Straightforward gain

configurations of 4 or –3 are stable, but there are several

others that allow the amplifier to be stable for lower signal

gains (the noise gain, however, remains 4 or more). One

example is the summing amplifier on the first page of this

data sheet. Each input signal has a gain of –1 to the output,

but it is easily seen that this configuration is equivalent to

a gain of –3 as far as the amplifier is concerned. Another

circuit is shown below with a DC gain of 1, but an AC gain

of 5. The break frequency of the R-C combination across

the amplifier inputs should be approximately a factor of 10

less than the gain-bandwidth of the amplifier divided by the

high frequency gain (in this case 1/10 of 150MHz/5 or

3MHz).

SI PLIFIED SCHE ATIC

6 OUT

LT1221 • SS

BIAS 2

BIAS 1

18

NULL

–IN

+IN 3

7

2

4