8

LT1207

SI PLIFIED SCHE ATIC

LT1207 • SS

OUTPUT

50

Ω

D2

D1

COMP

–IN

+IN

SHUTDOWN

1.25k

TO ALL

CURRENT

SOURCES

Q11

Q15

Q9

Q6

Q5

Q2

Q1

Q18

Q17

Q3

Q4

Q7

Q8

Q12

Q16

Q14

Q13

Q10

1/2 LT1207 CURRENT FEEDBACK AMPLIFIER

S

APPLICATI

I FOR ATIO

The LT1207 is a dual current feedback amplifier with high

output current drive capability. The device is stable with

large capacitive loads and can easily supply the high

currents required by capacitive loads. The amplifier will

drive low impedance loads such as cables with excellent

linearity at high frequencies.

Feedback Resistor Selection

The optimum value for the feedback resistors is a function

of the operating conditions of the device, the load imped-

ance and the desired flatness of response. The Typical AC

Performance tables give the values which result in the

highest 0.1dB and 0.5dB bandwidths for various resistive

loads and operating conditions. If this level of flatness is

not required, a higher bandwidth can be obtained by use

of a lower feedback resistor. The characteristic curves of

Bandwidth vs Supply Voltage indicate feedback resistors

for peaking up to 5dB. These curves use a solid line when

the response has less than 0.5dB of peaking and a dashed

line when the response has 0.5dB to 5dB of peaking. The

curves stop where the response has more than 5dB of

peaking.

For resistive loads, the COMP pin should be left open (see

section on capacitive loads).

Capacitive Loads

Each amplifier in the LT1207 includes an optional com-

pensation network for driving capacitive loads. This net-

work eliminates most of the output stage peaking associ-

ated with capacitive loads, allowing the frequency re-

sponse to be flattened. Figure 1 shows the effect of the

network on a 200pF load. Without the optional compensa-

tion, there is a 5dB peak at 40MHz caused by the effect of

the capacitance on the output stage. Adding a 0.01

µF

bypass capacitor between the output and the COMP pins

connects the compensation and completely eliminates the

peaking. A lower value feedback resistor can now be used,

resulting in a response which is flat to 0.35dB to 30MHz.