THS4081, THS4082

175MHz LOW POWER HIGHSPEED AMPLIFIERS

SLOS274D − DECEMBER 1999 − REVISED JUNE 2001

14

POST OFFICE BOX 655303

APPLICATION INFORMATION

driving a capacitive load

Driving capacitive loads with high performance amplifiers is not a problem as long as certain precautions are

taken. The first is to realize that the THS408x has been internally compensated to maximize its bandwidth and

slew rate performance. When the amplifier is compensated in this manner, capacitive loading directly on the

output will decrease the device’s phase margin leading to high frequency ringing or oscillations. Therefore, for

capacitive loads of greater than 10 pF, it is recommended that a resistor be placed in series with the output of

the amplifier, as shown in Figure 41. A minimum value of 20

Ω should work well for most applications. For

example, in 75-

Ω transmission systems, setting the series resistor value to 75 Ω both isolates any capacitance

loading and provides the proper line impedance matching at the source end.

+

_

THS408x

CLOAD

1.3 k

Ω

Input

Output

1.3 k

Ω

20

Ω

Figure 41. Driving a Capacitive Load

offset voltage

the corresponding gains. The following schematic and formula can be used to calculate the output offset

voltage:

V

OO +

V

IO

1

)

R

F

R

G

" I

IB

)

R

S

1

)

R

F

R

G

" I

IB–

R

F

+

−

VI

+

RG

RS

RF

IIB−

VO

IIB+

Figure 42. Output Offset Voltage Model