8

Op Amp Booster

The EL2003 or EL2033 can boost the output drive of almost

any monolithic op amp. Because the phase shift in the

EL2003 and EL2033 is low at the op amp's unity gain

frequency, no additional compensation is required. By

following an op amp with an EL2003 or EL2033, the buffered

op amp can drive cables and other low impedance loads

directly. Even decompensated high speed op amps can take

advantage of the EL2003’s or EL2033’s 100mA drive.

Driving capacitive loads with any closed loop amplifier

creates special problems. The open loop output impedance

works into the load capacitance to generate phase lag which

can make the loop unstable. The output impedance of the

EL2003 or EL2033 is less than 10

Ω from DC to about

10MHz, but a capacitive load of 1000pF will generate about

45° phase shift at 10MHz and make high speed op amps

unstable. Obviously more capacitance will cause the same

problem but at lower frequencies, and slower op amps as

well would become unstable.

The easiest way to drive capacitive loads is to isolate them

from the feedback with a series resistor. Ten to twenty ohms

is usually enough but the final value depends on the op amp

used and the range of load capacitance.

If the system requirements will not tolerate the isolation

resistor, then additional high frequency feedback from the op

amp output (the buffer input) and an isolating resistor from

the buffer output is required. This requires that the op amp

be unity gain stable.

DIRECT DRIVE

DOUBLE MATCHED

BACK MATCHED

OP AMP BOOSTER

OP AMP BOOSTER WITH CAPACITIVE LOAD

OS

10pF

17ns

10%

470pF

20ns

50%

0.001µF

30ns

35%

0.005µF

80ns

0

0.01µF

220ns

0

0.05µF

1.1µs

0

0.1µF

2.2µs

0

10

Ω is enough isolation and speed is

determined by the isolation resistor and

capacitive load time constant.

COMPLEX FEEDBACK WITH THE BUFFER TO DRIVE

CAPACITIVE LOADS

This works with any unity gain stable OA.

Snubber Circuit (51

Ω 470pF) is optional.

EL2003, EL2033