AD8546/AD8548

Data Sheet

Rev. C | Page 18 of 24

OUTPUT STAGE

The AD8546/AD8548 feature a complementary output stage

consisting of the M16 and M17 transistors (see Figure 61). These

transistors are configured in a Class AB topology and are biased

by the voltage source, VB2. This topology allows the output voltage

to go within millivolts of the supply rails, achieving a rail-to-rail

output swing. The output voltage is limited by the output imped-

ance of the transistors, which are low RON MOS devices. The output

voltage swing is a function of the load current and can be estimated

using the output voltage to supply rail vs. load current graphs (see

Figure 15, Figure 16, Figure 18, and Figure 19).

RAIL-TO-RAIL INPUT AND OUTPUT

The AD8546/AD8548 feature rail-to-rail input and output with a

supply voltage from 2.7 V to 18 V. Figure 62 shows the input and

output waveforms of the AD8546/AD8548 configured as a unity-

gain buffer with a supply voltage of ±9 V and a resistive load of

1 MΩ. With an input voltage of ±9 V, the AD8546/AD8548 allow

the output to swing very close to both rails. Additionally, the

AD8546/AD8548 do not exhibit phase reversal.

TIME (200µs/DIV)

VSY = ±9V

RL = 1MΩ

INPUT

OUTPUT

Figure 62. Rail-to-Rail Input and Output

RESISTIVE LOAD

The feedback resistor alters the load resistance that an amplifier

sees. Therefore, it is important to carefully select the value of the

feedback resistors used with the AD8546/AD8548. The amplifiers

are capable of driving resistive loads down to 100 kΩ. The Inverting

Op Amp Configuration section and the Noninverting Op Amp

Configuration section show how the feedback resistor changes

the actual load resistance seen at the output of the amplifier.

Inverting Op Amp Configuration

Figure 63 shows the AD8546/AD8548 in an inverting config-

uration with a resistive load, RL, at the output. The actual load

seen by the amplifier is the parallel combination of the feedback

resistor, R2, and the load, RL. For example, the combination of

a feedback resistor of 1 kΩ and a load of 1 MΩ results in an

equivalent load resistance of 999 Ω at the output. Because the

AD8546/AD8548 are incapable of driving such a heavy load,

performance degrades greatly.

To avoid loading the output, use a larger feedback resistor, but

consider the effect of resistor thermal noise on the overall circuit.

AD8546/

AD8548

R1

R2

RL

–VSY

RL, EFF = RL || R2

+VSY

VIN

VOUT

Figure 63. Inverting Op Amp Configuration

Noninverting Op Amp Configuration

Figure 64 shows the AD8546/AD8548 in a noninverting config-

uration with a resistive load, RL, at the output. The actual load seen

by the amplifier is the parallel combination of R1 + R2 and RL.

R1

R2

RL

–VSY

RL, EFF = RL || (R1 + R2)

+VSY

VIN

VOUT

AD8546/

AD8548

Figure 64. Noninverting Op Amp Configuration