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Figure 4

Dual

Supply

Bridge

EVALUATION KIT

FOR PA74 PIN-OUT

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Figure 5

Single

Supply

Bridge

across the load. To set the gain of the circuit you must deter-

mine the desired voltage across the load at Vin = full scale.

Inserting these values into the following equation will yield the

ratio of R1 to R5.

(V

The values of R 1,2,3, and 5 should be chosen such that

input bias current will not cause an error voltage that is unac-

ceptable. Set R2 equal to R3 to configure the slave amplifier

as a unity gain inverter.

Figure 5 shows a typical single supply bridge circuit for an AC

coupled input signal. DC coupled inputs may require a different

topology to accommodate proper gain and offset terms for a

desired transfer function.

The gain and output voltage capability for the single supply

bridge are determined the same way as the dual supply bridge

(see AN#2). The difference is the bias requirement for the slave

amplifier. The noninverting input of the slave amplifier should

be biased at mid supply, and must be bypassed.

BRIDGE MODE OPERATION

There are two types of bridge mode operation that will be

covered in this section; dual (or split) supply and single supply.

The PA74 is well suited for both types of bridge mode opera-

tion. If another vendor’s pin compatible part is to be compared

to the PA74, a close look at output swing and input common

mode range is in order. The features that make the PA74 an

excellent choice for bridge operation are not included in most

other amplifiers. A lack of common mode range may cause

permanent damage to other pin compatible parts and the in-

ability of other amplifiers to swing close to the supply rails may

cause a lack of available output voltage at the load as well as

increase internal dissipation.

The circuit shown in Figure 4 is a dual supply bridge using

the “master-slave” configuration. Resistors R 6,7,8,9,14,15

and J1 should be shorts. The available output voltage swing

is Vss–(2*Vsat). If operating a PA74A at 3 Amps and 30 Volts

total supply this translates to:

V

Of course this 23 volts may be applied in either direction