LT1678/LT1679

13

sn16789 16789fs

Rail-to-Rail Input

The input common mode range for the LT1678/LT1679

can exceed the supplies by at least 100mV. As the

– 0.7V), the tail current for the input pair (Q1, Q2) is

reduced, which prevents the input pair from saturating

(refer to the Simplified Schematic). The voltage drop

200mV, degrading the slew rate, bandwidth, voltage

noise, offset voltage and input bias current (the cancella-

tion is shut off).

When the input common mode range goes below 1.5V

above the negative rail, the NPN input pair (Q1, Q2) shuts

off and the PNP input pair (Q8, Q9) turns on. The offset

voltage, input bias current, voltage noise and bandwidth

are also degraded. The graph of Offset Voltage Shift vs

Common Mode shows where the knees occur by display-

ing the change in offset voltage. The change-over points

are temperature dependent; see the graph Common Mode

Range vs Temperature.

Rail-to-Rail Output

The rail-to-rail output swing is achieved by using transis-

tor collectors (Q28, Q29 referring to the Simplified Sche-

matic) instead of customary class A-B emitter followers for

the output stage. The output NPN transistor (Q29) sinks the

current necessary to move the output in the negative direc-

tion. The change in Q29’s base emitter voltage is reflected

directly to the gain node (collectors of Q20 and Q16). For

the gain. Figure 7 shows the change in input voltage for a

change in output voltage for different load resistors con-

nected between the supplies. The gain is much higher for

output voltages above ground (Q28 sources current) since

the change in base emitter voltage of Q28 is attenuated by

the gain in the PNP portion of the output stage. Therefore,

for positive output swings (output sourcing current) there

is hardly any change in input voltage for any load resistance.

Highest gain and best linearity are achieved when the output

is sourcing current, which is the case in single supply op-

eration when the load is ground referenced. Figure 8 shows

gains for both sinking and sourcing load currents for a

worst-case load of 600Ω.

–15 –10

–5

0

5

10 15

OUTPUT VOLTAGE (V)

INPUT VOLTAGE

(50µV/DIV)

16789 F07

MEASURED ON

TEKTRONIX 577

CURVE TRACER

12

03

5

4

OUTPUT VOLTAGE (V)

INPUT VOLTAGE

(10µV/DIV)

16789 F08

VOLTAGE GAIN SINGLE SUPPLY

MEASURED ON TEKTRONIX 577

CURVE TRACER

Figure 7. Voltage Gain Split Supply

Figure 8. Voltage Gain Single Supply

APPLICATIO S I FOR ATIO