LT1218CS8 datasheet(12/16 Pages) LINER

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LT1218/LT1219

APPLICATIONS INFORMATION

output stage with rail-to-rail swing. The amplifier is fabri-

cated on Linear Technology’s proprietary complementary

bipolar process, which ensures very similar DC and AC

characteristics for the output devices Q24 and Q26.

A simple comparator Q5 steers current from current

source I1 between the two input stages. When the input

common mode voltage VCM is near the negative supply,

Q5 is reverse biased, and I1 becomes the tail current for the

PNP differential pair Q1/Q2. At the other extreme, when

VCM is within about 1.3V from the positive supply, Q5

diverts I1 to the current mirror D3/Q6, which furnishes the

tail current for the NPN differential pair Q3/Q4.

The collector currents of the two input pairs are combined

in the second stage, consisting of Q7 through Q11. Most

of the voltage gain in the amplifier is contained in this

stage. Differential amplifier Q14/Q15 buffers the output of

the second stage, converting the output voltage to differ-

ential currents. The differential currents pass through

current mirrors D4/Q17 and D5/Q16, and are converted to

differential voltages by Q18 and Q19. These voltages are

also buffered and applied to the output Darlington pairs

Q23/Q24 and Q25/Q26. Capacitors C1 and C2 form local

feedback loops around the output devices, lowering the

output impedance at high frequencies.

Input Offset Voltage

Since the amplifier has two input stages, the input offset

voltage changes depending upon which stage is active.

The input offsets are random, but bounded voltages.

When the amplifier switches between stages, offset volt-

ages may go up, down or remain flat; but will not exceed

the guaranteed limits. This behavior is illustrated in three

distribution plots of input offset voltage in the Typical

Performance Characteristics section.

Overdrive Protection

Two circuits prevent the output from reversing polarity

when the input voltage exceeds the common mode range.

When the noninverting input exceeds the positive supply

by approximately 300mV, the clamp transistor Q12 (Fig-

ure 1) turns on, pulling the output of the second stage low,

which forces the output high. For input below the negative

supply, diodes D1 and D2 turn on, overcoming the satu-

ration of the input pair Q1/Q2.

When overdriven, the amplifier draws input current that

exceeds the normal input bias current. Figures 2 and 3

show typical input current as a function of input voltage.

The input current must be less than 10mA for the phase

reversal protection to work properly. When the amplifier is

severely overdriven, an external resistor should be used to

limit the overdrive current.

COMMON MODE VOLTAGE RELATIVE TO

NEGATIVE SUPPLY (mV)

– 800

–110

–90

–70

–50

–30

– 600

– 400

LT1218/19 • F03

–200

–10

–20

–40

–60

–80

–100

200

T = – 55

°C T = 25°C

T = 85

°C

–

T = 70

°C

MEASURED AS A FOLLOWER

Figure 3. Input Bias Current vs Common Mode Voltage

COMMON MODE VOLTAGE RELATIVE TO

POSITIVE SUPPLY (mV)

–500

–300

–100 VS

LT1218/19 • F02

100

110

300

500

T = –55

°C

T = 25

°C

T = 85

°C

T = 70

°C

MEASURED AS A

FOLLOWER

–

Figure 2. Input Bias Current vs Common Mode Voltage

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