AD743

HOW CHIP PACKAGE TYPE AND POWER DISSIPATION

AFFECT INPUT BIAS CURRENT

As with all JFET input amplifiers, the input bias current of the

approximately doubling every 10°C. Figure 30 shows the

relationship between bias current and junction temperature for

the AD743. This graph shows that lowering the junction

–60 –40

–20

0

20

40

60

80 100 120

140

–11

–10

10

–9

10

–8

10

–7

10

–6

10

10

10

–12

JUNCTION TEMPERATURE – °C

V = ±15V

T = 25°C

S

A

+

Figure 30. Input Bias Current vs. Junction Temperature

The dc thermal properties of an IC can be closely approximated

by using the simple model of Figure 31 where current represents

power dissipation, voltage represents temperature, and resistors

represent thermal resistance (

θ in °C/Watt).

= DEVICE DISSIPATION

= AMBIENT TEMPERATURE

= JUNCTION TEMPERATURE

= THERMAL RESISTANCE – JUNCTION TO CASE

= THERMAL RESISTANCE – CASE TO AMBIENT

P

IN

JC

CA

WHERE:

θ

θ

P

IN

CA

T

J

JC

θ

θ

JA

θ

Figure 31. A Device Thermal Model

θ

determined in a particular application by using Figure 30

together with the published data for

θ

The user can modify

θ

sink such as the Aavid #5801.

θ

the AD743 in chip form. Figure 32 shows bias current vs.

supply voltage with

θ

used to predict bias current after

θ

bias current will double for every 10°C. The designer using the

AD743 in chip form (Figure 33) must also be concerned with

both

θ

θ

θ

mount technology used.

Typically,

θ

for normal packages, this small power dissipation level may be

ignored. But, with a large hybrid substrate,

θ

proportionately more of the total

θ

300

0

100

200

5

10

15

T = +25°C

A

SUPPLY VOLTAGE – ±Volts

= 165°C/W

θ

A

= 115°C/W

= 0°C/W

θ

A

J

J

J

Figure 32. Input Bias Current vs. Supply Voltage for

Various Values of

θ

JA

Figure 33. A Breakdown of Various Package Thermal

Resistances

REDUCED POWER SUPPLY OPERATION FOR

lowering both the total power dissipation and second, by

reducing the basic gate-to-junction leakage (Figure 32). Figure

34 shows a 40 dB gain piezoelectric transducer amplifier, which

operates without an ac coupling capacitor, over the –40°C to

+85°C temperature range. If the optional coupling capacitor is

used, this circuit will operate over the entire –55°C to +125°C

military temperature range.

Figure 34. A Piezoelectric Transducer