UTC571

LINEAR INTEGRATED CIRCUIT

YOUW ANG ELECTRONICS CO.LTD

13

OPERATIONAL AMPLIFIER

The main op amp shown in the chip block diagram is equivalent to a 741 with a 1MHz bandwidth. Figure 18

shows the basic circuit. Split collectors are used in the input pair to reduce g M , so that a small compensation

capacitor of just 10pF may be used. The output stage, although capable of output currents in excess of 20mA, is

biased for a low quiescent current to conserve power. When driving heavy loads, this leads to a small amount of

crossover distortion.

Q1

Q2

Q3

Q4

Q5

Q6

D1

D2

I2

I1

-IN

+IN

OUT

Q7

Figure 14. Operational Amplifier

RESISTORS

Inspection of the gain equations in Figures 3 and 4 will show that the basic compressor and expander circuit

gains may be set entirely by resistor ratios and the internal voltage reference. Thus, any form of resistors that

match well would suffice for these simple hook-ups, and absolute accuracy and temperature coefficient would be of

no importance. However, as one starts to modify the gain equation with external resistors, the internal resistor

accuracy and tempco become very significant. Figure 15 shows the effects of temperature on the diffused resistors

which are normally used in integrated circuits, and the ion-implanted resistors which are used in this circuit. Over

the critical

0°Cto +70°C temperature range, there is a 10-to-1 improvement in drift from a 5% change for the

diffused resistors, to a 0.5% change for the implemented resistors. The implanted resistors have another

advantage in that they can be made the size of the diffused resistors due to the higher resistivity. This saves a

significant amount of chip area.

0.95

1.00

1.05

1.10

1.15

40

TEMPERATURE

-40

0

80

120

DIFFUSED

RESISTOR

140

¡/

1k

¡/

LOW TC IMPLANTED

RESISTOR

1% ERROR BAND

Figure 15. Resistance vs temperature