UTC571
LINEAR INTEGRATED CIRCUIT
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.
I1
I2
Q6
Q7
D1
D2
OUT
Q1
Q2
-IN
+IN
Q5
Q4
Q3
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°C to +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.
1.15
140
¡/
DIFFUSED
RESISTOR
1.10
1.05
1.00
0.95
1k
¡/
LOW TC IMPLANTED
RESISTOR
1% ERROR BAND
-40
0
40
80
120
TEMPERATURE
Figure 15. Resistance vs temperature
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YOUW ANG ELECTRONICS CO.LTD
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