20
Transconductance Amplifier
(U.S. Patent No. 4,091,333)
Assume that terminals I
hi
and I
lo
of Figure 3 are shorted, and 1 volt is applied to E
in
so that I
hi
is
positive. To equalize the inputs of IC5, IC4 must be driven to zero. This condition occurs only
when the voltage drops across R17 and R19 are equal to the drops across R18 and R20. For these
voltage drops to be equal, the output of IC5 must be at +1 volt. Since the output of IC4-8 must be
zero, the drop across R18 is 0.5 volts, making the inverting input 0.5 volts. The drops across R17,
R20 and R19 will also be 0.5 volts. Since the inputs to IC4 are essentially equal, its output is zero
(offset by the few µV required to drive IC5 to +1 volt). Under these conditions the sum of the
voltages across R17, R18, R19 and R20 equals the sum of E
in
plus the output of IC5.
Consider now that the short is removed from the I
hi
and I
lo
terminals and a 100-ohm resistor (R
L
) is
connected in its place. The current through R
L
increases the voltage at the input to IC4. A balanced
condition will be reached when the output of IC4 is equal to the non-inverting input of IC5. Again,
this condition occurs when the voltage drops across R17 and R19 are equal to the voltage drops
across R18 and R20. At this time the output of IC5 is 1.1 volts. The voltage drop across the Range
Resistor is 1 volt, just as it was when the output terminals were shorted. The current through R
L
is
10mA, just as it was through the jumper when the output terminals were shorted.
Analog to Digital Converter
The Analog to Digital conversion in the Model 4021 is aloud by IC 7 and its associated
components. The reference voltage applied to IC7-2 is 1Volts. 1.2Volts are generated buy D16
and adjusted by R40 before being applied to IC7-1. Due to this reference, the voltage across the
load is amplified X10 by IC6 and its associated components. IC8 generates a 120 kHz clock
signal for IC7-22; the frequency of the oscillator allows the 4021 a conversion rate of 3 readings
per second.
The digital portion of IC7 is applied to IC9 a BCD to 7 Segment Converter/Driver and to the
driver transistors Q5-Q9.
