19
The -5V supply is regulated by IC2 from the -10V supply mentioned above.
The +5Vb supply is regulated by IC3 from a +10V resultant from the DC-to-DC converter
composed of Q1, Q2, T1, D8, D9 and associated components.
Constant Current Source
The constant current source is composed of IC4, IC5, Q4 and the precision resistor R26 and R28 –
R32. The input to the constant current source is approxi1.05 volts, developed at IC4-7 and
connected to IC4-13 through R12 and R13.
The heart of the constant current source is the voltage-to-current converter, which incorporates a
Transconductance Amplifier. A simplified schematic of this unique circuit is shown in Figure 3 and
described in the next section. The amplifier of IC4-12 is an inverter, and its output is applied to
IC4-9. The amplifier of IC4-8 has unity gain due to the feedback through R18. Its output is applied
to the inverting input of IC5-3. The output of IC5-6 provides feedback to the non-inverting input of
IC4-10. This circuit operates to maintain the inverting input at IC5-3 and the non-inverting input at
IC5-2 at the same potential.
Figure 3 Transconductance Amplifier
