Spare Op Amp Generates Its Own Regulated Negative Supply
For systems that require low current (100mA or less) and include a spare op amp along with unregulated positive and negative voltages, the circuit of Figure 1 generates a regulated positive and negative supply voltage. The op amp then operates from the supply rails that it has helped to generate (±5V in this case). Figure 2 shows the power-up response. The circuit achieves regulation regardless of which unregulated voltage is applied first.
Figure 1. This circuit derives a regulated ± voltage from ± unregulated input voltages.
Figure 2. The power-up response for the Figure 1 circuit.
CH1 = regulated -5V output
CH2 = negative supply voltage
CH3 = regulated +5V output
CH4 = positive supply voltage
By controlling the base drive to Q1, the op amp maintains equilibrium at its inverting input, and thereby ensures that
VOP1+ = VOP1-Using Superposition:
VOP1+ = 0V
VOP1-' = VP(R3/(R3+R1)Therefore:
VOP1-'' = (VN(R1/(R1+R3)
VOP1- = VOP1-' + VOP1-''
VOP1- = VP(R3/(R3+R1)) + VN(R1/(R1+R3))
VN = -VP(R3/(R3+R1))((R1+R3)/R1)
VN = -VP(R3/R1)Alternatively:
IR1 = VP/R1Therefore:
VN = -IR1 × R3 = (-VP/R1)R3The allowable range of the unregulated negative supply voltage is -6V to -24V. For a wider input-supply range, either replace Q1 with a higher gain transistor or lower the resistance of R4 and R5. VN can deliver up to 100mA without distortion, but VP is limited by the reference of IC1 to a maximum of 30mA, with a VP reduction at that level of ~0.3V. (For higher positive load currents you can substitute a different positive regulator.)
IBIAS is negligible for the op amp shown. To minimize the IBIAS error for other op amps, R2 should have the value R1||R3.