Op-amp Regulator Incorporating Current Boosting with Zener Reference:

For larger load currents, the simple regulator illustrated above has to be modified as illustrated in Figure wherein a Zener diode is utilized to create reference voltage and transistor Q₁ has been added for increasing regulation action. Now, we have

          V _ref  = V_z            and  V₀  = (1+ (R _f  /R_i) V z

Figure: A Modified Op-amp Regulator

 Design Example

Let V₀ (required) = 15 volts DC we may design the circuit as follows.

 (a)       Select VZ, typically 1/2 or 1/3 of Vs, let VZ  = 5 volts,

∴          R_D = (V₀  - V_s )/i_D           = 15 - 5 /5 mA   = 2k

 (Supposing that a current of 5 mA is adequate to cause breakdown of Zener diode.)

(b)        Permit a current of 1 mA to flow in (R_f + R_i) so that :

               (R _f   + R _j ) =   V₀ / 1 mA      =   15   /1 mA       = 15k Ω

Since V0  =(R _f   + R _j ) /R_i . V_z   therefore, 15 = (15k / R_i) .5 gives R_i = 5 k and R_f  = 10 k.

(c)       Since V_s  ≥ V₀ + 2, we choose V_s  ≥ 17 volts,

(d)        Other design considerations :

                 (i)    We select VZ  and RD such that : (IRf + IRD) < < IL. Clearly,

                        I_{L (max)} ≅ h_FE I_{0 (max)} = 50 × 10 mA ≈ 500 mA

                 (ii)   Power dissipated in Q₁ : PD = V_{CE (max).} I_{L (max).} Therefore, if

                         I_{L (max)} = 500 mA; V_{s (max)} = 36, V_{0 (desired)} = 15 volts, then,P_D = (36 - 15) × 0.5 = 11.5 watts!!

obviously, appropriate heat sinking arrangements are required to properly dissipate the generated heat.

The above scheme may be further modified by incorporating 'current limiting' and 'overload protection' circuitry as we shall see in the following.