--- title: "Series regulator with error amplifier" subject: "Electronics Devices and Circuit" chapter: 6 key: "semester_3/electronics_devices_and_circuit:series-regulator-error-amplifier" source: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/series-regulator-error-amplifier site: BCT Engineering --- # Series regulator with error amplifier Electronics Devices and Circuit · Chapter 6: Power Supplies, Breakdown Diodes, and Voltage Regulators - Asked in: 2070 Ashad, 2+4+2 marks - Video: [Transistorised Voltage Regulator - DC Power Supplies - Application of Electronics Class 12](https://www.youtube.com/watch?v=FfpS3sSX20M) — Ekeeda - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/series-regulator-error-amplifier - Whole chapter: https://www.bctengineer.com/md/semester_3/electronics_devices_and_circuit/chapter-6 ## Question Draw the circuit diagram of variable series voltage regulator with transistor as error amplifier. And find its voltage stability factor and explain how change in output voltage can be improved. ## Short answer Circuit: series pass Q1, error amp Q2, Zener reference, divider R1,R2. Output Vo = (Vz+VBE2)(1+R1/R2). Stability factor Sv = (1+R1/R2)·rz/(R+rz). Improve by using op-amp error amp, Darlington pass, constant-current Zener bias, low rz Zener, temperature compensation, and output capacitor. ## Answer #### Circuit diagram: variable series regulator with a transistor error amplifier ```text Vi o----+------------------------+ | | C [R] | | B | +-----------+------------Q1 (series pass) | | | C | E Q2 B---+ +------+-------+-----o Vo E | | | | | [R1] [RL] (Dz) | | | | +-----------+ GND GND | [R2] (variable, pot) | GND ``` Connections: Q1 is the series pass transistor (collector to Vi, emitter to Vo). Q2 is the error amplifier: its collector is connected to the base of Q1 (and to Vi through R), its emitter is held at the Zener voltage Vz, and its base is connected to the divider R1-R2 across the output. Changing the variable resistor R2 changes the fraction of Vo that is sampled, and so the output voltage. #### Operation VB2 = Vz + VBE2 = Vo R2/(R1 + R2) (R2 = the lower part of the divider) **Vo = (Vz + VBE2)(R1 + R2)/R2 = (Vz + VBE2)(1 + R1/R2)** Varying R2 changes the ratio R1/R2, so Vo can be set to any value above the minimum (Vz + VBE2). If Vo rises: VB2 rises, Q2 conducts more, its collector voltage falls, the base of Q1 falls, Q1 conducts less and Vo falls back to its set value. The opposite happens when Vo falls. #### Voltage stability factor Sv = ΔVo/ΔVi at constant load. When Vi rises by ΔVi, the base of Q1 stays at about Vo + VBE1, so the current in R rises by about ΔVi/R. This extra current flows through Q2 into the Zener and changes the Zener voltage by ΔVz ≈ ΔVi · rz/(R + rz) (≈ rz ΔVi/R, since rz ≪ R) and Vo = (Vz + VBE2)(1 + R1/R2), so with VBE2 constant, **Sv = ΔVo/ΔVi = (1 + R1/R2) · rz/(R + rz)** Example: rz = 10 Ω, R = 1 kΩ, R1 = R2: Sv = 2 × 10/1010 = 0.0198 (about 2 %, that is 19.8 mV per volt). #### Improving the change in output voltage 1. **Use a higher-gain error amplifier** (a differential pair or an op-amp in place of the single transistor), which reduces the variation of Vo through the large loop gain. 2. **Use a Darlington pair** as the pass element, so that the base current drawn from the error amplifier is smaller (less load regulation error). 3. **Make the reference more constant:** feed the Zener from a constant-current source or from the regulated output (bootstrap) instead of a resistor, and choose a Zener with low rz (about 6 V, near zero temperature coefficient). Raising R or lowering rz reduces Sv. 4. **Use a temperature-compensated or bandgap reference.** 5. Add an output capacitor and current limiting for stability and protection. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the output voltage of this variable series regulator? A. Vo = Vz - VBE2 B. Vo = (Vz + VBE2)(1 + R1/R2) C. Vo = (Vz + VBE2)(R2/(R1 + R2)) D. Vo = Vz(1 + R2/R1) Answer: B. VB2 = Vz + VBE2 = Vo R2/(R1 + R2), solved for Vo. (Key point: Output voltage expression) 2. How is the output made variable? A. By changing the Zener B. By changing RL C. By changing Q1's β D. By moving the pot slider, changing the fraction of Vo sampled by Q2 Answer: D. A different divider ratio requires a different Vo to make VB2 = Vz + VBE2. (Key point: Varying the output) 3. What is Sv? A. Sv = (1 + R1/R2) x rz/(R + rz) B. Sv = rz/(1 + R1/R2) C. Sv = (R + rz)/(rz(1 + R1/R2)) D. Sv = R/(R + rz) Answer: A. ΔVz = ΔVi rz/(R + rz), multiplied by (1 + R1/R2). (Key point: Stability factor) 4. Which change improves the regulation? A. Use a Zener with higher rz B. Reduce the series resistor R C. Use a higher gain error amplifier (op-amp or differential pair) and feed the Zener from a constant-current source D. Remove the Zener and use a resistor reference Answer: C. More loop gain and a steadier reference reduce the output variation. 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