Last reviewed 16 Sept 2026 · 7 min read
DC power supply
AC mains → transformer (step down) → rectifier (AC to pulsating DC) → filter (smooths ripple) → regulator (constant output) → DC load.
Rectifiers
FormulaRectifier performance (ideal diodes, resistive load; Vm = peak secondary voltage)
| Parameter | Half-wave | Full-wave centre-tap | Full-wave bridge |
|---|---|---|---|
| Diodes | 1 | 2 | 4 |
| Average (DC) output | |||
| RMS output | |||
| Ripple factor | 1.21 | 0.482 | 0.482 |
| Rectification efficiency (max) | 40.6% | 81.2% | 81.2% |
| Peak inverse voltage (PIV) per diode | |||
| Ripple frequency | |||
| Transformer | Simple; DC saturation of core | Centre-tapped needed | No centre tap; better transformer utilisation |
- Ripple factor .
- With practical silicon diodes, subtract about 0.7 V per conducting diode from (one for half-wave and centre-tap, two for a bridge).
- Bridge rectifier is the most common (no centre tap, PIV only ).
Filters
| Filter | Principle | Features |
|---|---|---|
| Shunt capacitor (C) filter | Capacitor charges to peak and discharges slowly through load | Simple; good for light loads (high ); ripple increases with load current |
| Series inductor (L) filter | Inductor opposes current change | Better for heavy loads (ripple decreases with load current) |
| LC (choke input) filter | Combines both | Good regulation, ripple nearly independent of load |
| π (CLC) filter | Capacitor input, inductor, capacitor | Very low ripple, higher output voltage, poorer regulation |
FormulaCapacitor filter (full-wave)
(For half-wave: .) Peak-to-peak ripple voltage (approx.) (full-wave).
Voltage regulators
- Zener diode shunt regulator — simple, low-power (see Semiconductor Physics & Diodes).
- Series pass transistor regulator — transistor in series controlled by a Zener reference — higher currents.
- IC regulators — 78xx series (positive fixed output, e.g. 7805 → +5 V, 7812 → +12 V), 79xx (negative), LM317 (adjustable); need input a few volts above output (dropout); include thermal and short-circuit protection.
- Switched-mode power supplies (SMPS) — high-frequency switching with small transformers/inductors — high efficiency (commonly 80–90%+), compact; used in computers, chargers, LED drivers.
- Load regulation ; line regulation — change in output for change in input.
Amplifiers
An amplifier increases the power level of a signal using energy from a DC supply.
Gain and decibels
FormulaGain in decibels
- Gain ×2 in power = +3 dB; ×10 in power = +10 dB; ×10 in voltage = +20 dB
- Cascaded stages: overall gain = product of individual gains = sum of gains in dB
Common emitter (CE) small-signal amplifier
- Voltage divider biased with emitter resistor bypassed by a capacitor (), coupling capacitors at input and output.
- Voltage gain (bypassed emitter): , with — the minus sign shows 180° phase shift.
- Unbypassed : (lower but stable gain).
Frequency response
- Gain is flat over the mid-band and falls at:
- Low frequencies — coupling and bypass capacitors.
- High frequencies — transistor junction and stray capacitances.
- Cut-off (half-power) frequencies and — gain falls to 0.707 of mid-band (−3 dB).
- Bandwidth ; gain–bandwidth product is roughly constant.
Coupling of cascaded stages
| Coupling | Features |
|---|---|
| RC coupling | Cheap, good frequency response in audio range — most common for voltage amplifiers |
| Transformer coupling | Impedance matching, higher efficiency; bulky, poorer frequency response |
| Direct coupling | Amplifies DC and low frequencies (op-amps, ICs); drift problems |