Describe the circuit arrangement for series voltage regulators and discuss its operation.

Q.) Describe the circuit arrangement for series voltage regulators and discuss its operation.

 Subject: electronic devices and circuits

Series Voltage Regulators

A series voltage regulator is a type of linear voltage regulator that maintains a constant output voltage despite variations in the input voltage or load conditions. It is called a "series" regulator because the regulating element, typically a transistor, is placed in series with the load.

Circuit Arrangement

The basic components of a series voltage regulator include:

  1. Reference Voltage Source: Provides a stable reference voltage, often derived from a Zener diode.
  2. Error Amplifier: Compares the output voltage with the reference voltage and amplifies the difference (error signal).
  3. Series Pass Transistor: Acts as a variable resistor, adjusting its resistance to maintain a constant output voltage.
  4. Load: The device or circuit that receives the regulated voltage.
  5. Feedback Network: Senses the output voltage and feeds it back to the error amplifier.

Step by Step Operation

  1. Reference Voltage Generation:

    • A Zener diode is used to generate a stable reference voltage, $V_{ref}$, which is independent of the input voltage, $V_{in}$, and load changes.

  2. Error Detection:

    • The output voltage, $V_{out}$, is fed back through a voltage divider network to the error amplifier.
    • The error amplifier compares $V_{out}$ with $V_{ref}$ and generates an error signal proportional to the difference.

  3. Amplification of Error Signal:

    • The error signal is amplified by the error amplifier to drive the series pass transistor.

  4. Regulation by Series Pass Transistor:

    • The series pass transistor adjusts its resistance in response to the error amplifier's output.
    • If $V_{out}$ is lower than desired, the transistor's resistance decreases, allowing more current to flow to the load, thus increasing $V_{out}$.
    • If $V_{out}$ is higher than desired, the transistor's resistance increases, reducing the current to the load, thus decreasing $V_{out}$.

  5. Stabilization:

    • This process continues with the error amplifier constantly comparing $V_{out}$ to $V_{ref}$ and adjusting the series pass transistor accordingly.
    • The result is a stable $V_{out}$ that is largely unaffected by changes in $V_{in}$ or the load.

Example Circuit Diagram

Here is a simplified example of a series voltage regulator circuit:

Vin ----|>|---+----[Q1]----+--- Vout
        ZD   |             |
             |             |
             +--[R1]--[R2]-+
             |             |
             +----[AMP]----+

Where:

  • Vin is the input voltage.
  • ZD is the Zener diode providing the reference voltage.
  • Q1 is the series pass transistor (e.g., BJT or MOSFET).
  • R1 and R2 form the voltage divider for feedback.
  • AMP is the error amplifier.
  • Vout is the regulated output voltage.

Differences and Important Points

Feature Series Voltage Regulator
Position of Regulator In series with the load
Regulation Element Series pass transistor (e.g., BJT, MOSFET)
Efficiency Lower than switching regulators at high current due to power dissipation in the series element
Complexity Relatively simple
Output Voltage Stability High, due to continuous feedback and adjustment
Response to Load Changes Good, but slower than switching regulators
Ripple Rejection Excellent, due to low-pass filtering effect of the series element
Heat Dissipation Can be significant, requiring heat sinks

Formulas

The output voltage $V_{out}$ can be determined by the feedback network resistors $R1$ and $R2$ and the reference voltage $V_{ref}$:

$$ V_{out} = V_{ref} \left(1 + \frac{R2}{R1}\right) $$

The error signal $V_{error}$ is the difference between the feedback voltage and the reference voltage:

$$ V_{error} = V_{feedback} - V_{ref} $$

Where $V_{feedback}$ is the voltage at the junction of $R1$ and $R2$.

Conclusion

Series voltage regulators are widely used for their simplicity and excellent voltage regulation capabilities. They are particularly suitable for applications where a stable and low-noise power supply is required. However, they are less efficient than switching regulators, especially at higher currents, due to the power dissipation in the series pass element.