What are the advantages and disadvantages of a Widlar current source?
The correct answer and explanation is :
Advantages and Disadvantages of a Widlar Current Source
The Widlar current source is a modified form of a simple current mirror that generates a low current from a higher reference current by using an emitter degeneration resistor. It is commonly used in integrated circuits for biasing low-power transistors.
Advantages
- Lower Output Current Without Large Resistors
- Unlike a simple current mirror, the Widlar current source can generate much smaller currents without requiring large resistor values or high supply voltages. This makes it practical for IC implementation.
- Improved Power Efficiency
- Since it can produce low currents without wasting significant power, it is suitable for low-power applications such as analog circuits and biasing networks.
- Better Control Over Output Current
- The addition of an emitter resistor provides finer control over the output current by altering the voltage drop across the transistor, allowing designers to set precise biasing conditions.
- Reduced Sensitivity to Transistor Mismatches
- Compared to a simple current mirror, it reduces errors caused by transistor parameter variations (like β and V_BE mismatch) because the resistor helps stabilize the current.
Disadvantages
- Non-Ideal Output Resistance
- Unlike an ideal current source, the output resistance of the Widlar current source is lower, which means the output current may vary slightly with changes in output voltage.
- Requires Additional Resistor
- The inclusion of an emitter resistor increases the circuit complexity compared to a basic current mirror, consuming more silicon area in ICs.
- Lower Output Voltage Compliance
- The voltage drop across the emitter resistor reduces the available voltage range for the output, limiting its effectiveness in low-voltage applications.
- Temperature Dependence
- The output current is affected by temperature variations due to changes in V_BE and resistor properties, requiring careful compensation in precision applications.
In summary, the Widlar current source is an effective method for generating low currents in integrated circuits while maintaining reasonable accuracy and efficiency. However, it comes with trade-offs in terms of complexity, output resistance, and temperature stability.
