Manipulating Transistors Using Current-Regulating Sources
Below, we delve into Aaron Danner's 29th video of his Transistors Series, discussing the ingenious use of current sources to bias transistors in common-emitter and common-collector amplifiers. This fascinating method, as presented by Danner, replaces conventional biasing resistors (along with accompanying capacitors).
Employing a current source carries several advantages over conventional resistor biasing. One of the chief benefits is the elimination of capacitors, primarily due to their price in integrated circuits and their typical poor quality in designs. Additionally, the bias resistor's gain can be conserved with this method.
The current source used in this video by Danner goes by the name of a current mirror. Current mirrors, with their ability to match currents, prove particularly useful in improving the consistency and accuracy of the biasing process.
Here's a quick recap of the perks that current mirror biasing offers over conventional resistor biasing:
- Enhanced Consistency: Current mirrors offer reduced dependence on the supply voltage and are less sensitive to process and temperature variations. This leads to uniform amplifier performance.
- Improved Circuit Functionality: Current mirrors contribute to higher output impedance, offering better signal integrity and maintaining high gain, especially in multi-stage amplifiers.
- Increased Noise Resistance: By providing a stable bias current, current mirrors exhibit superior noise immunity compared to resistors which can be more susceptible to noise and voltage fluctuations.
- Space and Power Efficiency: Current mirrors can replace large resistors in integrated circuits, leading to a more compact design. Additionally, they can employ less power than equivalent resistor biasing methods.
- Versatility: Current mirrors facilitate the production of multiple output currents, making them adaptable to various circuit requirements.
- Easier Matching: In integrated circuits, matching transistors is generally simpler and more accurate than matching resistors.
| Feature | Current Mirror Biasing | Resistor Biasing ||------------------------|----------------------------|----------------------------|| Sensitivity to Vcc | Low | High || Matching | Excellent (on-chip) | Moderate/Poor || Output Impedance | High | Low || Noise Immunity | Good | Moderate || Chip Area Usage | Low | High (for large Rs) || Power Dissipation | Variable, can be low | Variable, can be high |
In conclusion, current mirrors provide numerous benefits when biasing transistors in amplifier circuits, enhancing the circuit's performance, precision, and functionality, especially for integrated circuits where compacity and power use are crucial.
The use of a current source, such as a current mirror, in biasing transistors offers increased noise resistance due to its stability in providing a bias current, compared to resistors which can be more susceptible to noise and voltage fluctuations. Additionally, current mirrors contribute to higher output impedance, offering better signal integrity and maintaining high gain, especially in multi-stage amplifiers.
