# 1. Current Source vs Current Sink (Using a MOS Transistor) ## Current Source - **Definition:** Supplies a constant current to a load regardless of the load voltage (within compliance range). - **Function:** "Pushes" current into the load. - **Example:** NMOS source configuration. ### MOSFET as Current Source #### Circuit - **PMOS as Current Source:** - Drain to load, source to \( V_{DD} \), gate at lower voltage than source. - **NMOS as Source:** - Source at lower voltage, load connected to drain, gate at fixed voltage. #### Illustration ``` VDD │ │ [LOAD] │ │ D │ |——| NMOS (current source) │ S │ GND ``` - **Gate** is biased to force the MOSFET in saturation, setting the drain current. ## Current Sink - **Definition:** Draws (sinks) a constant current from a node or load. - **Function:** "Pulls" current from the load to ground. - **Example:** NMOS sink configuration. ### MOSFET as Current Sink #### Circuit - **NMOS as Current Sink:** - Source to ground, drain to load, gate at fixed bias. #### Illustration ``` VDD │ │ [LOAD] │ │ D │ |——| NMOS (current sink) │ S │ GND ``` - **Gate** is biased for saturation, setting the drain current. ### Key Differences | Aspect | Current Source | Current Sink | |------------------|------------------------------|---------------------------------| | Function | Supplies current to load | Draws current from load | | Common MOS Used | PMOS (as source) | NMOS (as sink) | | Load connection | Between source and load | Between drain and load | | Direction | Out of source to load | Into sink from load | --- # 2. Standard Cascode Current Sink ## Working Principle A **cascode current sink** uses two stacked NMOS transistors (`M1` and `M2`): - **M1:** Sets the reference current (like a basic current sink). - **M2:** Sits above M1, with its gate at a fixed bias (often from a bias circuit). ### Circuit Diagram ``` VDD │ [LOAD] │ │ D2 │ |——| M2 (cascode transistor) │ S2 │ D1 |——| M1 (current-setting transistor) S1 │ GND ``` - **Gate of M1:** Biased to set current. - **Gate of M2:** Biased to keep M2 always in saturation. ### Operation Steps 1. **M1** is biased to set a reference current \( I_{REF} \). 2. **M2** boosts the output impedance by shielding M1’s drain from output voltage swings. 3. M2's gate voltage ensures it remains in saturation, regardless of output voltage variations (as long as \( V_{OUT} \) is above a certain minimum). ## Comparison: Cascode vs Basic Current Sink | Feature | Basic Current Sink | Cascode Current Sink | |-------------------------|------------------------|-----------------------------| | Output Impedance | Moderate | Much higher (better) | | Output Current Stability| Good (within limits) | Very stable | | Minimum Output Voltage | Lower | Higher (needs headroom) | | Complexity | Simple (1 transistor) | Higher (2+ transistors) | | Power Supply Rejection | Lower | Higher | ### Summary Table | Parameter | Basic Sink | Cascode Sink | |------------------|-----------------|--------------------------| | Output impedance | \( r_{o} \) | \( r_{o1} \times g_{m2} r_{o2} \) | | Voltage headroom | \( V_{DS(sat)} \) | \( V_{DS(sat,1)} + V_{GS} - V_{th,2} \) | #### Cascode Advantages - Better output current regulation. - Less sensitive to output voltage and supply fluctuations. - Preferred for analog circuits requiring high accuracy. --- **Final Answers:** 1. **Current source** supplies current to the load (often uses PMOS or NMOS with source to reference), while a **current sink** draws current from the load (usually NMOS, source to ground). *A single NMOS can serve as either, depending on configuration.* 2. **Cascode current sink** stacks two NMOS: the lower sets current, the upper boosts output resistance for better regulation. Compared to a basic sink, the cascode offers higher output impedance and better performance, at the cost of increased voltage headroom and complexity.