Folded Cascode

Folded Cascodes

Key Idea: The circuit utilizes two transistors with opposite polarities (e.g., NPN-PNP or NMOS-PMOS). This arrangement isolates the input transistor's gain (Q1) from the output impedance and noise of the second transistor (Q2).

Circuit Details:

• Current Source: I1 supplies operating current to both transistors.

• Current Sharing: Q2's current is derived from I1 minus Q1's current.

• Gain Mechanism: As Vin increases, Q1's current rises, leading to a decrease in Q2's current and consequently, Vout (inverting gain).

• Emitter Voltage Stability: Q2's emitter voltage changes slightly with its current due to varying Vbe. Using a current source (not a resistor) for I1 ensures precise and stable overall current.

• Frequency Response: Q2's constant emitter voltage minimizes feedback capacitance impact on Q1, improving frequency response.

• Device Requirement: Q2 needs to be a different type than Q1 but not necessarily a perfectly "complementary" device.

• Currents: Unlike a basic cascode with equal DC currents in both transistors, the folded cascode allows different DC currents in Q1 and Q2. However, AC currents remain the same in magnitude but opposite in polarity.

Advantages:

• Improved output impedance and noise isolation compared to standard cascode.

• Stable performance due to controlled current source.

• Better frequency response.