MOSFET - Current Mirror Explained

Summary

This educational video from ALL ABOUT ELECTRONICS dives into current mirrors, crucial for ICs, using MOSFETs. It explains the significance of stable biasing voltage for amplifiers, the challenges of maintaining it in battery-operated devices, and temperature effects. The video illustrates how current mirrors generate stable reference currents, adjusting them as multiples or fractions via the W/L ratio. It also touches on the channel length modulation effect and its minimization, and concludes with applications in biasing amplifiers and the cascode current mirror for improved gain and stability.

Takeaways

• 🔬 Current mirrors are used in integrated circuits as stable current sources for biasing MOS amplifiers.
• 💡 The advantage of using a constant current source for biasing is high voltage gain and improved biasing stability.
• 🔋 In battery-operated devices, current mirrors are crucial as supply voltage decreases over time, affecting the bias current.
• 🌡️ The biasing current is temperature-dependent, and current mirrors help maintain stability despite temperature changes.
• 🔄 Current mirrors can generate copies of a reference current source, which can be the same or a multiple/fraction of the reference.
• 🚀 The design of a current mirror using MOSFET involves controlling the gate-to-source voltage to change the drain current.
• 📉 The drain current ID in a MOSFET is given by the expression involving the device parameter k, Vgs, and Vt.
• 🔗 The current mirror circuit uses a diode-connected transistor to generate a Vgs proportional to the reference current.
• 🔍 The channel length modulation effect is considered in the discussion, which affects the drain current with changes in Vds.
• 🔗 The W/L ratio of MOSFETs determines the relationship between the reference current and the drain current in the mirror.
• 🛠️ The Early voltage (Va) is introduced to quantify the change in drain current due to changes in Vds, considering the channel length modulation.

Q & A

What is the purpose of a current mirror circuit in electronics?

A current mirror circuit is used to generate a stable current source that can be a copy or multiple copies of a reference current source, ensuring stable biasing for amplifiers in integrated circuits.

Why is it important to bias amplifiers with a constant current source?

Biasing amplifiers with a constant current source provides high voltage gain and improves biasing stability, which is crucial for maintaining consistent performance in battery-operated devices where supply voltage can reduce over time.

How does a current mirror circuit help in ICs with multiple amplifiers?

In ICs containing hundreds or thousands of amplifiers operating at different points, a current mirror circuit helps generate a stable biasing voltage for each amplifier, overcoming the challenge of generating a stable voltage for each.

What is the role of temperature in the operation of a current mirror circuit?

Temperature affects the drain current of a current mirror circuit because the mobility (μn) and threshold voltage (Vt) are temperature-dependent. As a result, the biasing current can change with temperature.

How does a MOSFET operate as a voltage-controlled device in a current mirror circuit?

A MOSFET operates in the saturation region where the drain current (ID) is controlled by the gate-to-source voltage (Vgs). By controlling Vgs, the drain current can be adjusted to match the reference current.

What is the significance of the W/L ratio in MOSFET current mirror circuits?

The W/L (width-to-length) ratio of a MOSFET determines the drain current it can carry. By adjusting the W/L ratio, the current mirror circuit can generate currents that are multiples or fractions of the reference current.

How does the channel length modulation affect the drain current in a MOSFET?

Channel length modulation causes the drain current to change with the change in voltage Vds. This effect can be minimized by increasing the length of the MOSFETs, thus reducing the impact on the drain current.

What is the Early voltage (Va) and how is it related to channel length modulation?

The Early voltage (Va) is the reciprocal of the channel length modulation coefficient (λ). It represents the change in drain current due to a change in Vds, and it helps to minimize the effect of channel length modulation by increasing the length of the MOSFETs.

How can a pMOS current mirror be used in an IC?

A pMOS current mirror can be used similarly to an nMOS current mirror to generate a stable current source. It ensures that the source and drain voltages (Vsg) are equal, and the drain current (Id1) is determined by the W/L ratio and the reference current.

What is the advantage of using a cascode current mirror as an active load in an amplifier?

Using a cascode current mirror as an active load in an amplifier improves both voltage gain and biasing stability. It provides a higher output impedance and transconductance, enhancing the amplifier's performance.