CURRENT MIRROR AND LEVEL SHIFTING

Summary

This video provides an in-depth explanation of current mirror circuits and level shifting in electronic design. It explores how current mirrors function to mirror input currents between transistors, ensuring stability and precision in current flow. Key concepts such as Kirchhoff’s Current Law (KCL) and Voltage Law (KVL) are used to analyze the circuit. The video also covers the importance of level shifting in maintaining proper DC levels to avoid signal distortion in coupled stages. The session concludes with discussions on the limitations of basic current mirrors and the benefits of modified versions like the Widler current source.

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Q & A

What is the main purpose of a current mirror circuit?

The main purpose of a current mirror circuit is to copy the current through one device and control the current through another, ensuring that the output current is a mirror image of the input current.

What does it mean for the current to be a 'mirror image' in a current mirror circuit?

In a current mirror circuit, the term 'mirror image' means that the output current is equal to the input current. The circuit is designed to ensure that the current flowing through the output device matches the current flowing through the input device, as if it were a reflection or copy.

How does Kirchoff's Current Law (KCL) apply to the analysis of the current mirror circuit?

KCL is applied at the nodes (A and B) of the current mirror circuit by ensuring that the sum of incoming currents equals the sum of outgoing currents at each node. For example, at node B, the incoming current I2 is equal to the sum of outgoing currents (I and Ic4), while at node A, the incoming current I is equal to the sum of the outgoing base currents (Ib3 and Ib4).

What is the significance of transistors Q3 and Q4 in the current mirror circuit?

Transistors Q3 and Q4 are crucial in the current mirror circuit because they are identical, meaning their base currents (Ib3 and Ib4) are equal, and their collector currents (Ic3 and Ic4) are also equal. This allows for the mirroring of the input current (I2) to match the output current (Ic3 or Ic4).

Why is the assumption that the transistor's beta (β) is very large important in the analysis of the current mirror?

Assuming that the transistor's beta (β) is very large simplifies the analysis, as it implies that the base current (Ib) is negligible. With an ideal β approaching infinity, the base current becomes zero, allowing the current mirror to function without significant loss due to base current, thus ensuring the mirrored output current closely matches the input current.

What are the drawbacks of a basic current mirror circuit?

The drawbacks of a basic current mirror circuit include its unsuitability for low current values and the potential for instability. If the output current (I2) is low, the resistance (R2) must be large, which increases power dissipation across R2 and can affect the stability of the circuit.

How does the Widlar current source improve the stability of the current mirror circuit?

The Widlar current source improves the stability of the current mirror by adding a resistor (R3) at the emitter of Q3. This additional resistor helps reduce the impact of variations in the output current, making the circuit more stable, especially under conditions where the output current is low.

What is the purpose of the level shifting stage in an operational amplifier (op-amp)?

The level shifting stage in an op-amp is used to manage the DC level of the signal. Without level shifting, direct coupling between stages would cause the DC level of the signal to shift, resulting in distortions and limiting the output swing. The level shifting stage ensures that the output voltage maintains a zero DC average, reducing distortions.

What are the two main types of circuits used for level shifting?

The two main types of circuits used for level shifting are the basic emitter follower circuit and the modified emitter follower circuit. The emitter follower circuit allows the output to follow the input voltage, but its drawback is that the output voltage is limited to a 7V range. The modified emitter follower circuit uses a voltage division setup to improve this limitation and provide more flexibility in adjusting the output voltage.

What is the equation for the output voltage (V0) in a basic emitter follower circuit used for level shifting?

In a basic emitter follower circuit, the output voltage (V0) is related to the input voltage (Vin) by the equation V0 = Vin - Vbe, where Vbe is the base-emitter voltage, typically 0.7V. This equation reflects the voltage drop across the base-emitter junction, resulting in a shift in the output voltage.