Current Mirror Circuits

Introduction

• Focus: Understanding current mirror circuits.
• Importance of current sources for biasing amplifiers in integrated circuits.

What is a Current Mirror?

• A current mirror is an active circuit that senses a reference current and generates copies with the same characteristics.
• Use Cases:
  • Biasing amplifiers with stable current.
  • Stable current sources needed for integrated circuit amplifiers.

Need for Current Mirrors

1. Importance of Stable Biasing Current:
   • Emitter current independent of beta (β) and Rb.
   • Increases input impedance with increased Rb.
   • Current stability against temperature and supply voltage changes.
   • Example: Smartphone battery discharge affects supply voltage.

Designing a Current Mirror with BJT

1. BJT as Current Source:

   • Collector current (Ic) is a function of base-emitter voltage (Vbe).

2. Conversion Process:

   • Reference current sensed and converted to voltage.
   • Voltage applied between base and emitter to establish collector current proportional to the reference current.
   • Diode-Connected Transistor:
     • Collector and base terminals are connected.

3. Current Copying Mechanism:

   • Vbe calculation:
     • Vbe(ref) = Vt * ln(Iref / Is(ref))
   • Apply Vbe(ref) to second transistor (Q1) base:
     • Ic1 = Is1 * exp(Vbe / Vt)

4. Conditions for Current Replica:

   • Ensure collector-base junction remains reverse-biased.
   • Vbe for Q1 equals Vbe(ref).
   • Collector current Ic1 is proportional to Iref.

5. Mathematics of Current Mirror:

   • If both transistors are identical:
     • Ic1 = Iref
   • If areas differ (Is1 proportional to area):
     • Ic1 calculations based on area ratios.

Generating Multiple Copies of Reference Current

• Base of each transistor connected to the reference transistor.
• Depending on emitter-base junction area:
  • Example:
    • Reference T area A, Q1 area 5A -> Ic1 = 5 * Iref
    • Reference T area A, Q1 area (1/5)A -> Ic1 = (1/5) * Iref

Effect of Base Current on Replicated Current

1. Base Current Impact:

   • Base current (Ib) = Ic / β
   • KCL applied to include base current effects.
     • Result for Ic1: Ic1 = Iref / [1 + (2/β)]
   • Larger β makes base current negligible.

2. For multiple transistors (N):

   • Ic = Iref / [1 + (N+1)/β]
   • As N increases, collector current variation observed.

Improving Current Mirror Performance

• Adding an additional transistor to enhance current mirror performance:
  • New equations with improved β such that: Ic = Iref / [1 + (N+1)/β²].
  • dramatically improves reliability.

Conclusion

• Summary of current mirror operation and its configuration.
• Questions and suggestions are welcome for further discussion.
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