Differential Amplifier Design using BJT

Introduction

• Differential Amplifier: Amplifies the difference between two input signals.
• Usage: Common in integrated circuits, such as the first stage of an op-amp.

Differential vs Single-Ended Input

• Single-Ended Amplifiers: Input applied between input and ground; noise also amplified.
• Differential Input: Eliminates common-mode noise present at both inputs.

Advantages of Differential Amplifiers

• Noise Elimination: Common-mode noise is eliminated, improving signal clarity.
• Supply Hum Reduction: Differential configuration reduces common noise in circuits.
• No Coupling Capacitors Needed: When cascaded, coupling capacitors are unnecessary.

Circuit Design

• Components: Requires a matched pair of BJTs with equal collector resistors.
• Current Source: Emitter of transistors connected to a constant current source, often a current mirror in ICs.
• Discrete Components: May introduce mismatches.

DC Analysis

• Current Computation: Current Iee is divided equally between two matched transistors.
• KVL Application: Helps find emitter current, collector current approximated as emitter current:
  • Formula: Iee = (Vee - Vbe) / Re
• Collector Voltage: Vc = Vcc - α*Iee*Rc/2

Response to Signals

Common-Mode Signal

• Characteristics: Could be DC or common noise.
• Current Division: Iee equally divided between emitters, Vc remains balanced.
• Differential Output: Remains zero, unaffected by common-mode.

Differential Input Signal

• Current Steering: Large differential input steers entire bias current to one transistor.
• Small Differential Signal: Differential input voltage should be small (under 2Vt) for amplification.
• Virtual Ground: Node P acts as a virtual ground for small signals.

Voltage Gain

• Gain Formula: ΔVo / ΔVin = gm*Rc.
• Differential Output: Vo2 - Vo1 eliminates common-mode.

Conclusion

• Overview: Provided a general introduction to the differential amplifier.
• Future Discussion: Further analysis in upcoming content.

Next Steps

• Upcoming Video: Large and small-signal analysis.

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