Multi-stage BJT Op-Amp Design
Design and simulation of a multi-stage BJT operational amplifier with gain, bandwidth, and stability optimization
Overview
This project involves the design and simulation of a multi-stage BJT (Bipolar Junction Transistor) operational amplifier with specific performance targets for gain, bandwidth, and phase margin. The design process covers DC bias point analysis, small-signal modeling, frequency response optimization, and stability analysis.
Design Specifications
| Parameter | Target |
|---|---|
| Open-loop Gain | > 80 dB |
| Unity-gain Bandwidth | > 1 MHz |
| Phase Margin | > 60° |
| Slew Rate | > 1 V/us |
| Power Supply | +/-15 V |
Circuit Architecture
The op-amp consists of three main stages:
- Differential Input Stage: Long-tailed pair with active current mirror load for high CMRR
- Voltage Gain Stage: Common-emitter amplifier with active load for high voltage gain
- Output Stage: Class-AB push-pull output for low output impedance and high current drive
Analysis Methods
- DC Operating Point Analysis: Ensuring all transistors operate in the active region
- Small-Signal AC Analysis: Bode plot for gain and phase response
- Transient Analysis: Step response and slew rate measurement
- Monte Carlo Simulation: Process variation sensitivity analysis
Key Results
The design achieves the target specifications. Frequency compensation using Miller capacitance ensures dominant-pole behavior for stable closed-loop operation.
\[A_v = g_{m1} \cdot g_{m2} \cdot R_{out}\]where \(g_m\) is the transconductance of each stage.
Tools Used
- SPICE Simulation (LTspice / Cadence Virtuoso)
- MATLAB for frequency response plotting and analysis