Transimpedance amplifier output waveform width

The output waveform width of a transimpedance amplifier is primarily determined by the TIA's bandwidth, which is influenced by the feedback resistor, parasitic capacitances, and the op-amp's gain-bandwidth product.

Factors Affecting Output Waveform Width

1. TIA Bandwidth The output pulse width is inversely related to the TIA's bandwidth. A higher bandwidth allows the amplifier to respond faster to input current changes, producing narrower output pulses. The closed-loop bandwidth is limited by the op-amp's gain-bandwidth product (GBP) and the total input capacitance, including the photodiode capacitance (CD), op-amp input capacitances (CCM and CDIFF), and parasitic board capacitance (CPCB) .
2. Feedback Resistor and Compensation The feedback resistor (RF) sets the transimpedance gain (VOUT = I × RF). However, RF interacts with parasitic feedback capacitance (CF) to form a low-pass filter, which slows the response and broadens the output pulse. Proper compensation with a feedback capacitor can stabilize the TIA and control overshoot, but it also affects the pulse width .
3. Parasitic Capacitances Parasitic capacitances at the input and feedback loop create additional poles in the frequency response, effectively reducing the TIA's bandwidth. This results in longer rise and fall times, increasing the output waveform width .
4. Op-Amp Slew Rate and Time Constants The op-amp's slew rate and internal time constants also influence the output pulse. If the input current pulse is very fast, the op-amp may not reach the full output voltage instantaneously, further widening the output waveform .

Estimating Output Pulse Width

A rough estimate of the output pulse width (full width at half maximum, FWHM) can be obtained from the TIA's -3 dB bandwidth (f3dB): Pulse Width ≈ 0.35 / f3dB For example, a TIA with a 100 MHz bandwidth would produce output pulses approximately 3.5 ns wide. This approximation assumes a single-pole response and neglects higher-order effects from parasitics .

Practical Considerations

• Minimize parasitic capacitances in PCB layout to reduce pulse broadening.
• Choose an op-amp with sufficient GBP to achieve the desired bandwidth at the required transimpedance gain.
• Use feedback compensation carefully to balance stability and speed.
• For high-speed optical applications, the combined bandwidth of the photodiode and TIA determines the ultimate output pulse width . By carefully selecting RF, compensating for parasitics, and using a high-speed op-amp, the TIA output waveform can closely follow the input current pulse, producing narrow, well-defined voltage pulses suitable for high-speed detection.