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--- electronics:opamps:opamp-feedback [2026/05/15 12:07] – created oscar
+++ electronics:opamps:opamp-feedback [2026/05/15 12:21] (current) – oscar
@@ Line 27: / Line 27: @@
   * noise requirements
+===== How to calculate? =====
+==== CASE 1 — Simple bandwidth limiting ====
+Adding Cf creates a low-pass filter in the feedback path.The cutoff frequency is approximately:
+  fc = 1 / (2π * Rf * Cf)
+So:
+  * larger capacitor → lower bandwidth
+  * larger resistor → lower bandwidth
+==== CASE 2 — Compensation for input capacitance ====
+This is more subtle and very important in fast/high-impedance circuits. Suppose the inverting node has capacitance:
+  * op-amp input capacitance
+  * PCB stray capacitance
+  * cable capacitance
+  * transistor gate capacitance
+This capacitance together with Rin creates a pole:
+  fp = 1 / (2π * Rin * Cin )
+This pole can destroy phase margin. A feedback capacitor introduces a zero that compensates it.
+A common approximation is:
+  Cf ≈ Cin * (Rin / Rf)
+This is widely used as a starting point.
+==== CASE 3 — Transimpedance amplifiers ====
+For photodiodes and probes, the feedback capacitor is often mandatory. Without it: huge peaking, oscillation, ringing, etc.
+The optimal value depends on:
+  * op-amp GBP
+  * total input capacitance
+  * feedback resistor
+A common approximation:
+  Cf ≈ SQRT( Cin / ( 2π * Rf * GBW  ) )
+where:
+GBW = op-amp gain-bandwidth product
+Cin = total input capacitance
+But in practice this is usually refined using:
+  * SPICE simulation
+  * phase margin analysis
+  * bench tuning