I understand notch filters, I can write transfer functions and draw Bode diagrams to see how they work. As far as I know, the notch filter only has: C1, C2, C3, R1, R2, R3.

The schematic I have here includes:

  • C1 = C2 = 0.05uF
  • C3 = 0.1 uF
  • R1 = R2 = 40k Ohm
  • R3 = 20k Ohm

(It does not let an 80Hz signal pass.)

Additionally, there is C4 = 0.05uF. I have found the transfer function of the schematic with and without C4 - the Bode diagrams are the same.

What is C4 for?

enter image description here

  • 1
    \$\begingroup\$ If you have a source Rs then it’s HF LPF \$\endgroup\$ Jul 6, 2021 at 3:32
  • \$\begingroup\$ Worth noting in the question that this particular filter is known as a "Twin-T" notch filter. When drawn normally now, it's easier to see why it's called that, and that C4 is working strictly before the notch. \$\endgroup\$
    – J...
    Jul 6, 2021 at 14:45
  • 1
    \$\begingroup\$ @J... thank you for drawing a new diagram \$\endgroup\$
    – Dat
    Jul 6, 2021 at 15:32
  • \$\begingroup\$ Real resistors have self inductance. Real capacitors certainly have "self resistance" (a.k.a. ESR). Any real circuit has stray inductance and capacitance depending on its physical layout. Consider what C4 does to RF frequency noise where those unwanted side-effects are important - for example it stops the "filter" (plus whatever is connected to the input and output) becoming an RF oscillator. \$\endgroup\$
    – alephzero
    Jul 6, 2021 at 15:36

2 Answers 2


You can't determine the effect of C4 without taking into account the output impedance of the previous stage.

C4 is directly across the input terminals - assuming your analysis used zero output impedance for the previous stage C4 will have no effect.

In a real world system the previous stage would have a finite resistance, C4 will then add a first order low-pass filter to the overall response. That is probably the intent.


C4 is between VIN and GND -- therefore it plays no role in the VIN..VOUT transfer function, although it does act as a load on VIN, and depending on the output impedance of VIN, it may indirectly affect the apparent transfer function, but won't affect the notch location.

Why was it added ? Perhaps it acts to filter other signals ?


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