About This Circuit

A bridged-T is a T network with a component bridged across its top. This one is a resistor-topped bridged-T: R3 and R4, the resistors, form the top arms of the T, C2 is the bridge across the pair, and the middle of the T is loaded by R5 and C3 to ground. The name matters because the topology comes in two flavors. Swap the roles, capacitors on top with a resistor bridging them, and you get the capacitor-topped bridged-T, a different animal that gets its own calculator. Drop this network into an op-amp's feedback loop and it becomes an EQ block: the op-amp has to work hardest at the frequencies the network passes least, so the stage's response is the inverse of the network's, and the dip in the network becomes a boost at the output.

The physics pins both ends of the curve at 0 dB. At DC, C3 is open, no current can flow through the feedback network, and the stage acts as a unity follower. At very high frequencies C2 becomes a short from output to inverting input, which also forces unity. The boost lives in between: R5 and C3 set where the shunt leg starts stealing current, and that against the C2 bridge sets the center frequency. Raising R4 makes the peak taller, raising R5 or C2 pulls it back down, and R3 shapes how the skirt returns to flat.

Drag a slider or click a value and type your own. 22k, 4k7, 50p, 0.1uF and 1MF all work, and Enter commits while Esc cancels. The solid curve is your exact values and the dotted curve is the same circuit built from the nearest standard E12 parts, so you can see what real off-the-shelf components do to the response before you solder anything. One thing to notice while you experiment: this topology only boosts. The feedback network can't push the gain below unity, so to cut instead you'd flip the network into the input path.

Response modeled with an ideal op-amp and the bridged-T network alone (C2, R3, R4, R5, C3). The fixed coupling and bias parts set the stage's own low-cut corners and are not part of this curve.