Frequency Response

How to use this calculator

An amplifier's gain is flat only over a midband; it rolls off at low frequency (set by the coupling capacitors) and at high frequency (set by the parasitic junction capacitances). The low cutoff is fL = 1/(2π·Ccs·Rs) and the high cutoff is fH = 1/(2π·Cbe·(Rb‖Rc)). Enter the coupling capacitor, source resistance, base-emitter capacitance and the base/collector resistances, and the calculator computes fL, fH, the bandwidth (fH − fL), and draws an approximate Bode magnitude plot.

What it means in a real circuit

For Ccs = 10 µF, Rs = 1 kΩ, fL ≈ 16 Hz — the amplifier passes audio well. For Cbe = 20 pF and Rb‖Rc ≈ 1.67 kΩ, fH ≈ 4.8 MHz. The bandwidth is ≈ 4.8 MHz, and the midband gain (from the Gain calculator) is what you get between fL and fH. A larger Ccs lowers fL (good for audio); a smaller Cbe or higher Rb‖Rc raises fH (good for high-frequency work). Use this to check that an amplifier covers your signal bandwidth, to diagnose a stage that's "muddy" (too high fL) or "slow" (too low fH), and to balance capacitance against speed.