Side-by-side comparison
| Parameter | Series | Parallel Resonance |
|---|---|---|
| Resonant Frequency | f0 = 1 / (2π√LC) — same formula | f0 = 1 / (2π√LC) — same formula |
| Impedance at f0 | Minimum — purely resistive (Z = R) | Maximum — purely resistive (Z = L/CR, called dynamic resistance) |
| Current at f0 | Maximum (I = V/R) | Minimum (circulating current is Q times supply current) |
| Q-Factor | Q = (1/R)√(L/C) = ω0L/R | Q = R√(C/L) = R/ω0L (R is tank resistance) |
| Bandwidth | BW = f0/Q = R/(2πL) | BW = f0/Q = 1/(2πRC) |
| Voltage Magnification | Voltage across L or C = Q × V_source (can be dangerous) | Not applicable — current magnification instead |
| Current Magnification | Not applicable | Circulating tank current = Q × I_supply |
| Application | Series RLC filters, IF transformers at 455 kHz | Tank circuits in LC oscillators, RF amplifier loads at MHz range |
Key differences
At resonance, both circuits present purely resistive impedance, but the values are opposite extremes. Series resonance gives Z_min = R (often a few ohms); parallel resonance gives Z_max = L/CR (can be hundreds of kilohms). Voltage magnification of Q times occurs across the inductor or capacitor in a series circuit — at 455 kHz IF with Q = 50, a 100 mV input produces 5 V across the capacitor. Parallel resonance magnifies circulating tank current by Q, not voltage. Q and bandwidth are inversely related in both cases.
When to use Series
Use series resonance when you need to pass or amplify a specific frequency with maximum current — for example, in the 455 kHz intermediate frequency transformer stage of an AM superheterodyne radio receiver.
When to use Parallel Resonance
Use parallel resonance when you need high impedance at a specific frequency to act as a high-Q bandpass filter or oscillator tank — for example, in a Colpitts oscillator at 10 MHz where the LC tank defines oscillation frequency.
Recommendation
For exam problems, always use f0 = 1/(2π√LC) for both circuits — that formula is the same. Then choose: series for minimum impedance and maximum current at f0; parallel for maximum impedance and minimum current at f0. That single distinction resolves nearly every MCQ on resonance.
Exam tip: Examiners always ask for dynamic resistance of a parallel circuit — remember Z_dynamic = L/(CR), not infinity, because real inductors have series resistance R.
Interview tip: Interviewers at analog design companies like Texas Instruments or Analog Devices ask about Q factor and bandwidth in the same breath — state that Q = f0 / BW and that a higher Q means sharper selectivity but narrower bandwidth.