--- title: "Hartley vs Colpitts for FM carrier" subject: "Electronics Devices and Circuit" chapter: 5 key: "semester_3/electronics_devices_and_circuit:hartley-vs-colpitts-fm" source: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/hartley-vs-colpitts-fm site: BCT Engineering --- # Hartley vs Colpitts for FM carrier Electronics Devices and Circuit · Chapter 5: Signal Generator and Waveform-Shaping Circuits - Asked in: 2076 Chaitra, 5 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/hartley-vs-colpitts-fm - Whole chapter: https://www.bctengineer.com/md/semester_3/electronics_devices_and_circuit/chapter-5 ## Question Among Hartley and Colpitts LC oscillator, which one do you choose to implement in FM stations to generate carrier wave signal? Why? Draw its circuit diagram. ## Short answer Colpitts oscillator is chosen for FM carrier generation due to better frequency stability, lower harmonics, and suitability for high frequencies. A varactor diode across a tank capacitor allows easy frequency modulation without tap issues. ## Answer **Choice: Colpitts oscillator** is preferred for generating the carrier in an FM transmitter (together with a crystal or a variable-capacitance diode for modulation). #### Reasons 1. **Better frequency stability.** The Colpitts frequency depends on a fixed inductor and capacitors, and is less affected by stray effects. The Hartley needs a tapped coil, and the mutual inductance M changes with temperature and construction, causing drift. 2. **Purer sine wave.** The capacitor divider shunts the harmonics to ground, so the waveform has lower harmonic distortion than Hartley. 3. **Works well at high frequency (up to hundreds of MHz).** The FM band (88 to 108 MHz) is high. A single coil without a tap is easy to make at VHF, while a tapped coil is difficult. 4. **Easy tuning and FM modulation.** A varactor diode can replace or sit across C1 or C2. The audio voltage changes its capacitance, and so changes the carrier frequency (frequency modulation). The feedback ratio C1/C2 remains almost constant. 5. **Stable amplitude** with feedback ratio set by capacitor ratio. | Point | Hartley | Colpitts | |---|---|---| | Tank | Two inductors + C | Two capacitors + L | | Frequency | 1/(2π√((L1+L2+2M)C)) | 1/(2π√(L C1C2/(C1+C2))) | | Frequency stability | Lower | Higher | | Harmonics | More | Less | | Best for | RF, wide tuning | VHF/UHF, FM carriers | #### Circuit diagram (Colpitts) ```text Collector o----+------+ | | [C1] | | [L] Emitter/GND o--+ | | | [C2] | | | Base o--+------+ ``` f = 1/(2π √(L C1 C2/(C1 + C2))), with a CE transistor stage with bias network and an RF choke in the collector. In an FM transmitter a varactor diode is added across C2 or in parallel with the tank. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which oscillator is preferred for FM carrier generation? A. Hartley B. Colpitts C. RC phase shift D. Wien bridge Answer: B. Colpitts has better stability, purer sine and works well at VHF. (Key point: Choice) 2. Why is Hartley frequency less stable? A. Its tapped coil's mutual inductance M changes with temperature and construction B. It has no inductor C. It has no capacitor D. It uses a crystal Answer: A. Drift in M changes Leq = L1 + L2 + 2M. (Key point: Stability) 3. Why does Colpitts have lower harmonic distortion? A. The capacitor divider shunts harmonics to ground B. It uses a bigger inductor C. It uses negative feedback D. It has a tapped coil Answer: A. Capacitors present low impedance to harmonics, shunting them. (Key point: Harmonics) 4. How is FM achieved in the Colpitts carrier oscillator? A. A varactor diode across C1/C2 or the tank changes capacitance with audio voltage B. Audio changes VCC only C. Audio changes the coil taps D. A crystal is cut Answer: A. Changing capacitance with audio shifts the carrier frequency. 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