Loudspeaker with Simple Amplifier

This simple* analog electronic amplifier design demonstrates the importance of multi-discipline system modeling. A swept frequency response test, from 40 Hz to 1000 Hz, shows the effect of the complex amplifier loading by the voice-coil and speaker-cone dynamics**. The electro-mechanical resonances strongly affect the current that must be supplied, in order to maintain a flat (controlled) output voltage over the specified frequency range. For example, the current in the voice-coil reaches a null at time 0.1 seconds, which corresponds to the effective "spring-mass" resonance frequency. The loudspeaker reaches its minimum impedance around 600 Hz, or near 0.6 seconds, where the peak load current is observed.

Normalized component stress monitoring signals are provided in all “datasheet specified” electronics models. For example, the simulation results show that the average power (bjt1/pwr_avg) in the BDP947 NPN BJT exceeds its 5 Watt rating across the entire range, but especially at lower frequencies. The corresponding stress monitor (bjt1/stress_ratio_power_avg) normalizes the transistor's average power relative to its 5W rating, so it is easy to see that the component is stressed (i.e. stress_ratio_power_avg > 1.0). Also, the red "hot part monitor", with the junction to solder-point thermal resistance set to 10 C/Watt as given in the datasheet, shows the part temperature rising to well over 100 C. These diagnostic indicators make it obvious that we need a bigger transistor!

* Note1: This is not intended to be a practical amplifier design. There is no blocking capacitor at the output, so it allows undesirable DC current into the voice coil. The purpose is to focus attention on the dynamic characteristics of the loudspeaker and not the circuit itself.

** Note2: Please refer to this companion example, that shows the input impedance frequency response of the loudspeaker alone:

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Mike Donnelly

Joined November 8, 2013
Design added Monday, August 17, 2015 | 11:19 am PDT