Do 12V MR16 LED bulbs contain active components?
MR16 bulbs are most commonly 12VAC powered, dimmable, and use active components.  A true Halogen retro-fit MR16 LED lamps are compatible with ELVT and ELV dimmers

   

LM3444 MR16 driver
  

LM3401 MR16 driver



  SSL MR16 lamps compatibility concerns with ELVT and ELV dimmers (true
  retro-fit)
  Electronic transformers modulate (PWM) the input AC voltage with a
  frequency of 35 kHz to150 kHz. This waveform is step-down from 120V or
  230V (typical) to 12VAC with a transformer. The higher switching
  frequency allows for the smaller magnetic components, and the overall
  smaller design. As mentioned earlier, the electronic transformer is a
  self driven resonant half bridge topology.  The self resonance
  half-bridge topology requires the converter to have a minimal load at
  all times to function properly. Common minimum loads for ELV dimmers
  are from 6W – 12W depending on manufacture, and maximum power rating
  of the ELVT. With traditional Halogen lamps, the minimal load is of no
  concern, common Halogen MR16 lamps use about 50W of power per lamp.
  These lamps are very inefficient, and 10W of Halogen power produces
  very little light.
  
  With the current efficacy of the LEDs above 100 lumens per watt, 6W of
  SSL power is equivalent to about 40W to  50W of Halogen power. One can
  quickly see the compatibility issue of SSL MR16 lamps and the ELVT’s.
  If the  output power of the ELVT reduces below the minimum
  requirement, the ELV dimmer will stop operating. The  turning on, and
  off of the ELVT will cause visible flicker from the SSL MR16 lamp, and
  could also cause reliability  issues with the lamp or ELVT.
  
  Source: LM3401 MR16 Reference Designs for Non-Dimming &amp; Dimming LED Applications