Class G Power Amplifier

Before you get too far into this page, beware that, while this amplifier has been built and works, my budget severely limits my ability to give you quantitative test results. This is currently a WORK IN PROGRESS! It is not my fault if you blow anything up.

 Class G Amplifier

Class G amplifiers are really cool. Literally. All other things equal, they run cooler than an equivalent Class AB/B amplifier. Unfortunately, there isn’t a ton of information out there on the design of high-efficiency linear amplifiers, so I will do my best to give some explanation of the principles at work and point you to the two best sources of information out there: the ESP website, and Douglas Self’s book, Audio Power Amplifier Design Handbook.

Taking advantage of physics

Let’s start with some math. The instantaneous power dissipated by an amplifier at any given point in time is the supply voltage minus the load voltage, multiplied by the current.

P = ( VS – VL ) x I

Looking at this, we can see that the only parameter here that we can reasonably change without affecting the actual amplifier output is the supply voltage. The problem is that our supply voltage is the limiting factor in how much power we can ultimately deliver to the load.

We always want more power.

The simplest solution is to keep the supply voltage as low as possible until we need to deliver more voltage to the load. We can do this by drawing current from two or more supply rails at different voltages throughout the output waveform.

For example: If you have a class B amplifier with a 50 volt supply rail (we’re only looking at the positive half-cycle here) and an 8 Ω load, the worst case amplifier dissipation occurs with a 25 volt output, 78 watts into the load and 78 watts dissipated in the amplifier.

PL = 252 / 8 = 78.125WPA = ( 50 – 25 ) * ( 25 / 8 ) = 78.125W

Contrast that with a class G amplifier that has a 50 volt supply rail and a second supply rail at 28 volts. When the output voltage is lower than about 25 volts, the amplifier will draw current from the lower supply rail, drastically changing the power dissipation.

PL = 78.125WPA = ( 28 – 25 ) * ( 25 / 8 ) = 9.375W !!!

That’s right, we just reduced the power wasted in the amplifier by a whopping 88%! That was easy, right? Well, kinda.

Implementing Class G

In order to actually use multiple supply rails, the basic class B configuration needs to be modified in the output stage and biasing.

TBC

bug list:

  • Ringing near the transition voltage. Try larger Class G bias voltage drop (blue LEDs?)
  • Protection cutting in early, despite lower rail voltage. Due to reactive loading? Replace 1N4148s with red LEDs for visual indication of protection operating? Reduce to simple I limiting, V not needed for Class G.

 

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