Added Complexity
    The Class-A mode of operation and the follower configuration both make the single-ended buffer amplifier a perfect candidate for adding a DC servo-loop. At all times, an IC strives to keep its positive and its negative inputs at the same voltage, which moves its output to swing positively or negatively in response.
   Consequently, when the buffer's output drifts too positive the IC's negative input pin will see a greater positive voltage than the ground referenced positive input, which will provoke the IC's output to go negative until the its input once again match in DC voltage. DC is a key adjective here, as the .22µF capacitor effectively absorbs the AC part of the signal presented to the IC's negative input; if a scope's probe is attached to this juncture, all that will be seen are the frequencies below 1 Hz.

Similarly, the Op-Amp does not need to swing anymore than a few volts positively at its output to keep the buffer's DC offset in line with ground. For example, a DC offset would likely never exceed 100 mV and the top MOSFET would at most only require 5 volts DC to set idle current to that of the bottom MOSFET.

          Push-pull buffer with a DC servo loop

     The next addition might be the creation of an active current source made up of the bottom MOSFET and an additional IC. In the circuit below, we see the current flowing through the 1-ohm resistor being monitored by the IC's negative input, which it will compare it to its positive input and thus work to keep the voltage across the resistor constant. A constant voltage means a constant current. The current is easily set by dividing the reference voltage by 1 ohm, thus in this example, the constant current source is set to 2.5A, which equals 25 RMS watts and 20 volts into an 8 ohm load, whereas increasing the current to 3A will increase the wattage to 36 watts and the peak output voltage to 24 volts.
      This constant current source is a truly active one, with the IC fully in the circuit, as its feedback loop encompasses the bottom MOSFET.

        Single-ended buffer with a DC servo loop

     The two 15 volt zeners are there to displace half of the 60 volts made available by the bipolar power supply, as the IC is voltage limited (a high-voltage IC would not require these zeners, but such ICs are expensive.) Basically, the Op-Amp does not need to see at its input the full output voltage swing of the buffer's output, only the few millivolts of DC offset at the output.

< PREVIOUS

NEXT >