MOSFET push-pull buffer/amplifier

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Another approach would be to use some of the MOSFETs high transconductance to provide voltage gain. Configuring the MOSFETs as grounded-source amplifiers unloads the input transformer the task of providing signal gain. The MOSFETs present a high output impedance at their drains, so a feedback loop will be needed. The amplifier at the right uses two feedback loops to lower the output impedance and distortion. Furthermore, they allow us to easily set the needed bias voltage. Note that in this case, the input transformer’s secondary is loaded by the two 20k resistors in series, not by two 300k resistor strings in series. (The MOSFETs are configured to invert the signal at their gates, so the connection between resistors is effectively grounded.)

Another tack might to go after the output transformer. Once again we are back to using the MOSFETs in a source-follower configuration. This time, though, the output transformer has been replaced by a center-tapped choke. Because the current flowing through the two paths away from the center tap are equal at idle, the choke does not become magnetized, as it usually does when it experiences unidirectional current flow. Consequently, the choke’s core does not need to be air-gapped, although it can be.

All wire has some DCR, which against a high idle current will cause a DC offset voltage at the extremes of the choke’s winding.

However, as the output is not referenced to ground, the speaker will never see the DC voltage. This amplifier, unlike the previous MOSFET amplifiers, must be run in strict class-A, so the idle current will be quite high, at half of the peak current demand. How do we find the peak current demand? This amplifier is effectively the equivalent to a normal source follower amplifier with +/- voltage rails twice that of this amplifier’s single rail; the magic of inductors. In this case, we can assume that 12 of the power supply’s 15 volts will be deliverable into an 8W load, so 24 volts of peak voltage would require 3A of peak current (as one terminal goes up the other goes down). Thus, an idle current of 3A is needed (24V / 8W), 1.5A per MOSFET. (If a floating power supply were used, the negative output could be grounded and the positive output would still be at ground potential, a note for the advanced practitioner.)

Alternatively, we could eliminate the output transformer (or choke) altogether. In the amplifier below we see dual floating power supplies and two feedback loops in parallel. This circuit is the Circlotron remade. It may look odd, but it functions identically to a conventional bipolar power supply source follower amplifier, as shown below. The only difference is that only N-channel MOSFETs are used and two power supplies are needed. The first difference is more important than the second. Part of the simplicity of this amplifier lies in its use of identical output devices, as P-channel MOSFETs never exactly match their N-channel complements. Consequently, using the device for both output decrease greatly improves the natural balance of the amplifier. Unfortunately, we must use an input transformer to create the required balanced drive signal.

Using P and N-channel MOSFETs allows us to get away with a single-ended input signal. In the amplifier below we see complementary output devices configured as source followers. As this amplifier offers no voltage gain, the line amplifier will have to swing the full +/-30 volts needed to bring the amplifier to full power. Because it offers no gain, this amplifier uses all its output stage’s transconductance to provide a low output impedance and a low distortion.

Simple Circlotron Amplifier

Push-pull MOSFET amplifier with feedback

Push-pull MOSFET class-A buffer/amplifier

MOSFET push-pull buffer/amplifier