R4 make the current through Q3 increase and the current through Q4 decrease. This
makes the output voltage go positive. As the positive input voltage increases, you approach
the point at which Q3 is conducting 2.6 amps and Q4 is conducting 0 amps – and all of the
2.6 amps goes through the loudspeaker.
The power of 2.6 amps into 8 ohms is I^2 * R, or 2.6 * 2.6 * 8 = 54 watts. This is the peak
value, and the nature of an undistorted sine wave is that the peak wattage is twice the
average, so this circuit would operate 27 watts average Class A into 8 ohms. At currents
above 2.6 amps one of the transistors will shut off, leaving the other to continue to increase
beyond the 2.6 amps in what is known as Class AB.
This circuit employs feedback to improve the performance in a number of ways. Feedback
sets the gain, lowers the distortion, improves the bandwidth, and creates the output
impedance (damping factor) of the amplifier.
The feedback mechanism for this amplifier is R3 through R6, a dual pair of low impedance
voltage dividers which feed the output to the Source pins of Q1 and Q2. Low impedance
feedback has been (incorrectly) referred to as “current feedback”, and it is popular in simple
high-speed linear circuits. One of the charms of this arrangement is that unlike the classic
two transistor differential pair, the drive current available exceeds the bias of the input
stage.
Something different about this example is that each JFET has its own feedback – there are
two separate feedback loops to this amplifier, so that the loop of Q1/Q3 is independent of
the loop for Q2/Q4.
The circuit of Figure 5 illustrates the basics and does in fact work, but has a tendency
toward instability. Local parasitic oscillation is often seen, and the bias drifts with
temperature, requiring a lengthy adjustment period. This simplified circuit also has no
provision for adjusting against the variations to be seen between real FETs that you will
encounter.
Actual Working Circuit
Figure 6 shows a completely functional version of the circuit, with useful enhancements.
We have added R7 and R8 in parallel with R5 and R6 so as to share the dissipation,
allowing inexpensive 3 watt resistors to do the job.
You never know what the amplifier will be attached to, so the input of the amplifier now
sports R9 to avoid parasitic oscillation of the input JFETs, and R10 to ensure that the input
has a default reference to ground when there is nothing connected.
R11 and R12 are 3 watt power resistors added to the Source pins of the MOSFETs to
increase thermal stability and serve as convenient current sensing elements. R13 and R14
are placed in series with the Gates of the MOSFETs to prevent parasitic oscillations, just as
we did with R9 on the input stage.
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