As recently low noise monolithic transistor arrays become available through
Farnell, I couldn't resist the temptation to implement them in this next design.
One of these chips, the THAT340 (2xNPN+2xPNP), replaces the discrete
transistors Q2, Q3, Q7 and Q8. As the max. Vce of these transistors is
limited to 36V, two cascode stages are inserted to lower the collector voltage
to 19V, see Q6 and Q9. Because of these cascodes, no bootstrapped power
supply is needed, except for the current sources (Q1 and Q4). Instead, they
are bootstrapped by R8...R11. Also notice that the base currents of Q6 and
Q9 are fed back via Q1 respectively Q4 to the input stage, analogue to a
Hawksford cascode. As a result, the distortion caused by the Early effect is
Furthermore, the VAS output is buffered with two additional transistors (Q19
and Q20), not only to provide temperature compensation for the next stage,
but also to eliminate the nonlinear load (Cob) from the next stage. This is
achieved by bootstrapping the collectors of Q19/Q20. As a result, the closed
loop THD20 of the front-end is further reduced to a negligible 60ppb. More
MCP (Middle Class Power Amplifier)
While the PMP line of amps was actually meant as a design example, the MCP is a more complete amp and
intended to be really build. In the following aspects, this brand differs from the PMP amp:
1. Enhanced low noise input stage.
2. Offset compensation by means of a servo.
3. VAS output buffer.
4. TMC compensation in the output stage.
5. Protection of output devices.
Below some THD figures at 20kHz and 1, 10 and 100W into 8 Ohms. The output stage comprises
only one pair of MOSFETs, The red dots depict the harmonic components. (full scale = 1ppm)
Below THD-20 figures for an output stage equipped with four pair of MOSFETs and a load of 4 Ohms