MCP

The Front-end

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

highly reduced.

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.