Classic Amp Schematic (Preliminary)

Schematic (click to enlarge)

Differential Amplifier
The differential amplifier compares the input voltage with the output voltage via the feedback network, R8, R9 and C1. Any difference between the two voltages creates an error correction signal for the voltage amplifier. Q2 and Q3 are the differential amplifier. Q1 is the current source for the differential amplifier. Q4 and Q5 are the current mirror, so that the voltage amplifier, Q7 gets the benefit of both Q2 and Q3 from the differential amplifier.

D1 and D2 are the voltage reference for the current sources. R2 sets the differential amplifier at about 20 mA, or 10 mA per device. R5 and R6 set the voltage at Q7 base at about 1.2 volts. This will approximately balance the differential amplifier. R3 and R4 are placeholders that will be determined experimentally to provide the correct amount of emitter degeneration to reduce the open loop gain of the amplifier to the desired amount, and linearize the differential amplifier.

Voltage Amplifier
The voltage amplifier is responsible for providing the bulk of the voltage amplification (when compared to the few millivolt swing from the diff amp, and the nominally unity voltage gain of the output stage). Voltage amplifier Q7 works into constant current source Q6. R10 sets Q6 to provide approximately 10 mA. R11 sets Q7 base to approximately 1.2 volts at 10 mA, consistent with the current mirror at Q4. R11 provides emitter degeneration to linearize the voltage amplifier. R5, R6 and R11 are placeholders that will be determined experimentally for optimum open loop gain and linearity. They will all be chosen to balance the differential amplifier when idling. C2 provides the dominant pole for setting the power bandwidth and phase margins of the amplifier. The optimal value will be determined experimentally.

Power Output Stage
Q10 and Q11 are the pseudo emitter-followers for the output stage. They're connected as emitter followers, except that they get a power assist from Q12 and Q13, which are the actual power output devices. This is the Sziklai configuration. Being inside the local feedback loop established by Q10 and Q11, the current through Q12 and Q13 is entirely controlled by Q10 and Q11. Therefore, it is only necessary to temperature stabilize Q10 and Q11 -- and since Q10 and Q11 dissipate much less power than Q12 and Q13, temperature stabilization and thermal runaway are much less of a problem, and very easily controlled. R14 and R15 speed up the output bandwidth, and set the idling current of Q10 and Q11 at about 10 mA, to match Q8 and Q9 when idling. Finally, C3 and C4 may be required to control any tendency of the output stage to oscillate. The actual value, if needed at all, will be determined experimentally.

Bias and Temperature Compensation
R12 and R13 bias the output stage; Q8 and Q9 are diode-connected to provide temperature compensation for Vbe of Q10 and Q11. Diodes are normally used in place of Q8 and Q9, but Q8 and Q10, Q9 and Q11 can be placed on the board in close proximity so that they can be fastened together for excellent temperature tracking. With the temperature compensation from Q8 and Q9 and the constant current from Q6, the quiescent idle current through Q12 and Q13 is completely determined by R12 and R13. The values are chosen to idle the finals at about 20 mA, which is a little on the low side for class AB (see equation 21). I might increase R12 and R13 to about 2.2 Ohms, for an idle current of about 50 mA later on.

Feedback
Overall feedback is provided by R8, R9 and C1, and is applied only after the open loop linearity and bandwidth, slew rate, etc. have been optimized. The rule for good amplifier design is, minimize open loop distortions, and then apply feedback. Then the feedback will set the gain at about 26 dB, and further linearize the amplifier.

Leaving Options
There are a number of place holders in this design, so that the circuit board will have places for all the options. They can be left empty or jumpered out if not needed. After the PCB is manufactured, it is easier to eliminate extra parts than it is to add extra ones.