Design goals for the Kumisa III: I like high bandwidth and slew rate, because it gives a high security margin before phase errors appear in the high frequencies.
Also I like low output impedance and high current capacities. Looking at these demands, I thought of current feedback opamps as the only possibility. They are typically fast and having low distortion. I hate crossover distortion and therefore all my amplifiers run real class A, using around 7 W for the entire amplifier.
The amplifier schematic is shown in figure 1. Q2 and Q3 are the input transistors. They are set up in a standard “ground emitter” connection. The current through the collectors of Q2 and Q3 is 2.5 mA (giving 4.5 V over the two 1.8 Kohm resistors) and 11.4 mA (4.44 V over the two 390 ohm resistors) through the next stage (Q7 and Q8). Q5 and Q6 are a voltage source. At the emitter, is there either +2.7V (on the Q5) or -2.7V (on the Q6) This way the voltage over the 820 ohm resistor connected between Q2 and Q6 (the same with Q3 and Q5) is around 2 V. If there is no signal (0 V) on input the current through Q2 and Q3 is 2V / 820ohm = 2.4 mA on booth. This balance is moved if a signal is added to the input. If 1V is added to the input there will flow 3V / 820ohm = 3.7 mA through Q2 and only 1V / 820ohm = 1.2 mA through Q3.
The 390 ohm resistor on the drivers (Q7 and Q8) make sure that the current through these is always at least 4 mA. Using such high currents I´m sure that all transistors are used in their linear working zone and that they always are running pure class A. Only the output transistors can go from pure class A (push-pull) to ordinary class AB, and that only happens when the output current exceeds the idle current. This high current makes the transistors a little warm, but not more than 35 degrees C.
Q1/Q7 and Q4/Q8 make up the 2 current mirrors. Q1 is a diode compensating for the loss in Q7 and the same goes for Q4 and Q8. The Vbe of Q1 and the Vbe of the Q7 neutralize each other within an error of 60 mV, making this stage a good current mirror with 4.6 times current gain. At 1 V input the current through Q2 is 3.7 mA and then the current through Q7 is 17.1 mA since the gain is around 4.6 times. The current through Q3 would in the same situation be 1.2 mA and 5.5 mA through Q8. The current forced on to the resistor network (1.8 Kohm + 1.8 Kohm + 33 Kohm + 33 Kohm = 853 ohm) is 17.1 – 5.5 = 11.5 mA. The Q9 adds some bias voltage and the driver transistors + output transistors eats the bias voltage again. The voltage on the output will then be around 853 ohm * 11.5 mA = 9.8 V. The gain is 9.8 times (a little less in practise) = 19.8 dB.
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