# someone can help me to analyze this circuit?

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#### paulopca

Hi everyone! Someone can help me to analyze this circuit? It is a circuit of a power amplifier. This amplifier has already been assembled and I know that it works. Q13, Q15, D6, D7, R18 and R21 form two voltage stabilizers feeding U1. Q12, Q14, D5, D8, R16, R17, R19 and R20 form two current sources. Q11, R14, R15, R16 form a vbe multiplier. Q1, Q2, D1, D2, D3, D4, C1, C2, R1, R2, R3, R4 form a VI limiter. Q3 and Q7 are predrivers. Q4, Q5, Q6, Q8, Q9 and Q10 are output drivers.(2SC5200 and 2SA1943) U1 (tl071) is the input stage. All right so far? I don't understand how the op amp drivers the output stage and how the local feedback of the op amp works and how the global feedback (feedback between output and the op amp) works. I thought the voltage amplifier was the op amp, but the signal level at pin 6 (output) is much smaller than the signal level that drivers the predrivers. The VI limiter it's ok? If not, someone can help me to correct? The source is +40V and -40V and the load is 4ohms. What is the maximum power that this circuit can achieve with a load of 4ohms and what source? This circuit can work with a load of 2ohms? Under what conditions? I'll be very grateful to anyone who can help me.

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#### djoffe

Drive pin 3 of U1 positive. Pin 6 or U1, its output pin, goes positive, pushing current through R24. That current had to come thru the positive supply rail, pin 7 of the opamp, supplied by Q13. That current is mirrorer in Q12, and Q12 provides the big voltage gain. Thus the swing on the collector of Q12 is much bigger than on the output of U1. Similar action for negative inputs, Q15, and Q14.

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#### jaycee

It's a more crude implementation of the Alexander current feedback amplifier topology I think. The opamp is driving R24 as a constant load, and the current changes in it's power supply pins are being mirrored by Q12 and Q14 to produce the voltage gain, followed by a conventional emitter follower for the output stage.

The original Alexander amp is detailed in Analog Devices app note AN-211:

http://www.analog.com/static/import...tes/58052492001115525484056221917334AN211.pdf

I'd say use MJE15032/3 as the drivers instead of TIP41/42 and that you could use 2N5401/2N5551 or MPSA42/92 for the VI limiter transistors.

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#### sregor

Besides Alexander, search for Koruda amplifier.

#### sregor

Besides Alexander, search for Koruda amplifier.

My bad - should be Kuroda

#### paulopca

I thank all who responded.

#### godfrey

The VI limiter it's ok?
It looks OK.

What is the maximum power that this circuit can achieve with a load of 4ohms and what source?
Maximum power into 4 Ohms is about 150W.
It needs about 700mV rms input for full output.

This circuit can work with a load of 2ohms?
Yes. Maximum power will be about 270W.
It will need big heatsinks to prevent the output transistors and driver transistors getting too hot.

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