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analog_sa 26th September 2004 03:03 PM

Ok, I admit i don't make any sense out of this
PD was recently kind to post a fragment of the ML38 schematic.

For the last couple of hours i've been staring at the PS arrangement and feeling very dumb. Power to the collectors of the matched transistors seems to be supplied on both sides from CCS feeding a unity gain opamp. What's the point?

jan.didden 26th September 2004 03:47 PM


Very strange indeed. Could there be a drawing error in the interconnection of the terminals of those matched pairs? That doesn't make sense.

Jan Didden

Peter Daniel 26th September 2004 03:54 PM

I'm not sure if you read it right, but the dual transistor is in 8-DIP package, so it is represented by the frame outline on a drawing. The power is supplied to one of the collectors, the other collector is connected to the emiter (of the other transistor).

Eva 26th September 2004 03:59 PM

This appears to be a funny and exotic way of cascoding

All transistors appear to work at a fixed Vce of about one diode drop

jan.didden 26th September 2004 04:36 PM

But that transistor that has the collector to the emitter of the other, that places its collector-base diode in conduction, isn't it? That would influence the operation of the input, no?

Edit: that collector doesn't appear to get any supply, as it is connected (with the other emitter) directly to the output via 10 ohms.

Jan Didden

Eva 26th September 2004 05:00 PM

Left side transistors work with Vc = Vb so its Vce is one diode drop
Right side transistors work with Vc = Vb + diode drop so its Vce is two diode drops

Note that left side transistors have its bases tied together while right side transistors have its emitters tied together

Note also that small signal transistors may work with Vce as low as 0.1 V [ie: collector potential between base and emitter potential

Redrawing the schematic may help to understand it

Peter Daniel 26th September 2004 05:10 PM

2 Attachment(s)
Here's the redrawn version. It looks much clearer now.

jan.didden 26th September 2004 05:56 PM

Thanks Peter, that's MUCH better.
Eva, does that mean that the right side transistor works in what looks like bootstrapped common-base?

BTW, who designed this?

Jan Didden

andy_c 26th September 2004 05:59 PM

Is this better?
2 Attachment(s)
Here's yet another interpretation. It's a diamond buffer with the input collectors bootstrapped to the output emitters, and the output collectors bootstrapped to one diode drop away from their respective bases.

Peter Daniel 26th September 2004 06:04 PM


Originally posted by janneman

BTW, who designed this?

This circuit is used in Mark Levinson ML380 preamp, as well in some other ML units.

Is there a need for dual NPN/PNP transistors here?

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