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Old 16th October 2021, 01:33 PM   #41
hpasternack is offline hpasternack
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Join Date: Sep 2002
Location: New England
Connecting the collectors this way maintains a nearly constant, low voltage from collector to emitter of of the driver transistors. They are effectively cascoded. This has several advantages:

- Lowers driver transistor power dissipation
- Eliminates nonlinearity caused by Early effect
- Eliminates nonlinearity caused by nonlinear junction capacitance
- Lowers driver transistor effective input and output capacitance

Since the output transistors now carry the driver bias current, I think this helps the output stage stay in Class A.

I'm not aware of any disadvantages, but that doesn't mean there aren't any. Let's see what people say in your thread.

Edit: This has been asked before. See here: Questions regarding diamond buffers.

The driver transistors run at nearly zero Vcb. You want devices with a low saturation voltage here.

Edit: TBH, I didn't think of this latter point when I selected the TTA/TTC004B drivers. Their saturation voltage is given as 1.3V which is inadequate in this design, oops. But they are medium power devices with a maximum collector current of 1.5A. Saturation voltage will be much lower at 10mA. It seems to work well, but maybe changing to BC550/560 would give better results. In that case, you need to find a way to mount the TO-92 devices to the heat sink for thermal tracking.

Last edited by hpasternack; 16th October 2021 at 01:47 PM.
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Old 16th October 2021, 02:05 PM   #42
hpasternack is offline hpasternack
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Join Date: Sep 2002
Location: New England
SPICE predicts we have about 1.2V across the drivers, so we just get by by the skin of our teeth. Crisis averted, probably.

Edit: The plot of Vce sat versus Ic paints a more encouraging picture at low collector current.
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Last edited by hpasternack; 16th October 2021 at 02:08 PM.
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