new thermal trak transistors

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audioPT said:
yes,

OnSemi sent me samples of some of the newest devices and I only can say they sound better than Sanken SAP's...

Which schematic did you use?
I ask because at a quick glance the SAPs have several diodes integrated while ONSemi's seem to have just a diode and thus (like in their application note) there seems to be the need of multiple devices to achieve a correct polarization of the output stage...

Cheers

Andrea
 
Andypairo said:


Which schematic did you use?
I ask because at a quick glance the SAPs have several diodes integrated while ONSemi's seem to have just a diode and thus (like in their application note) there seems to be the need of multiple devices to achieve a correct polarization of the output stage...



It's not strictly necessary. Diodes embedded in onsemi devices may be used as a sensor for a second circuit (just the usual VBE mult a bit rearranged) that really "track" the idle current against temperature.
However is a very niceful idea! :) With appropriate circuit this device may really help to keep at minimun crossover distortion in Class AB amplifier especially when the output devices are the only that commute on zero crossing and when driver and predriver stages are keep in a continous conduction state (really a kind of non linear class A).

Normally, if crossover is restricted to the effective power devices, crossover distortion - in well trimmed amplifier of couse! - became hardly detectable without measurement. If bias control tracking with temperature is thermally improved as Onsemi device promises, crossover may became hardly detectable *even* with instrumental measurement. May be a real improvement toward theoretical minima of crossover distortion (for bipolar devices at least) without excessive entanglement of output stage topologies.

Ciao
Piercarlo
 
Mikeks
---What OnSemi ought to have done is include another transistor on the same substrate as the power device for biasing with a Vbe-multiplier.---

A dream for amplifier engineers since more than thirty years. Manufacturers seem to have the worst difficulities to understand it. At last they are on the right way, but still far from the real demand.
 
Andypairo said:


Which schematic did you use?
I ask because at a quick glance the SAPs have several diodes integrated while ONSemi's seem to have just a diode [...]


Piercarlo said:


It's not strictly necessary. Diodes embedded in onsemi devices may be used as a sensor for a second circuit (just the usual VBE mult a bit rearranged) that really "track" the idle current against temperature.
[...]

I have done just like Piercarlo said! A fancy VBE multiplier ;)

One 10uF Cap!

I've used OnSemi schematic, only from VAS to output. I've used a differential pair with current mirrors at the input.
 

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COPIED FROM LEACH AMP DOCUMENTATION

The collector bias currents in Q12 and Q13 flow through the VBE multiplier circuit shown in Figure 10. Q7 is connected as a dc voltage regulator employing shunt-series negative feedback. The dc voltage across Q7 is adjusted with P1 to set the bias currents in the output stage. D1 through D4 are mounted in holes in the heat sink with the output transistors to provide negative thermal feedback to the VBE multiplier. The diodes cause the voltage across Q7 to decrease as the heat sink temperature increases to prevent thermal runaway in the output transistors. C12 improves the voltage regulation across Q7 at high frequencies. In addition, it prevents any oscillations that might occur because of the shunt-series feedback around Q7.


Figure 10. VBE multiplier circuit.

Many amplifiers have the VBE multiplier transistor on the heat sink with the output transistors. This eliminates the need for the diodes. The wires which run from the circuit board to the transistor exhibit capacitance to ground which can affect the high-frequency response of the second stage. At worst, this could cause oscillation problems. With the diodes on the heat sink, resistors on the circuit board can be used in series with the wires to isolate this capacitance from the second stage. These resistors are R25 and R26 in Figure 10. If Q7 were mounted on the heat sink, isolation resistors could not be used because they would affect the voltage regulation between the collectors of Q8 and Q9.
 

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LineSource said:
COPIED FROM LEACH AMP DOCUMENTATION


Many amplifiers have the VBE multiplier transistor on the heat sink with the output transistors. This eliminates the need for the diodes. The wires which run from the circuit board to the transistor exhibit capacitance to ground which can affect the high-frequency response of the second stage. At worst, this could cause oscillation problems. With the diodes on the heat sink, resistors on the circuit board can be used in series with the wires to isolate this capacitance from the second stage. These resistors are R25 and R26 in Figure 10. If Q7 were mounted on the heat sink, isolation resistors could not be used because they would affect the voltage regulation between the collectors of Q8 and Q9.

Sounds like a black magic spell. :cool:
 
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