How to drive 8 pairs of MJL21193/94?

[sajti]

Here You can find the driver stage for the bridged amplifier. It will be fully balanced from input to output.

Sajti

[roender]

Originally posted by AndrewT
A pair of output devices with an Re of 0r1 requires 260mA for optimal ClassAB bias.
Reduces to 114mA with Re=0r22 and to 53mA with Re=0r47.

Multiple pairs need these bias currents per device.

Andrew, you seems to forgot the internal parasitic emitter resistance. Also, the base stopper has a influence too.
With 0r1 Re and 2r2 base stopper the optimum bias current is 180...190mA

[roender]

This is the output stage linearity in relation with various values of Re, for a single output pairs (ref D. Self)
Of course, the figure is much more better as the number of pairs is increased, but the 0r1 re value is always better if the Vbe variation among output BJTs is in the range of 1..2mV
Re=0r47 is only for the cases when no OPS BJT matching is performed

[sajti]

Originally posted by roender


Andrew, you seems to forgot the internal parasitic emitter resistance. Also, the base stopper has a influence too.
With 0r1 Re and 2r2 base stopper the optimum bias current is 180...190mA

But how can I count with the internal parasitic emitter resistance?

Sajti

[sajti]

Originally posted by roender
This is the output stage linearity in relation with various values of Re, for a single output pairs (ref D. Self)
Of course, the figure is much more better as the number of pairs is increased, but the 0r1 re value is always better if the Vbe variation among output BJTs is in the range of 1..2mV
Re=0r47 is only for the cases when no OPS BJT matching is performed

The distortion is much better with more output pairs, but the load impedance is lower, because of the bridge connection.

And the 0R1 emitter resistor gives less protection against thermal runaway.

Sajti

[roender]

Originally posted by sajti


But how can I count with the internal parasitic emitter resistance?

Sajti

You can't ...
You should measure the minimum distortion vs bias current.
The numbers posted above are from measurements and not from theory.

[AndrewT]

Hi Roender,
I had not forgotten about the internal part of Re nor the effect that Rb has on it.
Your and others' earlier lessons are well remembered.

[sajti]

First, I will try to reduce the number of the output devices, to reduce the capacitances, and get smaller size. I think that 4pairs for each amplifier module (8pairs for 900W/2ohms)will be enough. The peak current will be 8A for each pairs, 1ohm load for each module, with 32Vpeak output.
I put together some test circuit to measure the bias. 2pairs of FJL4X15 with 0.1ohm resistors, driven by MJE15032/33 biased to 80mA, predrivers are BC639/640 with 8mA. I use two transistors bias circuit with BD139/140.
The heatsink is about half sized I planned for one module.

The bias looks overcompensated. 26mV with hot heatsink, and if I cool down the heatsink it increase up to 38-40mV

Sajti

[Coastben]

Just checking something here...


Can I use an MJL21193/21194 to replace MJ15003/15004's


They only have a rating of 140V and wouldnt mind bumping it up to 250V. I would obviously have to compensate for a slightly lower W rating though...


Any thoughts?

[wg_ski]

Originally posted by Coastben
Can I use an MJL21193/21194 to replace MJ15003/15004's

Not if you want them to fit in the same socket. The MJ is a drop-in, the MJL is not.

In reality, in audio apps the MJ and MJL handle the same power. You just can't run the temp as high with the plastic pacakge.