Zen Amplifier by Bipolar Junction Transistor - diyAudio
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Old 5th October 2004, 04:44 AM   #1
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Default Zen Amplifier by Bipolar Junction Transistor

Hi,

I planned to design the zen amplifier that will apply BJT instead of MOSFET. Could anybody tell me which BJT should be applied for 10W zen amplifier?
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Old 5th October 2004, 04:45 AM   #2
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Have anybody tried to design Zen amplifier by BJT? Could you share me some further information?
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Old 5th October 2004, 06:51 AM   #3
PRR is offline PRR  United States
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>Zen amplifier by BJT?

Some things go well together. Pass-Zen and FET. Some things do not go well together. Pass-Zen and BJT.

The FET has a high input impedance (especially for DC and low audio), and a low but fairly linear transconductance. The BJT has a VERY low input impedance, a lot more transconductance, which varies directly as current.

When you push them hard (as in a Power amplifier), without feedback, FETs (and tubes) tend to 5%-10% THD, transistors over 25% THD and hard to judge because the waveform is so bent that it never really clips on one side, even while flat-lined on the other side.

Here's a 15-minute design:

Click the image to open in full size.

I didn't bother with a literal 1-transistor design: I knew from years of blunt pencils that the input impedance would be impossibly low, and bias would be too sloppy. So I used a Darlington, which is a lot like a single transistor with higher current gain.

Here is a SPICE-simulation run. Never trust SPICE, but it sometimes get the right answer on simple circuits like this. And from my experience, these answers are about right.

Click the image to open in full size.

The top trace, 2.0V in, is giving 11.3V peak in 8 ohms, or 8 watts (for 178 Watts supply power!). It may not look like it, but distortion is 11% THD: 11% 2nd, 0.65% 3rd, 0.6% 4th, 0.2% 5th, under 0.05% 6th thru 9th.

The bottom trace shows how it will clip horribly, yet stay round on the bottom. Actually the bottom is badly distorted, much weaker than the original. It is more like "the shape of the transistor" (its Gm curve) than the shape of the input.

Also note that the input impedance is a little over 1K ohms, which many folks would call "too low". Not just that it is low, but it varies over the cycle.

Damping is not large, but may be ample for many uses.

Bias stability is poor (though maybe no poorer than the simpler Zens that need a trim-pot).

I made a small attempt to optimize operating point. I expected 8 ohm DC load and 20V collector, but it likes the higher resistor and lower voltage better. It could be a little better, but not a lot.

Much of the distortion could cancel push-pull. But then you have to arrange push-pull drive, still into an annoyingly low input impedance.

And historically: if any 1-transistor plan worked, in days when transistors were expensive, it would have been done. In fact there were 1-transistor megaphones, but they used carbon mikes (VERY high output and DC tolerant), ran DC through their speakers, worked poorly, sounded awful.
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Old 5th October 2004, 11:43 AM   #4
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I've compared the BJT and MOSFETs in some simulations.. I such a simple circuit, the MOSFET seems to have about 6db less THD (0.5% cq 1% at 1Watt) , slightly higher gain, but slichtly lower output..
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Old 5th October 2004, 11:45 AM   #5
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and the BJT...
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Old 5th October 2004, 12:08 PM   #6
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I just realized that the 0.5 Ohm resistor is'nt needed sich the upper transistor determines the Iq.. Allthough it does stabelize Vdc, but this is less critical..

The THD measurements with Spice of the MOSFET evrsion are very accurate, as most of the circuit performance parameters. I don't know if BJT models are as accurate..

Now I could raise the input impedance a little. BTW.. damping factor is higher compared to MOSFET version..
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Old 5th October 2004, 08:26 PM   #7
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what about bride of zen with bipolar junction transistor.please give some information
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Old 6th October 2004, 01:39 PM   #8
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There are many good idea in here. I wanted to do my simulation but I had no more times . My work was so busy and busy.
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Old 6th October 2004, 01:41 PM   #9
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Quote:
The BJT has a VERY low input impedance
If I don't wrong remind, I thought common collect type has higher input impedance than common emitter. Is it right? Have anyone tried to design Zen amplifier by common collector type?
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Old 7th October 2004, 11:40 AM   #10
zinsula is offline zinsula  Switzerland
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Hi all

take a look on this thread:

simplest amplifier possible with BJT's?

there are some ideas too...

Ciao, Tino
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