"Lehmann" Buffer Output Transistors. - diyAudio
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Old 20th August 2014, 05:08 PM   #1
ammel68 is offline ammel68  United States
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Default "Lehmann" Buffer Output Transistors.

I have a question about the BD139 & BD140 output devices in the Lehmann buffer circuit below.

The schematic calls for the BD139 & BD140-16 devices which have the highest hFE.

What happens if you substitute the lower hFE devices like the BD139 & BD140-6 or -10?

Thank you...
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File Type: png BD140_hFE.png (75.2 KB, 271 views)
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Old 21st August 2014, 06:39 AM   #2
Mooly is offline Mooly  United Kingdom
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I would imagine that you would see a slight increase in distortion at higher output levels when driving lower impedance loads.
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Old 21st August 2014, 04:22 PM   #3
ammel68 is offline ammel68  United States
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I was concerned of saturating or "overloading" the output transistors since the BC transistors are also the highest hFE "C" grade devices.
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Old 22nd August 2014, 07:47 AM   #4
Calvin is offline Calvin  Germany
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Hi,

the BC drivers are biased just under 10mA, which is quite a lot.
even if the BDs were runnung on 100mA of bias current theyd require less than 1mA of Base current.
So nothing to worry about.
THD may rise a bit with lower hfe BDs but on a rather negligable niveau.

This Buffer is the second simplest form of an Diamond Buffer.
The drivers run on ~9mA bias, the outputs on 40-45mA.
Heat power loss of the BCs is ~140mW and ~550mW for the BDs.

THD improves when You connect V12s collector to V10s emitter and V9s collector to V11s emitter.
Also the drivers losses reduce considerably down to ~10mW, allowing for SMDs.
The BDs power losses incerease though by ~100mW to 650mW.

jauu
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Old 26th August 2014, 04:35 PM   #5
ammel68 is offline ammel68  United States
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Hi Calvin,

Does this look right?

Is there anything else you would change about this buffer to improve its performance?

Thank you...
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Old 26th August 2014, 05:23 PM   #6
Calvin is offline Calvin  Germany
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Hi,

not quite right, just one half
Follow the link to my website, bipolar Buffers and see Fig.5 and associated description

jauu
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Old 26th August 2014, 05:52 PM   #7
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Quote:
Originally Posted by ammel68 View Post
Is there anything else you would change about this buffer to improve its performance?
Bootstrapping. Not as good as CCS but simple and with lower voltage drop over the output transistors.
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Old 27th August 2014, 10:09 AM   #8
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Quote:
Originally Posted by Calvin
not quite right, just one half
He preciselly followed your instructions. He didn't notice small typing error

Quote:
Originally Posted by Calvin
THD improves when You connect V12s collector to V10s emitter and V9s collector to V11s emitter.
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Old 27th August 2014, 10:32 AM   #9
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Originally Posted by franzm View Post
lower voltage drop over the output transistors.
Should read: higher output voltage swing. Sorry for my bad English.
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Old 27th August 2014, 02:19 PM   #10
Calvin is offline Calvin  Germany
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Hi,

uups , oh ehm, yes, my fault.
Got my new glasses just later that day

jauu
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