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Old 18th November 2013, 03:22 PM   #1
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Default class A amp with transistors sound better with 200ma bias per trans x 5

When transistors are linear why cant one make class A with little bias as much as 200ma per transistor at 40v and use them in parallel like 5 of them? There will not be any thermal runaway in the case isnt it?

What else is prohibiting to use transistors in class A
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Old 18th November 2013, 03:47 PM   #2
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Originally Posted by rhythmsandy View Post
When transistors are linear why cant one make class A with little bias as much as 200ma per transistor at 40v and use them in parallel like 5 of them?
There's nothing stopping you apart from the fact that it would be a class-ab amp, class-a only up to 8 watts (into 8 ohms).

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There will not be any thermal runaway in the case isnt it?
That would depend on how big your heatsink was and how good your bias servo (Vbe multiplier) was.
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Old 18th November 2013, 04:13 PM   #3
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When u r paralling 5 of tbem then the total current will be 1A and total 2Apeak so I^2 x R is 4 x 8 = 32W peak with 22 W RMS so how come u calculated as 8W?
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Old 18th November 2013, 05:58 PM   #4
sreten is offline sreten  United Kingdom
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Hi,

There is essentially no problem designing an output pair for high
optimum aB bias and then using multiple pairs to increase the
nominally Class A region of operation.

There is trade off between low Re's for higher bias and current
sharing in the multiple output devices. I reckon 333mA each
and 3 output pairs is doable for 1A standing current.

rgds, sreten.

(aB = optimum B bias, AB = overbias with gm doubling)
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Last edited by sreten; 18th November 2013 at 06:01 PM.
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Old 18th November 2013, 06:12 PM   #5
Bigun is offline Bigun  Canada
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Quote:
Originally Posted by rhythmsandy View Post
When transistors are linear
It's a tradeoff because transistors aren't linear.

As I understand, when you drive more current through a transistor their intrinsic Re provides degeneration that tends to result in improved linearity. Higher currents can also affect other parameters such as increased transconductance and increased Ft. Using multiple paralleled power devices also increases the total capacitance that the driver stage has to cope with (not so bad with transistors compared with FETs but still it happens). More devices means more $ along with larger pcb (longer traces), and more holes to drill in your heatsink. However, with more devices you apply less voltage between base-emitters for the same total current into the load - which means you are operating over a smaller part of the I-V curve of the devices and this has a very nice effect of reducing distortion.
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Last edited by Bigun; 18th November 2013 at 06:15 PM.
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Old 18th November 2013, 06:29 PM   #6
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32W peak would give you 16W rms, not 22.
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Old 19th November 2013, 10:40 AM   #7
AndrewT is offline AndrewT  Scotland
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Originally Posted by toprepairman View Post
32W peak would give you 16W rms, not 22.
32Wpk is the same as 16W

It comes from the power formula for sinewave signals:
P=Ipk^2 * Rload / 2
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Old 19th November 2013, 10:47 AM   #8
AndrewT is offline AndrewT  Scotland
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10pair of MJE15034/5 each biased to 180mA (18mVre) gives a total output bias of 1.8A.
That is good for 52W of ClassA output into 8r0.
The stage could go on to supply ~100W into 4r0 in ClassAB and this without the gm doubling that happens with most push-pull ClassA output stages.
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Old 19th November 2013, 12:26 PM   #9
DF96 is offline DF96  England
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Transistors are not linear. They follow an exponential law for transconductance. This means that 5 transistors in parallel behave more or less exactly like one bigger transistor - the same nonlinearity.

Their current gain is approximately linear, but droops at the ends.

There are two ways to get linearity with transistors:
1. carefully balance off different exponential responses - only possible for low gain in small signal circuits with constant temperature - Barry Gilbert was the expert at doing this
2. use lots of negative feedback (including degeneration and followers)

If you don't use either of these techniqies then you are happy with nonlinearity.
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Old 19th November 2013, 01:05 PM   #10
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Quote:
Originally Posted by Bigun View Post
More devices means more $ along with larger pcb (longer traces)
Stereo power amp example from the late '80s, output stages with hundreds of TO92 devices. (height of 4 feet)
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