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Old 19th November 2005, 07:52 AM   #1
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Default Paralleling VAS devices for higher power

Awhile ago, I asked how to parallel transistors in the VAS stage for more power handling capability. I was told to degenerate the emitters of the common emitter and cascode transistors. Included below is a picture of how I would do it (the left is before, the right is after paralleled devices have been added). Is this the correct way?
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File Type: jpg dual1.jpg (29.8 KB, 313 views)
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Old 19th November 2005, 08:10 AM   #2
AndrewT is offline AndrewT  Scotland
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since the cascode and VAS are running at the same current then I would expect the emitter resistors to be similar.
Usually the cascode takes most of the voltage and thereby most of the power. If you check your voltages and power dissipations you may find that you only need to parallel either the cascode or VAS.
Your diagram looks right.
regards Andrew T.
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Old 19th November 2005, 08:15 AM   #3
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In upper pair is better to give resistors into base ( by my opinion... ).
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Old 19th November 2005, 02:26 PM   #4
Bensen is offline Bensen  Belgium
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HI rtarbell,

I attached a pic of how Elektor did this with the Titan2000 amplifier (500Wrms/4Ohm).
This seems to me the most logical way to do this, just paralleling three cascoded VAS'es. Without making connections inbetween like in your drawing.

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File Type: jpg parallelled vas.jpg (17.5 KB, 282 views)
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Old 19th November 2005, 02:40 PM   #5
jcx is offline jcx  United States
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bensen's post shows the way to go - no need for tieing the currents togeter and then adding the degen to balance the current division in the cascode transistors

just think of a cascode pair as a composite transistor, draw the circuit with single Qs in parallel with degen and then replace each with the composite cascode - then you only degerate the input transistor emitter, when paralleling the internal c-e nodes of each cascode are kept seperate
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Old 19th November 2005, 06:02 PM   #6
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Thank you! This is EXACTLY what I was looking for!

I didn't state it in the original thread, but the theory of this design is that the cascode bias voltage on the base will follow half way between the output and the supply rail ==> both the common emitter transistor and cascode transistor will dissipate power equally (or at least, in theory).

Thanks for the replies!
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Old 19th November 2005, 07:37 PM   #7
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Would not one single VAS transistor always perform better than two-three in parallel?

Multiple PCB traces and packages will add a couple pF and nH over one single device. Adding current balancing emitter resistors in series will reduce the effective gain.

I agree that it is much more difficult to find a great 2-5 watt VAS complementary pair than using the rich selection of low power complementary 0.5 watt input transistors. I've been searching for 2-watt devices with specs similar to BC549C/BC559C. Massive heatsinks on these tiny transistors is my temporary solution.

"common wisdom"
Parallel transistors only when higher power devices or acceptable cooling solutions are not available, like in output stages.
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