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Author Topic: 2 meter amplifier schematic question  (Read 640 times)
KD0XX
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Posts: 21




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« on: July 21, 2006, 04:31:27 PM »

Hi,
    Can someone educate me as to why 4 seperate caps are needed on the bases of the transistors? They are physically touching and in parallel in the amp.
    Why not just use a single cap since caps in parallel combine?

Here is the address to the schematic...

http://www.asburydevelopment.com/files/B-2530-G_PG3-4.jpg

Thanks,
RoD
KD0XX
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N2AXZ
Member

Posts: 90




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« Reply #1 on: July 21, 2006, 09:32:13 PM »

There are several possible reasons for this.  First, in RF work it is frequently desirable to distribute capacitance across several amall caps than to use a single, larger one.  The reason for this is that any capacitor always has a small amount of inductance associated with it that arises from its leads and the conducting surfaces that make up the plates. At low frequencies, what looks like a perfect capacitor can behave very differently at VHF.  The internal inductance of any capacitor will eventually lead to the capacitor itself becoming resonant at some high frequency.  To reduce this problem, using several smaller capacitors in parallel (the smaller capacitance will generally also mean a smaller self-inductance) is frequently employed.  Since parallel inductances will result in a smaller total inductance, the effect of inductance in the capacitors is greatly reduced.

Another reason for using several capacitors in parallel in this instance is for ease of impedance matching.  In this case, it appears as if the capacitance in the base lead is also functioning as the capacitive arm of an L-section formed with the inductance of the input to the stage.  When tuning such an amplifier, it is often useful to be able to adjust capacitance slightly by placing or removing small amounts of capacitance.  Using several small capacitors in this way makes it easier to get the impedance matching correct.

Hope this helps!

73,

David
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KD0XX
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Posts: 21




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« Reply #2 on: July 22, 2006, 12:06:46 PM »

David,
    Thanks for the help. That was oone of the best descriptions I have heard.
I have been away from repair for a few years and it's amazing how much I've forgotten.
I'm working on the amp in question and had replaced 2 of the transistors but they popped again as soon as I applied drive.
Bias and collector voltage looked correct but as soon as the amp draw was 8A or so, they shorted again. (base, collector short)
Any suggestions?

Thanks,
RoD
KD0XX
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N2AXZ
Member

Posts: 90




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« Reply #3 on: July 22, 2006, 01:30:17 PM »

Bipolar transistor power amplifiers can be very tricky, especially at VHF.  Blowing out numerous transistors is, unfortunately, a reality of working with these kinds of amps when adjusting them or attempting to troubleshoot them.  The schematic quality is a little hard to read, but without actually being in front of the amplifier itself, there are simply too many possibilities to explain your problem.  Some suggestions as to how you might try to proceed in your investigation might be:

1. When applying drive, don't use a 100% duty cycle, but pulse it at some low frequency (perhaps several hundred Hz or so).  This will help keep the average power dissipation down so the transistor will have less of a tendency to go into thermal runaway (if that is indeed the problem).

2. Check to see if the amplifier is oscillating.  Bipolar amplifiers like to oscillate at low frequencies, perhaps a few MHz or so, because the gain of the transistors there is so much higher than it is at VHF.  The best way to do this is non-invasively --- sometimes the loading of your scope probe will disturb the circuit enough so that everything appears to be okay when you look at it on the scope.  It is only after you remove the scope probe when all hell breaks loose.  If you have a spectrum analyzer, you can probe the circuit by winding a coil of wire with a few dozen turns around a pencil, and then use the coil (take out the pencil) as a field probe at the end of a piece of coax.  Sniff around the circuit while you are applying a very small amount of drive and see what the spectrum of the radiated energy is.  Do you see energy only at the driving frequency, or is there other stuff present.  As you increase the drive, if the amplifier suddenly breaks out into oscillation, the spectrum analyzer will pick it up immediately.  If it is a low frequency oscillation, it will generally be at a very high power level ---- much higher than the output you would expect to get at VHF.  If you see this happen, kill the drive and remove the power from the transistors, because they will not last long under the stress.

3. Make sure that the emitters are VERY WELL grounded.  The emitter leads should be as short as physically possible.

4. Recheck the bias circuitry.  Is the bias independent of the drive being applied?  If the bias point is moving around as the drive changes, this should be investigated further.

5. Always perform these tests with a good dummy load connected to the output.

Hope some of this helps.  As I said, VHF bipolar transistor amplifiers are very moody creatures.  Proceed gently, and always plan to blow a few transistors up in the process of getting it right.

73,

David
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W6GF
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Posts: 161




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« Reply #4 on: July 25, 2006, 11:55:25 AM »

I will keep my answer short.  Partially it is the potential inductance the cap shows, but disapation factor is also key.  A single cap with very low inductance and very low disapation may be more costly.  One has to avoid self resonance as well.
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