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I have had to add a resistor or ferrite bead when using my 7CT1N to
test bipolar cascodes. Bead selection is not critical for an
application like this. I have at least 6 different sizes and types
and all of them worked to suppress oscillation.

On Sun, 22 Jul 2012 16:52:12 -0400 (EDT), jerry massengale
<j_massengale@...> wrote:

>Hi,
>
>That has been on of my worries. I use a 2P5T rotary switch to select the base and collector of the uut but leave the four emitter pins tied straight to the emitter pin of the tester. The 7CT1N takes it to ground anyway. One configuration i used worked better if the unconnected metal enclosure top was grounded with a short lead. Other times the oscillation would dampen out if I held my fingers on the uut base/collector leads. I usually bus the 4 emitters together and run a short lead from pin 7 of the test socket to the emitter pin on the box. I wonder if it would help to make the connection at the pin13 subtrate. I did try running leads from all four emitter pins to the box emitter pin. That did not help. I do not have ferrite beads. What size and z should i get and try?
>
>
>Jerry Massengale
>
>-----Original Message-----
>From: Chris Trask <christrask@...>
>To: TekScopes <TekScopes@...>
>Sent: Sun, Jul 22, 2012 3:26 pm
>Subject: Re: [TekScopes] Re: CA3046 Curve Tracer Adapter
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>>
>> > I have been trying for sometime to make a curve tracer adapter that can
>> > test all 5 transistors in the CA3046 array used in 7K series mainframes
>> > and plugins. I have been fighting a problem of oscillations in the gain
>> > curves. I was reluctant to add base resistors as gain would be affected.
>> > After trying several cures I find that it is necessary to use 560ohms
>> > in series with the base of the UUT. Is this acceptable?
>>
>>
>> Depends on which gain you want to measure. For hFE, so if you drive the
>> UUT with base current, that resistor does no harm at all. But for Vb to
>> Ic conversion, with voltage driven base, 560R could easily be 10% and
>> much more of the dynamic base resistance (nominally 3.5k at Ic = 1 mA).
>> The larger Ic the worse this gets.
>> Can't you suppress oscillations with ferrite beads?
>>
>
>One thing essential to using the CA3046/3146/3246 is that pin 13 must
>always be at the lowest voltage. In the case of testing the individual
>devices on a curve tracer, that pin should be grounded.
>
>Chris
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