Re: question re performance of bidirectional amplifier (corrected)

ajparent1 <kb1gmx@...>
 

Hi,

I've found the MSP2222, PN2222, 2n2222A to be interchangeable
assuming good parts. However, depending on the source and how
they got in to the product stream they can be out of spec devices.
A good 2N2222a has an Ft of not less than 350mhz and good gain
at 5ma Ic. The 2n3904 however is a little quieter and slightly
better gain even though the Ft is the same. A 2n5179
or other VHF device will not achieve much more gain as the
amplifiers are of RC feedback type and component values set
the gain not the transistor used unless the transistor is
poor quality or simply not reasonable for the frequency
in use.

I have some bad commercial experience with parts that just were
not up to spec. I had this once back around '78 with MPS1764s,
bought a bag of them from Motorola and tested them with 100%
yeild to spec. Purchasing bought another 1000 from a cheaper
source and the failure rate was 25% to spec. Showed Motorola
and they wanted to know where they came from. Seems they were
purchased as known offspec for a non critical app the excess
were being resold as full spec. I've been similary burned with
2n3866s and 2n5109s (low Ft and beta), 2n5179s (noisy and low Ft)
to name a few.

So when talking parts like transitors simple DC tests do not tell
all. Measured in an RF test circuit it's easy to spot.


Allison
KB1GMX

--- In BITX20@..., Arv Evans <arvevans@e...> wrote:
Hi

On my first BITX (a BITX20) I used PN2222s and also found the gain
to be quite
low. After substituting a handfull of 2N3904s my gain became much
better.

Another early BITX builder found that his VFO was unstable, until he
replaced
a 2N2222 with a 2N3904.

There have been recent suggestions that VHF transistors be used in
the IF & RF
sections of the BITX. There seems to be some merit in that idea.

It seems that some providers of 2N2222 & PN2222 maybe assume that any
transistor that is NPN and not something else, is a 2N2222. My
2N2222s that
were purchased from DigiKey were all good in the BITX but those that
were
purchased from another source (100 each 2N2222s for $1.50 USD) show
very poor
HF gain, but they are just fine for audio applications.

Arv K7HKL
_._


On Sunday 21 August 2005 21:28, eternalesquire wrote:
1) Checked. These are 10 ohm resistors. I've made 2 stages so far
and had double checked all values of resistors going in.

2) I had been using 0.1 microfarad (value code 104) for all of the
capacitors of these units.

3) On a hunch that this was a bad type of transistor, I substituted a
2N2222A transistor. Gain was acutally worse, X6 rather than X10,
so I
put it back.

I strongly suspect that whatever mistake I am making, I am making it
consistently.

Now, the original schematic specifies 220 Ohm emitter bias resistors
in parallel with the emitter degeneration leg, for 2 of these units.
A 440 Ohm resistor is specified for the unit after the crystal
filter.

Were the original values wrong?

Should I make DC voltage readings and show them to you?

Thanks,

The Eternal Squire

--- In BITX20@..., "ajparent1" <kb1gmx@a...> wrote:
> Edited. inline comments.
>
> --- In BITX20@..., "eternalesquire"
<eternalsquire@c...>
>
> wrote:
> > I wired up a second stage and checked it against the
schematic as I
> > was doing it, every trace is accounted for.
>
> Another thought..
>
> One possible is that the 10 ohm emitter resistor (in series with
> capacitor) is not 10 ohms! It is sometimes easy to get 100ohm
> and 10 ohm by error. Rough calculation suggests that might be the

error.

> Another source of error is if any of the capacitors are not
> the correct value (too small).
>
> I'd also check the transistors used. Is the gain going TX and
RX the
> same? It could be possible one transistor is shorted collector to
> base internally.
>
> Just thinking out loud.
>
> Allison
> KB1GMX

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