Re: question re performance of bidirectional amplifier (corrected)

Arv Evans <arvevans@...>
 

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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