Re: Bidirectional amplifier, DC measurements.

ajparent1 <kb1gmx@...>

--- In BITX20@..., "eternalesquire" <eternalsquire@c...>

Here are my AC and DC measurements for the Q3 amplifier circuit. I
had used a scope to measure rather than my voltmeter, so measurements
are eyeballed.
Not useful.

With R set to 12V from an external power supply.
Anode of diode is 9 V
Cathode of diode/collector of Q3 = 8 V
Base = 3 V
Emitter = 3 V (probably a tad less due to BE voltage drop)
If you'd used a accurate voltmeter or DVM you would see more
like 2.7V emitter and 3.4V on the base and that small difference
can be telling.

From BFO through an attenuator, I am applying 10.7 Mhz signal of
5mV pp seen by scope injected at junction of input capacitors

25 mV pp same frequency seen at junction of output capacitors.
25/5=5, way low. for 5MV in I'd expect 100mV or more.

I get no amplification whatever when power is removed or reversed,
which is what we want.
What happens if you reverse the signal (TX direction) same?

Except for the AC gain, this appears to be a properly DC biased small
signal transistor amplifier.

On the other sets of amplifier circuits, I see 50 mV pp at junction of
output capacitors.

Could it be that I am using the wrong techniques to view the signal
with my scope, rather than there being a bug in the circuit? I'm
halfway tempted to try putting this circuit in LTSpice as a sanity

I do question scope calibration from range to range and at that frequency.

Me too. I'm building my second and it's mostly a straight up deal.
Sure I have made mods and used ferrite loaded rather than washers
for the transformers but those are not going to affect the amps.
I may add that I've built a large number of these style (RC feedback)
amps and they work or are broken. the latter is often due to bad part
or the occasional use of 100ohm where I meant to pull 10ohm from the
parts bin.

Just for reference I haywaired that stage on the desk to try it
standalone. Used 2n3904s as they were handy, MSP2222 and 2N2222
have roughly the same DC beta so the biasing will be similar.
With Q3/Q11 biasing (470ohm emitter reistor) at 10mhz I got a
voltage gain of just under 7.2 ( IE: 5mV in and 36 out). Voltages
from ground were emitter 2.4V, Base 3.1V, Collector 10.0V and other
side of diode 10.7V. I was using nominal 13.8V supply for this. When
I changed the biasing to Q1/Q13 (220 ohm) I got a bit more gain 8.4
as measured. Gain fall off at lower current is not unusual and the
overall design of BITx is to not use a lot of RF gain as it favors
stability and resistance to overload. The later is important as
theres no AGC in the base design so it must handle big signals
without distortion. So all in all those MPS2222s are usable but
maybe not the best choice.

I used a 50 ohm loaded scope probe and precision 50ohm step
attenuator for the tests. So the gains are likely lower than
in circuit as a result of the 50ohm loading. By calculation
and eyeball the numner I got numbers look right for that
loading at 10mhz.


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