#### Collector current design point for Q911+Q912?

Jim

Just what the title says -- what's the design point for the combined collector current going to Q911+Q912?  Looking to sub in a 2N5179 here, but only has a max collector current value of 50 mA.

Thanks, Jim KK0U

Hi,

From datasheet max is 400mA.

Hi Jim,

You can also check the voltage drop across the emitter resistor during transmission and then you can calculate the current used.

73,

Mark

Jim

Hi Mark -- I plan on doing that (needing three hands and a prehensile tail), but also wanted to know what the intended value was so I know the difference between design and application here.

Thanks!

Jerry Gaffke

Here's how calculate the quiescent current, so no signal present:
For DC, T9 is just a piece of wire, and the 2.2 ohms at R89 is small enough to be disregarded.
So we have 12v across R85,R86, and the current through those resistors is 12v/(100+1000) = 0.011 Amps
and the voltage across R85 is  100*0.0109 = 1.1 volts.
We're assuming that the beta of the transistors is high enough that the base current can
be ignored.  Let's also assume that the voltage drop from base to emitter when in the active
region is around 0.7 volts (the BE junction looks like a silicon diode),
so the emitter voltage is 1.1-0.7 = 0.4 volts.
Each 22 ohm emitter resistor at R87, R88 has  0.4v/22ohms =  0.018 Amps through it.
And since the base current is so low, that is also the collector current.

Jerry, KE7ER

On Mon, Feb 10, 2020 at 07:46 AM, Jim wrote:
Hi Mark -- I plan on doing that (needing three hands and a prehensile tail), but also wanted to know what the intended value was so I know the difference between design and application here.

Curt

Jim

before you do all of that - have you measured both the CW and SSB output levels into a 50 ohm load?  yes you will need some kind of audio oscillator, but this might provide a little further insight.

also consider how you might use a few-watt rig in the SSB mode?  certainly as solar flux is higher, it might cross oceans but I am thinking on the upper bands.

we have a tendency to overvalue watts over dB's.

Curt

Jim

What's this "SSB" you speak of?  ;)

Note that I am not asking the original question as a way to "hot rod" the output of the uBitx V6, but rather as an approach to level off the output power with increasing frequency, as per Allison's suggestions.  Once I'm satisfied, I'll back RV1 off so that I'm getting at least 5W out on 20m, and the rest can fall wherever.

QRP CW op (mostly) here, so I understand the ability to get a signal out at 5W or less.

Thanks all.

wrong number, it is 2n5109

At 10/02/2020, you wrote:
Just what the title says -- what's the design point for the combined collector current going to Q911+Q912? Looking to sub in a 2N5179 here, but only has a max collector current value of 50 mA.

Thanks, Jim KK0U

Jim

For Q90, a 2N5179 ought to work -- it has an fT of 1.4 GHz, can handle 50 mW and dissipate 200 mW.

Curt

Jim

the designer snuck in a mixing path in the transmit chain, an audio driver and some key-up bias into the PA .... and provided a slow QSK effect.  funny thing I have hundreds of CW QSOs with it and a few SSB.  I justed drilled the hole for my new receive bandwidth control (via an integrated NESCAF audio filter).

with a v6 you are missing all the fun we had with refinements to correct spurious with v3 or v4.

thanks for your thread to introduce me to C81.  I see that 0.047 pF is 48 ohms at 7 MHz -- so it would seem that 0,001 or 0.002 uF could provide a pleasant boost -- I need to check my levels again before tweaking this - I have only 3.5 watts out on 50m at the moment.  I am thinking the 40m dip is more troublesome than the frequency slope.  mine has been there since the beginning, and got worse with spurious mods.

73 Curt

Jim

Note that C81 is 470pF in the V6 (haven't looked at earlier versions to see what it is in those).  Before you change it to 0.01uF (which is what I've done), try a 680 pF or a 750 pF instead, to see what the effect is.  I tried a 330 pF, which really didn't do much for changing the weird outputs I was getting.  My assertion is that the 470pF was resonating with something around 21 MHz.  TP3 showed much higher output (1.2Vpp vs 0.2Vpp) at 21MHz vs other frequencies.  I think that was also driving the low output on 40m, as something was resonating at the third harmonic, which was naturally being filtered out by the time you probe the BNC.

I've got more tests to run tonight.  I think part of the issue is that the Si5351 CLK 2 runs out of gas as it goes higher in frequency (45-75 MHz).  TP12 certainly shows the voltage dropping on receive as you move from 80 through 10 meters.

Evan Hand

FYI

C81 is 470pF in both the v4 and v5 boards that I have.

73
Evan
AC9TU

Rick Price

v3 Schematic shows C81 as a 0.1u

Rick Price
KN4AIE

From: BITX20@groups.io [mailto:BITX20@groups.io] On Behalf Of Evan Hand
Sent: Wednesday, February 12, 2020 11:52 AM
To: BITX20@groups.io
Subject: Re: [BITX20] Collector current design point for Q911+Q912? #ubitx #v6

FYI

C81 is 470pF in both the v4 and v5 boards that I have.

73
Evan
AC9TU

barry halterman

Can someone check the capacitance of c81 in circuit and see what value you get. I get 12.nf when I go across that particular cap. Far from 470pf but then again there is other circuitry to mess with the LC meter.
Barry

On Wed, Feb 12, 2020, 11:57 AM Rick Price <rickprice48@...> wrote:
v3 Schematic shows C81 as a 0.1u

Rick Price
KN4AIE

From: BITX20@groups.io [mailto:BITX20@groups.io] On Behalf Of Evan Hand
Sent: Wednesday, February 12, 2020 11:52 AM
To: BITX20@groups.io
Subject: Re: [BITX20] Collector current design point for Q911+Q912? #ubitx #v6

FYI

C81 is 470pF in both the v4 and v5 boards that I have.

73
Evan
AC9TU

barry halterman

Evan, I know that the value is shown as 470pf, but have you measured it in on the board, in the circuit, by any chance?
Like I said earlier, I measured it at 12nf. Someone posted that he changed the value to .01uf (10nf) and his power went up... I too have the low power syndrome with my V6. My v4 kicks butt at over 18 watts on 80 and 11 watts on 40. My V6 on 80 is 6 watts and 4 watts on 40. I don't recall the power on the higher bands.
Trying to figure out what is different in the power chain to cause such a big difference between versions.
Thanks for responding.
Barry
K3Bo

On Wed, Feb 12, 2020, 2:56 PM barry halterman via Groups.Io <kthreebo=gmail.com@groups.io> wrote:
Can someone check the capacitance of c81 in circuit and see what value you get. I get 12.nf when I go across that particular cap. Far from 470pf but then again there is other circuitry to mess with the LC meter.
Barry

On Wed, Feb 12, 2020, 11:57 AM Rick Price <rickprice48@...> wrote:
v3 Schematic shows C81 as a 0.1u

Rick Price
KN4AIE

From: BITX20@groups.io [mailto:BITX20@groups.io] On Behalf Of Evan Hand
Sent: Wednesday, February 12, 2020 11:52 AM
To: BITX20@groups.io
Subject: Re: [BITX20] Collector current design point for Q911+Q912? #ubitx #v6

FYI

C81 is 470pF in both the v4 and v5 boards that I have.

73
Evan
AC9TU

Evan Hand

Barry,

I did try to measure the C81 in circuit.  I got strange results: 26 nF for one of my v4 boards, and the same for my v5  As you stated before, we are measuring in circuit, so I would not trust the values.

v5 board: measuring CW - 6.7 watts at 7.050 MHz and 14.6 watts at 3.550 MHz.
v4 board: measuring CW - 7.4 watts at 7.050 MHz and 13.7 watts at 3.550 MHz.

Both with 13.8 volts to the PA measured into a 50 ohm dummy load with a Missei digital SWR meter.

I too have the power drop on 40 meters.  Have not been doing much with the ubitx, have recently built a Hermes Lite 2 and been playing with that with time I have for the hobby.  Most of my time is taking care of my wife who is having medical issues and needs to go into surgery for an intestine problem on Monday.

73
Evan
AC9TU

Evan Hand

Barry,

Did another quick test to see if the Si5351 drive might be the issue (this was one of the theories put forth before).  Measure both signals at TP2 with my scope.  Only real difference was the frequency, though the 3.55MHz signal was closer to a square wave, as would be expected. Values were about 480-490 mVpp.

73
Evan
AC9TU

Curt

I reported on another thread that adding 330 pF to C81 solved my low power issue on 40m, with minimal impact on the other bands.

Do measure your output on each major band into a good load, I used a homebrew dummy load. My drop was down to 2 to 2.5 watts. If your drop is not as much,  try adding around 100 pF and remeasure each band.

73 curt