Re: Transmitter Mods

Gordon Gibby <ggibby@...>

howard -- thank you for that review!   The inductor I find is up into the 2-3K  (ohms reactance) range by 20 MHz so it definitely changes the feedback ratio at the higher frequencies.  Capacitors across the emitter resistor likewise reduce the negative feedback at higher F.    This little improvement is one of the coolest things I've read yet about the uBitx power issues --- and also the idea of adding C across the power amplifier output transformer to better match --- 

I'm encouraging our Local ARES group to move into these rigs as a learning platform and also for some, their first HF rig -- can't afford the pricey ones --- wonderful platform for LEARNING.     Getting 10W or so on 80/40/30 would be great for NVIS emergency communications for many of these people.   At at a far far cheaper price than commercial fancy qrp rigs --- so next is to put one of mine in an ammo can for protection.


From: <> on behalf of Howard Fidel <sonic1@...>
Sent: Thursday, April 19, 2018 10:00 AM
Subject: Re: [BITX20] Transmitter Mods
The theory is very simple. The closed loop gain of Q911 and Q912 is set by the ratio of R86/R85, or a gain of 10. Adding an inductor into the path in the feedback loop increases the effective impedance at higher frequencies, adding "peaking" to the circuit, thus making the closed loop gain higher. I think the poor frequency response is in the transformer as well as the transistors hfe falling to under 25 typically at 30 MHz. Also, the open loop gain is also increased by bypassing the emitter resistors for higher frequencies.

The transformer is a 2 to 1 step down, (it looks like a trifilar winding) so the theoretical gain of the stage is 5.


On 4/19/2018 12:36 AM, ohwenzelph via Groups.Io wrote:
How and why does this mod work? What was the calculation that came up with 220pF and 27 uH? 
For the inductor does it matter how many ma it can handle or what it’s self resonant frequency is?
(doubt I will be able to follow the answer but still curious)

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