Re: ubitx - stop press
toggle quoted messageShow quoted text
I found the statement included below a bit puzzling. I think what Farhan is saying is that he took one of the bidi gain blocks from the uBitx and terminated it with a 220 ohm resistor instead of the 50 ohm load for which it was designed. The measured input impedance didn't change much from 50 ohms with the new load resistor value.
As W7ZOI and K3NHI point out here http://www.phonestack.com/farhan/bidiamp.pdf the bidi amps in the Bitx do not have this isolation between input and output impedance.
LZ1NEF mentioned in his post in this thread that a simulation showed a 90 ohm input impedance for the uBitx bidi amps at 10 mhz and varying with frequency, that may be worth looking into. Farhan's biasing approach can perform exactly the same as the W7ZOI/K3NHI biasing scheme with the proper selection of component values. The W7ZOI/K3NHI circuit uses more parts but allows bias current to be adjusted without affecting feedback and vice-versa.
On Sat, Apr 8, 2017 at 08:32 am, Ashhar Farhan wrote:
i have measurer the input impedance of these stages to be appox 50 ohms when terminated in the 220 resistor in the output.