[hpsdr] Munin lll

[Kjell Karlsen]

Hi Bob.

The problem is that when I built the first prototype of Munin III, I used  
a pair of RD100HHF1 from an old batch of transistors that have a low Vth  
(Gate treshold voltage) This voltage can be from 1,5-4,5V (see the date  
sheet page 1). If the Vth is in the upper part of the voltage range, R9  
and R10 (1,5K) must be replaced with 2,2K resistors.

I have received reports from 2 Hams that have had this problem. It seems  
as the transistors delivered today have higher Vth than before.

Sorry for the problem but there has not been any issues before as far as I  
know.

73, Kjell, LA2NI




På Fri, 02 Mar 2018 06:33:26 +0100, skrev rwfish <rwfish56 at gmail.com>:

> ***** High Performance Software Defined Radio Discussion List *****
>
> Hello,
> I recently finished building a Munin lll amplifier. I am having trouble  
> setting up the bias for T1 and T2. I assume that the devices should be  
> biased at 1A each as they are in the Munin 2. I can’t seem to get the  
> the current to 1A on either device with the associated pot. I am  
> measuring the total current to the board with a DMM in series with the  
> +13.8 line. I thought perhaps the 10K thermistor circuit was loading  
> down the bias circuit so I lifted one leg of the thermistor. This did  
> raise the current somewhat however still not enough. I also have a bit  
> of an imbalance between the devices (ie different gate voltage to get  
> same source/drain current). The current maxes out at 800ma on one device  
> and 400ma on the other. I then lifted the gate connection on each device  
> and measured the range of the bias voltage. Both bias channels are  
> identical at 2.5 to 4.3vdc. I then used an external dc power supply to  
> bias the Mosfets. I was able to swing both mosfets all the way up to 5  
> or 6 amps or higher independantly with an external bias supply, but I do  
> have to go slightly higher in voltage than 4.3v. Maybe 5v or so. I am  
> stumped, any ideas?
>
> Sorry for the long winded post but I wanted to include most of the  
> details.
>
> Thanks & 73,
>
> Bob. K6GGO
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