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ChasW
Re: Variable Gain Amplifier For AD9850 DDS
December 12, 2018 03:10PM
Hello.

I found this article to be very interesting and quite useful given the increasing popularity of these low cost DDS modules. My comment is regarding the feedback amplifier you reference in Experimental Methods in RF Design. The circuit model for this amplifier is Figure 2.57 and Figure 2.64 which shows the negative feedback bypass capacitor in parallel to the bias resistor, both in series with the feedback resistor R-f to the transistor base. I noticed your arrangement was quite different. Was this by design?

Best regards,
ChasW
Mike, WA6ISP
Re: Variable Gain Amplifier For AD9850 DDS
December 12, 2018 03:11PM
I can not get any usable gain. I built with good RF practices. I used 3/8" - #43 torrid. Output Z is more than 50 ohms. Primary winding is about 50 uH. I even have problems simulating with given values in LTSpice. I have spent a couple days now, checking my Spice and Breadboard? Too bad, I spent a lot of time? Mike
Maven Merkel
Re: Variable Gain Amplifier For AD9850 DDS
December 12, 2018 03:12PM
First of all thank you for this contribution. Secondly, I wanted to if I could use an output impedence of 8Ohms instead of 470Ohms in the amplifier stage. The thing is I have a Hammond audio transformer lying around with impedance ratio of 8:600 and my final load is of 600. I was wondering if I could use it.
Cameron, VK2CKP
Re: Variable Gain Amplifier For AD9850 DDS
January 07, 2019 12:06AM
Interesting article however can you confirm the part number of the Schottky diode (2N5819 in schematics) as when I lookup that part it appears to be a PNP-BJT. Also, can you confirm the size of wire you used to wind the transformer (12:4 turns ratio)?
Re: Variable Gain Amplifier For AD9850 DDS
January 07, 2019 10:32AM
Hello Cameron,

Yes, the Schottky diode is incorrectly shown as a 2N5819 BJT. Thanks for pointing that out. It's just for reverse polarity protection, so pretty much any diode with current rating over 200ma or so should do, Use as Schottky for lower forward voltage drop, or omit it altogether if you are confident that you wont reverse positive and negative.

Regarding the wire gauge on the 12:4 transformer: IIRC I used thin wire-wrap wire on a 43-type material binocular core, which is probably not ideal as the wire has a thick plastic coating which could be more lossy than enamel coated. I don't think that it's too critical though: as thick as will fit perhaps? Anywhere from 20AWG to 26AWG should be just fine.

Cheers,
Steve VK2AAV
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