Bench Amplifier

I’ve been working with some AD9850 ebay modules that are serving well as function generators, but they’re a bit limited output power wise. I needed something to boost its signals into the 1/4 to 1/2 watt range and the designs at SM0VPO’s page look to be pretty close to what I need.

Building off the design posted at the very bottom of the page, I’ve come up with my own board based on SMT parts because apparently I have a masochistic need to work with 805 components, but also because it would meant that even with the clumsiest layouts I could still fit it into the small die cast aluminum project boxes that I like.

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Using the toner transfer method and the HCl+H2O2 DIY etchant process I had the PCB laid out in just a couple hours, it’s not the prettiest board but it should do the job. I’ll post the eagle files at some point for anyone who’s interested… though it can very likely use some optimization… 🙂

Part placement went not too but there was one rather significant hangup. I had two types of transistor on hand, some BC847 and some 2SC4081. I thought I was placing the 847s, which would line up with SM0VPO’s design, but it turns out I was actually placing the 2SC4081s. This might have been a waste of time but I figured it was worth a go.

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When the board was fired up and fed a 1Mhz signal it did manage to amplify but it had a pretty distorted output. With the biasing set the way it was it seems like these little chips weren’t able to do what I was asking of them.
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I ended up swapping out these little guys for some higher power capable 2n4401 chips that I had to scrounge around for. I also added a pot for the biasing on the input transformer to see if I could tune in a little better. The emitter resistor also needed to be changed out. The value stayed the same but it turned into a 1/4W axial style in order to better deal with the power dissipation, even if only during my messing around with biasing values. Perhaps on revision 2 I might actually get a full SMT version of this amplifier, but this will do for now.

Here’s the whole package put together and in it’s new home.

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With a 0.5 Vrms input I get about 3.3 Vrms output (both into 50ohms), so that’s not a bad performance for a rev A boards I’d say 🙂

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