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  • Author Author: michaelkellett
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Fast Test Amplifier - Part 1

michaelkellett

A while ago (July 5th) in the discussion

 

https://www.element14.com/community/message/241748/l/programmable-frequency-generator#241748

 

I suggested an approach to building a low cost high frequency power amplifier.

The idea was to keep the design as simple as possible and base it on the classic 1970s bipolar transistor based HiFi power amplifier.

 

I've taken the idea a bit further and designed some boards, and finally got round to putting the parts on one and doing a bit of testing today.

 

The final schematic looks like this:

 

image

And the board looks like this:

 

image

 

I've been testing it cautiously using +/- 12V supplies (should be OK up to +/-18) and a 20 ohm load.

So far it looks quite good, small signal bandwidth is up to 10MHz but it can't do anything like full power at that frequency.

All the tests below were 12V pk-pk into 20 ohms:

 

Sine wave at 1MHz:

image

 

And the spectrum of the same:

 

image

 

Square wave at 500kHz:

 

image

 

And a close up to look at the slew rate:

 

image

 

So far it doesn't look too bad - quite a bit more testing to do and almost certainly some tweaking. I'm hopeful of meeting my target spec of 10W into 8 ohm load up to 2 MHz.

 

If anyone wants to build one I'll be posting the BOM and will put the Gerber files on Dropbox or similar.

 

MK

  • Good stuff. Effectively, it's a high-power op amp, which I suppose is where those amplifier designers were getting some of their ideas from.

     

    Look forward to seeing if you can make the thermal side work.

  • in reply to Jan Cumps

    I too have been using a picoscope, just the basic 10MHz version, but I love the big screen, data capture and analysis capabilities.

     

    DAB

  • in reply to michaelkellett

      wrote:

     

    Here's a plot of the small signal (1.7Vpk-pk) frequency response, still 20 ohm load. Some more work perhaps to tame the HF peaking (not too grim at about 3dB but should be less).

     

    image

     

    I've been using a Picoscope 5444 to do these measurements and used the free response plotting app that will work with it and any other signal generating Picoscope.

     

    I'm very impressed with the Picoscope, the hardware of the front end isn't quite up with the big four (Keysight, Tek, R & S or LeCroy) but the price runs at a small fraction (maybe 10 - 20%).

    The basic control software is good and things like decoders or FRA are free.

     

    ..l

     

    MK

    That ‘ll be a lesson for me. I used to see the picoscope as “not real compared to real scopes”. You’re opening my eyes.

  • in reply to shabaz

    I had a few boards made -  I need to do a bit more work before I'm happy to inflict it on third parties but if you let me know when you are ready to play with one I'll put parts on another and send it to you.

     

    The board is designed to be bolted flat down onto a heatsink ( it needs serious thermal management) which is why I'm running it into 20 ohms and off lower than planned supplies.

     

    I'm off on hols for a week starting tomorrow so not much more will be done before the end of August.

     

    I won't get the control board done much before the end of September.

     

    MK

  • in reply to DAB

    Here's a plot of the small signal (1.7Vpk-pk) frequency response, still 20 ohm load. Some more work perhaps to tame the HF peaking (not too grim at about 3dB but should be less).

     

    image

     

    I've been using a Picoscope 5444 to do these measurements and used the free response plotting app that will work with it and any other signal generating Picoscope.

     

    I'm very impressed with the Picoscope, the hardware of the front end isn't quite up with the big four (Keysight, Tek, R & S or LeCroy) but the price runs at a small fraction (maybe 10 - 20%).

    The basic control software is good and things like decoders or FRA are free.

     

    Back to the amplifier - my plan is to use it with a control pcb which will buffer and bandwidth limit the input  as well as manage DC offsets and power supply regulation. You may have noticed the tap off point at R33/34 - this will allow the control board to measure DC offset on input and output and inject a compensating DC offset so that there is no overall DC  error but without resorting to AC coupling. Because this requires an offset injecting DAC it will also be possible to deliberately dial up any DC offset you like. The Rigol offers this feature and I wonder if they have gone down the same path to get there ...

     

    MK