AD603 Thoughts

By | 6th July 2017

Having done some research in to the AD603 which which was drawn to my attention by G6LBQ, I found a supplier who offers free next day delivery – RS Components for the sum of £9.60 each plus VAT.

I will put together a breadboard lash up of the AD603 circuit published in the EI9GQ “Building a transceiver” handbook and will post test results and schematics in due course.

AD603 Datasheet

Key points about this device:

  • Low external component count.
  • Fixed gain amplifier so no non-linear components created by AGC bias changes.
  • Voltage controlled attenuator – neat.
  • Input and Output impedance seems to favor 50 Ohm system architecture.
  • Not obsolete.
  • Available in DIP 8 discrete or SMD package.

Downside so far is the price, however the trade off in what appears to be published performance over variable bias amplifiers far outweighs the cost to me – watch this space!

9th of July 2017 – Proof of concept testing.

I have built a breadboard lash up of the AD603 variable gain amplifier following the schematic published in the Analogue Devices datasheet, so far it looks good but still not 100% as I am not getting the gain expected  – currently seeing around 16dB, for a 1.3V peak to peak input I am seeing a 8.96V peak to peak output – not bad but not what I would expect based on the spec of the AD603 device, I also need to spend some time figuring out how the gain control works as applying a positive voltage decreases the gain but only by 10dB – see the video below for an explanation.

4 thoughts on “AD603 Thoughts

  1. Andy

    Hi Robert

    I would recommend putting a bandpass filter between each of the AD603 stages. I am about 80% done laying out a PCB and rather than purchasing some of the non preferred resistor values for the offset voltage I am going to use Multi-Turn presets which will allow some flexibility with voltage adjustment and the sequential gain.


    Andy G6LBQ

  2. Robert Mullen Post author

    Thanks for the suggestion Andy, I have some spare 3 pin type 10.7MHz Crystal filters so I guess I could use those between the AD603 stages, it would save me having to muck around making more tuned circuits. 🙂
    The AD603 devices are due to arrive today, so once I have created a test setup based on the design I am using I will pop a short video up here.


    Rob 2E1IIP.

  3. Andy

    Hi Rob

    The filters I have designed use one standard 10.7 Mhz 10mm Toko type IF coil and a few capacitors, the filter is matched to the 100 ohm impedance of the AD603’s input and output pins.

    I am guessing that your 10.7MHz Crystal Filters have a higher impedance than 100 ohms ? If this is the case you will have to fabricate two inductors. You can match the crystal filters to the AD603 I/O ports with either a pair of LC matching networks or matching transformers.

    I will email you a schematic of my AD603 IF sometime tomorrow.


    Andy G6LBQ

  4. Robert Mullen Post author

    Hi Andy, thanks for the info and much appreciated.
    The filters are 7.5KHz wide with a 1.5K I/O impedance so a simple L Match either side should do the trick – maybe I have over thought this as I am sure the component count will be much higher than a 10K coil and some capacitors, I suppose the added benefit of the Toko coil is the ability to adjust it.
    I am going to breadboard the circuit below taken from the AD603 datasheet and see what it looks like compared to my test results yesterday, it will be interesting to see the approach you have taken Andy.


    AD603 Test Circuit

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