SM2000 Part 4 – Tx Amplifier Chain

This week I’ve been working on class A and class C amplifiers for the transmit side of the SM2000 design. Yesterday I worked up a gain budget that would take me from -20dBm at the output of the 1st mixer to +30dBm (1W) at the output of the PA. I need two 20dB (ish) driver stages and one 10dB (ish) PA, 50dB of gain in total.

The prototype amplifiers all had Z-match networks that matched to 50 ohm input and output. When combining the amplifiers I combined the Z match networks, removing the intermediate 50 ohm step. So we go from 2500 ohms at the output of the first stage to 10 ohms at the 2nd stage input. Then 250 ohms at the 2nd stage output to 10 ohms 3rd stage (PA) input.

I cranked the handle on the Z-match calculations and soldered the thing together:

In the photo you can see the extra large rectangular pads I used to give Q2 and Q3 some heatsinking. I measured and tweaked all the home made inductors using the spec-an method from the last blog post. I estimate they are within +/- 15% of the values on the schematic.

It worked first time but the overall gain was a bit low at 43dB. So I entered “experimental mode”, and started testing individual stages. To test stage 1 and stage 2 alone this meant changing the Zout network to match the 50 ohms of the spec-an.

Couple of changes:

  • The 1st stage gain was a bit low. I changed L1 from 24nH (the calculated value) to 57nH, this gave me +3dB. I don’t know why, and it’s annoying me! However I appreciate that at VHF the calculations can only get you to within 20%.
  • While testing stage 1 and 2 there was some instability, which went away when I rotated L2 45 degrees away from L3.
  • In my first pass I use a 18 ohm resistor to bias Q3 at 0V. However when I changed this to a RF choke the output power jumped up to 1W with -20dBm drive, and I can get 0.5W with -30dBm drive.
  • The power supply filtering (R7 and R8) causes about 1V of drop to the power supply of Q1 and Q2, which leads to a few dB loss of drive. This could be improved by recalculating the Q1 and Q2 bias points.

The BFQ19 is not meant for 1W so I only ran it for a few seconds there. I killed one when I raised Vcc to 14V, it’s rated at a Vce(max) of 15V and with Vcc=14V the collector would be seeing at least 2Vce = 28V. However it runs OK for a few minutes at 0.5W.

Next Steps

I figure this design has plenty of drive to deliver 1W with the right output transistor. Now I need to select and obtain a suitable transistor and test it. Think it will need to have a Vce(max) of 36V, 2W dissipation, and >10dB gain at 150MHz.

I need to integrate this PA with the PIN diode TR switch and come up with some switching earlier for the BPF output just after the mixer. The tx and rx chains share the 1st mixer (which is bi-directional) and BPF. So I need a way to switch between the input of this tx amplifier chain and the LNA output.

The PA has a 2nd harmonic just -38dB down so we’ll need a little more filtering, spurious 50dB down would be nice.

3 thoughts on “SM2000 Part 4 – Tx Amplifier Chain”

  1. Maybe look into using ferrite beads rather than resistors for R7 and R8? They’ll get you a few hundred ohms at 100MHz while being below an ohm at DC and are (mostly) resistive so you don’t have to worry too much about instability.

  2. Hi David,

    Amazing work :).

    So I have a ‘dumb’ question:

    Your amplifier has roughly 50 db of gain.

    The input is -20 and output +30.

    I have always wondered about SNR and noise floor of the transmitted signal. It would seem that the SNR of the transmitted signal MUST get worse as more gain is added between the original signal source and the antenna? [And more noise means a higher error rate.]

    I don’t have any real data, but for me it is an interesting question. I run my transmitter and several ‘channels’ away the SNR is degraded [for a lot of people]. So a simple ‘broadband’ amplifier ‘should’ be MUCH worse than a transmitter like this one you have designed.

    I watch 20 meters and I can often see broadband noise sources [spread spectrum signals?] degrade the SNR of the whole band I am watching.

    I guess analog filtering is about all that can help? This is done in this case by the various interstage coupling elements.

    Just a curious question.

    John

  3. Hello John,

    It’s a good question. I haven’t thought about tx noise myself.

    Each amplifier stage will inject some noise, and degrade the SNR.

    Say the final stage has a gain of 14dB, a power output of 30dBm, and a noise figure of 10dB. So the noise power at the input to the amplifier is 10dB above the thermal noise floor of -174dBm/Hz. Lets say we are transmitting a carrier with a 1 Hz bandwidth which at the input to the final stage is +16dBm. So the SNR of the final stage is 16–174=190dB, which is rather high!

    So perhaps it’s like a chain of Rx amplifiers – the noise figure is effectively set by the first stage, which in this case is the DAC that generates modem signal. Subsequent stages degrade the SNR, but not in any meaningful way as the signal is so far above the noise introduced by each stage.

    What you may be seeing are non linear (mixer overload) or phase noise effects of your nearby, high power tx in yr rx.

    – David

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