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.
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.