LNA design (stability) - Page 1
Forum section: electronics / rf-microwave

author: vectorotter
author_rank: Contributor
timestamp: July 21, 2021, 09:39:49 am
content: I had a crack at following various Qucs and other tutorials relating to LNA design. The amplifier is intended for 1.7GHz HRPT signal reception. I hope someone might be able to make some comments about how to bring it to unconditionally stable as K is below 1 for much of my region of interest in frequency. K is >1 at high frequency.I picked what I think should be a suitable transistor, an BFP842ESD and went through ngspice simulation to find a suitable operating point for low NF.Here are the noise, stability circles and reflection coefficients of the transistor at the chosen operating point and @1.7GHzInitially I chose to (mis)match the input for the reflection coefficient given by the intersection of the NFmin circle with the highest GA circle value (toward the exterior of the SG input stability circle) and subsequently recomputing with the input match in place and then conjugate matching the output seemed to go OK but with a double inductor L match for the output - I presume the calculator tool decided a capacitor was too small to be realisable.Here is the schematic and Smith Chart plots for stability circles, reflection coefficient and noise figure circles and a frequency sweep of K, NF, S21, S11, post application of both input and output matching networks:I noted that S11 was outside the input stability circle and S22 inside but very close to the output stability circle. Output seems well matched to 50 ohm.Being prudent, I took the second intercept point of NFmin and GA at a lower gain and repeated the matching exercise yielding:At the expense of some gain I go with option 2 (in fact, I have tried both but post is rather long already) since the L network contains capacitors that can act as DC blocks it's convenient. I use my previous ngspice results to add a suitable bias network, using the inductors as the DC path and RF decoupling them so RF sees the original matching network. See schematic:and the plots:I note that S11 is outside the input stability circle. S22 is inside the output stability circle @f_0 . The frequency sweep clearly shows K < 1 around my area of operation. Referencing Infineon's app note AN_1805_PL32_1806_113119, I have derived a circuit fairly close to what they have in structure.I wonder about the value of my C4. I chose it to appear short to RF. In the app note they have a few pF (different matching network values too but not such order of magnitude different) I notice that moving it to a value where L2, C4 are series resonant @ around 3.5GHz (guessing that the series resonant frequency of this pair to ground is significant somehow...) I see improvement in K. What is going on here?Also, in the app note they have put shunt 160R direct across the output port - I see also improves the K situation at the cost of some gain.Here is the frequency sweep with C4=10pF and R10 activated:Hmm ran out of attachments. Anyway, K < 1 only between 1 and 2 GHz at the cost of a fair amount of gain, now 17.8dB @1.7GHz and slightly worse NF = 0.515 dB.I'll try degenerating the emitter with some inductance and see what that brings. Anything else I can try? I know it's not unconditionally stable but is it likely to be OK?Also, regarding realisation, would I be better off using double stub matching with microstrip at these frequencies since the C's and L's are exact values and design is very sensitive to these and I could in theory cut PCB trace to tune? Or better start the matching network from scratch with known nominal value L's and work from there?

author: vectorotter
author_rank: Contributor
timestamp: July 21, 2021, 09:48:52 am
content: As it turns out, bothering to write everything down got me to the emitter degeneration with 200pH the amplifier appears unconditionally stable...up to 6GHz ... hmmmAt the expense of a fair amount of gain but a second stage after a BPF can deal with that I reckon.

author: vectorotter
author_rank: Contributor
timestamp: July 21, 2021, 11:57:32 am
content: Here is my attempt at modelling a microstrip realisation (well, the matches are not physically realised):Is this even close to a good enough model to decide if it's going to work or not?What should happen in practice at the transistor pins and do I need to model a transition?The layer stackup dictates very thin ~0.198mm width and thin 35 um height T lines. Should I be able to worry a bit less about stability since there is a resistive damping effect everywhere?

author: neilhao
author_rank: Regular Contributor
timestamp: July 21, 2021, 06:33:59 pm
content: You will be surprised by adding a resistor at the input or output. Feedback topology could also be used for implementing unconditionally stable, however it will also reduce the gain a little bit.Generally, if one adds resistors at the input, then the noise figure of an amplifier will increase, however the stability can be improved. Adding resistors at the output will reduce the gain but can make the amplifier unconditionally stable. There are three possible ways to add resistances to the output:1) In series with the output, thus increasing the impedance that the amplifier sees.2) As a shunt across the output, thus decreasing the impedance that the amplifier sees.3) As a T or Pi Pad, thus keeping the impedance that the amplifier sees constant.

author: cdev
author_rank: Super Contributor
timestamp: July 21, 2021, 09:13:10 pm
content: I often have found that putting an LNA between two slightly longer pieces of coax makes it stable. Also if you have some antistatic foam, you can try putting it in a very small plastic bag and pressing the antistatic foam against the active device. That may help. Another useful thing to have are ferrite beads. Or sleeves. You can slip them over power leads, etc. Youre just trying to reduce the gain slightly so that its lower than unity gain, to stop the feedback. Could you upload a pic of both sides of the PCB?

author: vectorotter
author_rank: Contributor
timestamp: July 21, 2021, 10:05:13 pm
content: It's theoretically not unconditionally stable. Not unstable. Nothing has been proven yet.-editWith emitter degeneration it looks pretty good (see above 2nd post) but that will depend on if the value I plucked from the air, some 200pH is physically true once I get it on a board. The microstrip realisation is more of interest now for the reasons above around L, C values and sensitivity.