EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: ezalys on March 25, 2022, 01:22:27 pm
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Suppose I want a DC coupled amplifier with a -3 dB roll off of maybe 600 MHz. It’s a bit tempting to buy a nice HEMT and resistively bias it as a standard common source Class-A with source degeneration. Clearly though, most RF amplifiers are not designed this way. At what frequency does this naive design tend to stop working, and what can be done to push it up to higher frequencies? If this is a bad strategy, does anyone have any examples of common source Class-As that are DC coupled and have good bandwidth up to 600 MHz or advise how to do so? I have effectively infinite power and thermal budget, if this is a concern.
Also by DC I should be clear and say I mean 100 Hz. I have no qualms with a somewhat complex bias servo for the gate, if the 22 uF needed into 50 ohms is unavailable with a high enough SRF. For the drain bias I have no idea how you’d couple it down to 100 Hz and keep it matched to 50 ohms.
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Easy -- use shunt feedback instead of series. A common sight is a resistor from D-G (maybe with coupling cap), as "neutralization", but it's not actually neutralizing anything, it's just reducing the gain to improve stability. Which comes with reduced distortion (it's negative feedback!), and at the expense of whatever power is dissipated in it (a bit of an embarrassment for power amps using this, needing big fat power resistors :) ).
Since you're DC coupling, this also serves as DC stability, i.e. as part of the gate voltage divider.
Do you really need much? Or is this at modest power levels so it's just not something jellybeans can serve? I made a wideband amp a long time ago,
(https://www.seventransistorlabs.com/Images/WidebandAmp2.jpg)
which despite the rather loose layout, rolls off in the 600MHz range using BFR92AWs. (Although not this exact layout, I did two variants; one with peaking components and some tweaks to stabilize the poor layout, the other with more of a, whatever that thing is called*, architecture.)
*It's cascaded transconductance and transresistance amplifiers, there's a name associated with that that I can't remember.
Tim
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I want maybe 10 volts into 50 ohms — so it needs to be pretty beefy.
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10 volts peak to peak or rms?
Best,