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Registered Member #207
Joined: Sat Feb 18 2006, 05:14PM
Location:
Posts: 45
Hello everyone,
I propose the following question -- I'm sure someone (with their sagely wisdom) will be able to help me:
The attach document is a partial schematic of a VHF LNA design. For reference, it is sandwiched between two bandpass filters and acts (obviously) as a gain stage before the signal is mixed into an IF and digitized. It's being used in a standard superheterodyne type of setup.
Alright, now, the company that designed this receiver chose to design their LNAs discretely. This isn't surprising and is probably due to the cost-savings associated with avoiding an IC based LNA solution. No biggie.
Here's my problem: What is Q3254? From the schematic, I can decode it to be an NPN and PNP transistor pair (probably matched?). However, I have no idea what it does. Why is it there? Does it aid in biasing the transistor amplifier? (Q3255). I'm sure this is probably some sort of standard setup that i'm simply ignorant of.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
Q3254 is just two transistors in one package, the npn transistor is connected as a diode (collector connected to base) to compensate for the pnp base-emitter voltage. So, as you guessed, Q3254 is to bias Q3255. Any small signal transistors will work, having the npn/diode and the pnp in the same package ensures that the dVbe/dT of the two devices track each other, you could thermally bond (glue) together two discrete transistors but a dual transistor isn't expensive and would be better.
If this is for hobby use I suggest something like a BF964 dual-gate fet, only 1db NF and easy agc. (Google BF960, 961, .....) or some newer/better device.
Registered Member #207
Joined: Sat Feb 18 2006, 05:14PM
Location:
Posts: 45
Thanks for the help, Sulaiman. I appreciate it.
One more question:
What does the PNP transistor do, then? What's the advantage of biasing Q3255 in this fashion?
To be honest, i'm a little rusty with discrete amplifier design. I understand why biasing a transistor is important. But, what would this method of biasing provide above and beyond simply using a a voltage divider to achieve an appropriate bias voltage?
Why do they include the PNP portion of Q3254 to achieve a correct bias voltage?
Registered Member #29
Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500
Since noise figure in transistors is mostly determined for a given collector/drain current, a constant current biasing scheme gives the best results... That is what this is.. Current is determined by the voltage divider R3257, R3259 and the voltage drop across the paralleled resistors R3255, R3258.
I use this type of bias scheme all the time in LNAs (mostly GaAs PHEMTs). Note that microwave FETs will require a 2-5V negative supply for gate biasing...
Registered Member #207
Joined: Sat Feb 18 2006, 05:14PM
Location:
Posts: 45
Sorry for the double post, but one more thing is causing me trouble.
So, given what was stated above, what would you call the basing scheme of Q3255 then? Given that constant current it's being provided (via Q3254 et al.), is it safe to say that Q3255 is being biased in a "collector feedback" style, or a "base bias" style?
Or, is the concept of limiting the bias current in this fashion the "biasing method" itself. Whereas, this constant current is reliable enough that our source signal (through C3936) can then constructively and destructively interfere (eg: modulating our bias). Thus, resulting in the amplification.
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Decoding an existing LNA design. Unfamiliar transistor biasing?
