2SC2078 Matching Circuit

[jamfletch]

Hi again all,

I'm trying to build a 2SC2078 amplifier, I thought I'd start with the application circuit given in the datasheet (https://www.futurlec.com/Transistors/2SC2078.shtml). I've attached a screenshot of the test circuit. Just wondering if anyone knows what is meant by "90 4T" for the inductors? I initially thought maybe this meant "4 turns on a Type 90 ferrite core", but doesn't look like Type 90 is actually a thing.

Any suggestions?

[OZ1LQB]

hi
it is not 90 but 9Ø which means 9mm diameter..
have fun

[Wallace Gasiewicz]

Those are just coils without any cores. "air core"    Since this is a 27MHz amp you may wish to look at some of the common CB Radios for insight. Get a schematic of a simple cheap CB. I think that 2078 was used in CBs before MOSFETs were common and less expensive.     
With prices these days, maybe think about a IRF 520 MOSFET based amp. 520s are really cheap.
I think that the 2078 is usually provided with a DC Bias on the gate also. Through a higher value "choke"

[jamfletch]

Managed to built the circuit, I've tried driving the Base of the transistor with a 27MHz source. I've got about a 2V pk-pk sine wave drive at the base, which I thought would've been enough for driving the transistor on (Measure Vbe to be about 0.6V) . Don't suppose anyone has any ideas? Collector is biased to 12V, but I'm getting no current draw from my 12V supply.

[PA0PBZ]

(Measure Vbe to be about 0.6V)

If you look at the 3rd graph under the schematics in the datasheet the transistor is just 'waking up' with a Vbe of 0.6V. I think you have to drive it a little harder!

[OZ1LQB]

You have to remember that the vast majority of 2sc2078 on ebay and ali are fake transistors, so you should probably try to find an old CB and pick from there, believe me I have pulled out a lot of my sparse hair because of fake transistors from ebay/ali

[jamfletch]

Yeah tbh I can't even remember where I bought these from.

I've just tried driving it with an IRF510 amp I made, so about 23/24dBm output into 50 ohm. Obvious some loss due to poor return loss to the 2SC2078.

Gain wasn't great though, maybe only a few dB. Max output of barely a Watt. Datasheet says it can deliver 4W, but maybe my output match is just extremely poor.

So maybe they are just poor counterfeit transistors, or I have a crap input/output matching network