Please see the previous posts to understand I am trying to replace obsolete transistors in my standard Class A amplifier chain with something readily available today. Especially for the output stage.
Having settled on a driver transistor I thought that replacing the 2N2219 with a generic buffer transistor was going to be without issue. However, this assumption was wrong and the final stage testing has been delayed.
The 2N2219, used as the buffer, has a claimed transition frequency of at least 250MHz. I've spent the last 40 years playing with small signal NPN transistors and regularly noticed in the datasheets Ft of 300MHz or more. So I assumed that pretty much any transistor I dropped into the buffer stage was going to work.
It wasn't that simple. I started with something marked G1. This was ~1.2dB worse that the 2N2219. Hmmn.
Grabbed something marked 1GM. This was ~1.5dB better than a 2N2219. Hang on. If this was a MMBTA05LT1, a Motorola NPN part marked 1GM, it should have been comparable with a claimed Ft of 330MHZ at 25mA.
An element of confusion was setting in. I decided to put the junk box recovered parts away and grab some nice new and fresh from packet NPN transistors.
The first was something labelled a BC549 from a long forgotten Aliexpress vendor. I agree, this could still have been a junk box part. It was marked G1 and was better than the first "G1" I tried and the 2N2219 itself. Rolloff was improved by 1dB over the 2N2219.
After trying many such parts I tried a 2SC3356R, Ft of 7GHz. The rolloff at 20MHZ was now 2dB better than the 2N2219 version. A similar outcome was achieved when I recycled something marked R22 from a Philips FM900 VCO board. Some claim this marking is a 2SC3356. I'm not sure this was a 2SC3356 however it was a high Ft device.
Clearly, the higher the Ft the better. Which I hadn't expected given the feedback networks being used.
I crunched up a spreadsheet with a Hybrid Pi model of a Class A amplifier without collector base feedback to see what difference Ft made. With all assumptions unchanged, shifting Ft from 100Mhz to 330MHz results in a lot more gain at 20MHz. Collector base feedback reduces this gain, but the numbers supported selecting a buffer transistor with a higher Ft.
Later I fired up RFSim99 and loaded up the S parameters for a Renasas 2SC3356. Fiddling with the emitter and feedback resistors suggested a minor variation in values would be better to improve the match to 50 ohms. I will use those values going forward.
There are many 2SC3356 transistors on LCSC. Since Renasas has discontinued this part I figured it would be prudent to include some in my next order so I opted for this 2SC3356 since it had curves that resembled those in the Renasas datasheet.
Conclusion:
Recycle, or spend a few extra cents if you're buying a transistor, and get something for the buffer with an Ft greater than 1GHz. This may be counter to the conventional wisdom that HF circuits do not need such a high Ft. But the difference could be as much as 3dB per stage. So for low level amplifiers with feedback to prevent oscillation I argue it makes sense since it could save an entire stage.
Now I can test a few final transistors.
73's
Richard
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