PNP Transistor Radios — What your father knew but didn't tell you

Emailers have encouraged me to publish my off-cuts. About 50% of my ideas fail. This is a tough hobby grasshopper. I first chose to not publish my VHF PNP experiments from 2024 because few people ever make PNP transistor circuits --  and as the kids say; "like  - who cares?".  Perhaps no one indeed.  But here goes.

I love PNP circuits and the rich history they hold as radio transitioned from tube (valves for you Brits) to solid state design.

Solid state amateur home brew is ancient craft

In the 1960s, using small signal geranium transistors, home brewers were making single side band suppressed carrier transmitters + also receivers with crystal filters, phase-shift networks, & even Weaver networks.  Nothing new under the sun?

Above — Click to see a solid state complete 40 meter band SSB transceiver using a Collins mechanical filter for IF filtration. The A01 transistor and all others were PNP. Still, they got it done without all the fancy programs and test equipment we've got today. I feel humbled by old circuits like this. These images came from a printed book called the Transistor Radio Handbook. This serves as my all-time favorite radio book to read since forever.  Someone has it online here.   I treasure my printed copy.

 

Above — Inspired by this Figure, I decide to make a low current, PNP RF preamplifier for my battery operated weather receiver at 162.55 MHz. Please read - the Philco 2N1742 datasheet - an fT of 150 MHz -- and whoa, look at the current gain. I'll use a few more modern silicon BJTs in my design lol.  

I love silicon transistors -- although I once build a custom fuzz 'stomp box' for a professional guitarist using geranium transistors when I served as his (tube) amp tech.

 

From reading the  Transistor Radio handbook, even in the 1960's, home brew players had enough common sense to stick their projects in metal boxes and understood how to bypass and decouple their circuits.  Somehow, despite easy summations like Mark Montrose's book above, some modern analog builders seemingly feel that the laws of physics don't apply to their projects. Curious.

 

Above — My base schematic. A PNP preamp with tuned input + output, no negative feedback (or neutralization) and a collector current of 2.4 mA. I tried old-school neutralization as shown in the inspiration schematic , but found that tapping the output coil as shown worked much better to stabilize the amp.

Input Tank Circuit 

Above — Sweep of circuit adapted to allow assessment & tuning of the input tank circuit.

For the collector coil, I temporarily placed a FT-23-43 ferrite with 10 turns on it in-situ.  I shunted that coil with a 56 Ω resistor to give a decent output Z for my sweep circuit. Thus, I knew that the frequency transfer response was due the input circuit. I tweaked the trimmer capacitor so it peaked at 162.55 MHz.

Q1 = MPSH10, a 650 MHz part still surprisingly available in Mouser's catalog for 2025. Even with the "temporary" 56 Ω resistor | coil, this amp provides a power gain of  ~ 4.6 dB.

The rest of the circuit

 

Above — "Rookie mistake". To show what might happen if one decides to not include the 39 ohm 'UHF snubber' resistor on the collector,  plus -- not tapping the collector coil as performed in the original schematic.

 

Above — The transfer function of the original schematic as shown with a MPSH10 BJT. Love this.

Most modern builders would never put a BJT as a low noise amp in a VHF project. Plus they would run a lot of current for a high dynamic range.

BJTS at VHF and above bands in the 1960s led to JFETS -->MOSFETS -->GaAsFETS | MESFETS --> pHEMT and so forth over time. But, today, we are making battery powered radio circuitry & using BJTs circa 1963.  Feels refreshing.

 

 Above — What happens  when we stick in a 2N3906 PNP?  Power gain = 2.4 dB

 Above — My fastest leaded PNP part, the BF509 soldered into the Q1 slot.

 

Above — It's fun to read the datasheet of older parts. I've got several PNP SMD parts with an fT up to 9 GHz. Even most of these are obsolete and were collected years ago when parts were real ( not so bootlegged ) and relatively cheaper.

PNP circuits are really not that different or difficult than NPN circuits. I encourage you to experiment with PNP transistors for fun and learning.

Best to you --  Spring is coming!