In our posting about The Four Body Problem, I cited the four major elements of a SSB Transceiver. These were identified in alphabetical order. Thinking about it and as a memory aid I rearranged the letters to read FOAM. Now that may be easier to remember like FOAM on a Beer!
But today's post is connected to FOAM and raises the question as to why most of the radio circuits we see use NPN transistors to the almost exclusion of any PNP devices. A notable exception was the SBE-33 and 34 SSB transceivers where most of the circuitry was not only PNP but also Germanium devices.
I have no good answer as to why we are not using PNP devices? If you look at the PNP datasheets for devices like the 2N2905 or the 2N2907 they certainly pass the smell test as to Ft (cutoff frequency) and Device Dissipation.
So, with that in mind we turn firstly to simulation software (LT Spice) to see if there is something not seen. The circuit simulated below with a 1-volt input has 19dB of gain at 20M. I have even designed a circuit board for this Driver stage and that is shown too.
Does the answer get us to something like well it seemed strange to ground the Collector or it always has been that way. I suspect both are part of the answer. But we have proven the circuit in simulation and now is the time to build the hardware.
A pictorial of the parts layout is provided for your dining and dancing pleasure. Gulp, you could even do this with Manhattan squares and the awful glue down process.
A good test of the use of PNP transistors in a homebrew SSB design would be to convert the PSSST-20 (N6QW website) design to PNP devices. In place of the 2N2222A, I would use the 2N2907 (TO-18). Here is a simulation of the IF Module converted to the 2N2907 PNP device. The 2N2907 has an Ft = 200MHz, a device dissipation of 400 milliwatts and can do a Collector current of 600 milliamps. Thusly, for the 2N2222A we use the 2N2907 and for the 2N2219A we use the 2N2905. What I have not explored is a P Type MOSFET to replace the IRF510.
With an IF of 9MHz, we have 42 dB of gain. Considering a loss of about 6dB in the filter this module is good for 36dB as a gain block. I am super encouraged.
I have seen some devices on eBay that a bag of 10 Motorola units is going for $1 each. You only need five units for the conversion.
If I get some time, I will tackle the conversion of the Mic/Audio Amplifier stages using the 2N2907.
This is the value of LT Spice -- design conversion. No Nano VNA's were used in this process and my soldering iron is off.
73's
Pete N6QW
(The home of innovation)