Exploring PNP Transistor Microphone Amps
This is a first for me in terms of using PNP transistors for microphone amps. In fact more recently it was a first for me to use a single NPN transistor as a microphone amp. For the longest time my standard building block was the NE5534 for microphone amps.
On the air signal reports for the Junk Box rig were very good with the added comment that there seemed to a favoring of the highs with very little in the way of low frequencies. Well Duh, a couple of minutes with an LT Spice Simulation confirmed the why this was so. This post will explore the why.
[I should note that I have a bag of vintage PNP audio type transistors that I have often wondered what can be done with these treasures of old? Now I know!]
I ginned up the circuit I was initially using and it worked and just moved on but given the reports of the lack of lows I actually simulated the original circuit and found that there was a substantial lack of gain (like 10 to 15 dB) for frequencies below 1 KHz. Above that frequency things were much better.
Since there were no LT Spice Library entries for the 2N996 I picked the 2N4403 and then verified the same results with a 2N3906. Two capacitors play a key role in boosting the low end response and those are C1 and C4. Initially I had C1 at 100NF and C4 was 10 ufd and of course we could see that lack of the low end response. With a bit of cut and try these final values of 10 and 100 ufd, really boosts the low end. Now there is less than about a 3 dB change from 300 to 3000 hertz. I used a generator with a 100 millivolt output and swung the frequency from 10 hertz to 5000 hertz.
For those picknitters in the readership the + side of the 10 ufd is toward the signal input and the minus toward the base. On the 100 ufd, the + side is connected to the emitter of the device. I installed a socket on this board so I can actually turn this into a PNP transistor checker. For those who are still uncertain about electrolytic cap polarity-- the answer is to purchase and install non-polarized electrolytic capacitors.
A little more than the 15 minutes I spent with LT Spice could even further improve the low end so that the circuit is flat to 5 kHz. At this point I say Basta and just move on.
|Schematic of the PNP Microphone Amplifier|
Finally for the doubting Thomas crowd here is the expected performance of the circuit. I should also caution that R4 is really a 10K Trim Pot and the 220 NF (0.22 ufd) cap is connected to the center wiper. The output end of the 220 NF is paralleled with the audio amp input where both are connected to pins 3& 4 of the SBL-1.
|Projected Output from the PNP Microphone Amplifier Stage.|
The bottom line is that the PNP Microphone amplifier stage is good for about 20 dB of gain over the audio range and confirms the suitability of the device in homebrew SSB transceiver projects. Somehow many of us sort of gravitate to NPN devices --but good circuits can be built using PNP devices.